27 June 2009. Though the amyloid-β protein has been widely implicated in Alzheimer disease pathogenesis, proof for how it might trigger such devastation at a molecular level has been hard to come by. In the June 25 issue of Neuron, researchers led by Dennis Selkoe, at Brigham and Women’s Hospital in Boston, offer up evidence for a new mechanism. “This paper is an attempt to contribute to the finer analysis of synaptic dysfunction in what we view as the earliest phase of AD—when soluble Aβ is rising, and plaques are just beginning to be seeded and formed,” Selkoe told ARF. In a series of electrophysiology experiments, his team showed that low concentrations of soluble Aβ interfere with glutamate uptake at the synapse. This leads to overstimulation of NMDA-type glutamate receptors and makes neurons more prone to long-term depression, the weakening of synaptic responses.
Last year, Selkoe’s group isolated Aβ oligomers directly from brain tissue of AD patients and showed they are indeed neurotoxic (Shankar et al., 2008 and ARF related news story), validating numerous reports of similar effects using synthetic Aβ peptides or Aβ derived from cultured cells. Furthermore, when applied to mouse hippocampal slices, the human Aβ peptides facilitated long-term depression (LTD). In the new paper, first author Shaomin Li and colleagues tried to work out the mechanism by which soluble Aβ oligomers push hippocampal neurons toward this LTD state.
The researchers treated mouse hippocampal slices with several forms of human Aβ—synthetic Aβ1-42, soluble extracts of human AD cortex, and size-exclusion chromatography fractions of secreted Aβ from a cell line expressing mutated human amyloid precursor protein (APP). All three preps principally contained small Aβ oligomers and lacked insoluble aggregates. All treatments were done in brain tissue from wild-type mice, Selkoe noted, as they wanted to mimic what might happen in the vast majority of human AD cases that have no mutations in known genetic risk factors.
Consistent with his lab’s earlier findings (Shankar et al., 2008), exposure to soluble human Aβ prompted induction of mouse hippocampal LTD under low-frequency stimulation conditions that ordinarily would not induce LTD. Also as shown previously, the LTD-promoting effects depended on activation of metabotropic glutamate receptors (mGluR), since antagonists of mGluR prevented LTD induction by cell line-derived Aβ. N-methyl D-aspartate (NMDA) receptors also seemed necessary, though high doses of a potent NMDA antagonist were required to see similar protection from Aβ-induced LTD. The researchers saw similar results when they performed the experiments using synthetic Aβ1-42 or human cortical extract.
The involvement of glutamate receptors in Aβ-induced LTD led the researchers to look more closely at the role of this neurotransmitter. They did this by treating hippocampal slices with a glutamate scavenger enzyme prior to incubation with soluble Aβ. In this context, even a weak NMDA antagonist was able to block Aβ-induced LTD. The finding suggested that Aβ induction of LTD depends on extracellular glutamate.
A surprise came when Selkoe and colleagues made whole-cell recordings to measure glutamate receptor-mediated currents in pyramidal neurons from Aβ-treated mouse hippocampal slices. The researchers found that both NMDA and AMPAR (α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor) currents were reduced, which might seem paradoxical given their finding that extracellular glutamate may be necessary for LTD induction. But they reasoned that the decreased glutamatergic transmission would make sense if receptor desensitization was occurring. This hunch proved correct—treating the slices with an inhibitor of AMPAR desensitization (CTZ, or cyclothiazide) prior to Aβ exposure reverses the drop in AMPAR current. However, CTZ had no effect on induction of LTD by Aβ, suggesting that AMPA receptors are not involved in that process.
Receptor desensitization could be due to an excess of glutamate. To address whether soluble Aβ leads to the extracellular accumulation of the neurotransmitter by inhibiting its uptake by neurons, the researchers performed the LTD protocol on mouse hippocampal slices in the presence of the glutamate uptake blocker TBOA (threo-β-benzyloxyaspartate), which inhibits the excitatory amino acid transporters (EAATs) that scavenge glutamate released into synapses. The effects were remarkably similar to those seen when the slices were treated with soluble Aβ. Moreover, if TBOA was applied first, subsequent exposure to Aβ did not have much effect on LTD induction, and vice versa. To characterize the glutamate effects more precisely, the researchers measured the uptake of radiolabeled glutamate by synaptosomes purified from mouse hippocampus. “We were able to show directly in synaptosomes that soluble Aβ, at low concentrations relevant to human disease, interferes with uptake of glutamate,” Selkoe said.
By performing experiments in media with varying calcium concentrations or in conjunction with compounds that block release of intracellular calcium stores, the researchers showed that Aβ-mediated LTD requires excess extracellular calcium but does not involve intracellular stores. Similarly, using inhibitors of signaling pathways normally activated by extracellular calcium influx, they found that Aβ-induced LTD triggers the protein phosphatase 2B (PP2B)- and glycogen synthase kinase 3 β (GSK3β)-mediated pathways in common with conventional LTD but differs in its apparent failure to activate p38 mitogen-activated protein kinase (MAPK) signaling.
Drawing from all these data, Selkoe and colleagues propose that Aβ oligomers block glutamate uptake by neuronal transporters, which leads to overaccumulation of extracellular glutamate. The excess glutamate “could induce NMDAR desensitization and/or activation of presynaptic mGluRs, thereby reducing the peak amplitudes of the NMDA currents,” the authors write.
Lennart Mucke of the Gladstone Institute of Neurological Disease in San Francisco, California, finds the new paper “very interesting and thorough” and said it identifies “a relatively upstream mechanism for how Aβ may trigger electrophysiological changes in neurons.” Mucke said the new data seem consistent with recent work from his own group showing that Aβ at the network level triggers intermittent aberrant excitatory activity that manifests as non-convulsive seizures in APP-overexpressing mice (Palop et al., 2007 and ARF related news story). “There may be excess spillover of glutamate to extra-synaptic sites and receptors which could trigger aberrant excitatory activity,” he told ARF. “That might set in motion adjustments in the cell that would impair overall synaptic transmission.”
This idea also finds support from research published separately by co-author Dominic Walsh, of University College Dublin in Ireland. In that study (O’Shea et al., 2008), the researchers placed microdialysis probes into the brains of moving, conscious rats, and measured neurotransmitter levels after infusion of the same soluble Aβ oligomers used in the new study by Selkoe’s group. Walsh and colleagues found that Aβ treatment led to acute elevation of extracellular glutamate, but not GABA or aspartate.
Previous studies have proposed alternative explanations for how Aβ might induce synaptic dysfunction. In particular, research by several groups has suggested that Aβ decreases the number and activity of NMDA and AMPA glutamate receptors (see, e.g., Almeida et al., 2005; Snyder et al., 2005 and ARF related news story; and Hsieh et al., 2006 and ARF related news story). These findings seem to run counter to Selkoe’s new data, which suggest that Aβ first leads to enhanced activation of NMDARs.
The picture gets even dicier. Earlier this year, scientists offered perhaps the most provocative theory to date on how Aβ might affect learning and memory at the molecular level. They showed that Aβ oligomers bind cellular prion protein, and that this interaction was required for Aβ-mediated suppression of long-term potentiation (Laurén et al., 2009 and ARF related news story).
So how does one put into context these various candidate receptors and proteins that different labs have put forward as potential mediators of Aβ-induced synaptic changes? Selkoe offered several ways to consider the complexity. “One is that people are using different methods and different preps of Aβ,” he said. “It could also depend on how long the neurons are exposed to Aβ.” In the paper, he offers another possibility. “It could be that none of these receptors are the real cognate target of Aβ, but rather that hydrophobic Aβ oligomers bind to membrane lipids and that this secondarily perturbs a number of different receptors,” he told ARF. “But that is pure speculation.”
Regardless of the precise molecular mechanism behind Aβ-mediated induction of LTD, a key issue Selkoe’s group plans to address in future studies is whether Aβ’s effects on LTD can be mitigated by environmental manipulations in living animals. In previous studies (see, e.g., Lazarov et al., 2005 and ARF related news story), AD mice housed in more stimulating environments (i.e., bigger cages with running wheels and colorful toys) had lower Aβ load and fewer plaques. Such enhancements seem to benefit wild-type mice, too. Earlier this year, researchers showed that mice in enriched cages had elevated levels of neuronal Wnta/b, signaling proteins involved in synapse assembly (Gogolla et al., 2009 and ARF related news story). Considering these data, Selkoe said his group would like to take hippocampal slices from mice that got some form of environmental enhancement, to see if they would exhibit amelioration of Aβ-induced LTD, compared with animals who did not receive the environmental manipulation.—Esther Landhuis.
Li S, Hong S, Shepardson NE, Walsh DM, Shankar GM, Selkoe D. Soluble Oligomers of Amyloid Beta Protein Facilitate Hippocampal Long-Term Depression by Disrupting Neuronal Glutamate Uptake. 25 June 2009. Neuron 62:788-801. Abstract