Soluble Aβ sends neurons into a hyperactive frenzy, but scientists don’t know why. New evidence suggests that the peptide causes the excitatory neurotransmitter glutamate to build up in synapses, making neurons hyperactive. In the August 9 Science, researchers led by Arthur Konnerth, Technische Universität München, Germany, suggest that after a neuron fires, Aβ dimers block clearance of glutamate released into the synaptic cleft. This makes the neurons even more active. The evidence comes from both live mice and hippocampal slices. The results may explain overactive neurons seen in mouse models of amyloidosis and in people with prodromal AD.
- Aβ dimers, both synthetic and human-derived, make neurons hyperexcitable.
- Compounds that block glutamate reuptake cause similar hyperexcitability.
- Aβ dimers may interfere with glutamate clearance, leaving more of the neurotransmitter in the synapse.
“This is a nice paper that mostly reinforces what we know about the interaction of Aβ at the synapse, but adds significant insight into the interaction of low-molecular-weight soluble oligomers of amyloid with specific components of the synapse,” wrote Brian Bacskai, Massachusetts General Hospital, Boston.
Scientists have known for years that various forms of soluble Aβ make neurons more excitable (May 2012 news). But the toxic species of Aβ and the mechanism by which they cause the hyperactivity are still in question. One hypothesis, from the lab of co-author Dominic Walsh at Brigham and Women’s Hospital, Boston, contends that Aβ dimers are the most toxic species (Nov 2018 conference news). As for the mechanism, evidence suggests glutamate might be involved. Researchers from Dennis Selkoe’s lab at the Brigham reported that Aβ interfered with reuptake of glutamate from the synapse, and that this impaired plasticity in hippocampal slices (Li et al., 2009; Li et al., 2011). Might poor reuptake cause hyperexcitability as well, and does this take place in live mice?
Exciting Compounds. Calcium imaging shows that active neurons (blue and green dots at left) fire more often (red, orange, yellow; middle) with the addition of either synthetic Aβ dimers ([AβS26C]2) or an antibody that impairs glutamate reuptake (GLT-1 AB). After washout, cell activity returns to normal (right panels). [Imaging courtesy of AAAS/Science.]
First author Benedikt Zott and colleagues started to address this by testing how neurons in 2-month-old wild-type mice responded to injected Aβ dimers. They used a special cranial window to monitor neurons by two-photon Ca2+ imaging while they pipetted various substances onto the hippocampal surface (Busche et al., 2012). Synthetic Aβ dimers quadrupled Ca2+ transients relative to baseline (see image above).
Interestingly, the same dimers applied to hippocampal slices had no effect. However, if the researchers stimulated the hippocampal slices by adding glutamate, blocking inhibitory inputs, or elevating the extracellular K+ concentration, Aβ dimers revved up the activity even further, just as they did it vivo. The results suggested that the mechanism whereby Aβ drives hyperexcitability relies on synapses that are already active.
To see if impaired reuptake of glutamate might be responsible for the Aβ effect, the authors tried to mimic it by pipetting the glutamate uptake blocker DL-threo-b-benzyloxyaspartic acid (TBOA) onto the hippocampal CA1 region of wild-type mice. They saw a similar quadrupling of Ca2+ transients as in Aβ-treated neurons. Further, both the Aβ dimers and TBOA transiently increased the extracellular glutamate concentration, suggesting they acted by a similar mechanism.
Still, TBOA might act via a different mechanism. To test this, Zott repeated the TBOA experiment in 2-month-old APP23xPS45 mice (Busche et al., 2008). These animals are too young to have developed plaques but old enough to have plenty of soluble Aβ in their brains and their hippocampi are markedly hyperactive. TBOA had no effect on hippocampal activity. The authors believe this is because the endogenous Aβ had already fully blocked glutamate reuptake.
How is that block achieved? Because astrocytes normally clear glutamate from synapses, the researchers wondered if Aβ might affect GLT-1, the main excitatory amino-acid transporter in these cells. GLT1 hangs out on the plasma membrane of astroglial projections that protrude into synapses. When a neuron fires, GLT1 quickly binds the newly released glutamate, then immediately diffuses along the projections away from the synapse, dragging glutamate with it. The neurotransmitter then passes through the transporter into the astrocyte (Murphy-Royal et al., 2015). Researchers can impair this process using an antibody to cross-linking GLT-1, anchoring the transporters in the membrane. Treating the CA1 region of hippocampi in wild-type mice with this antibody induced hyperexcitability on par with Aβ dimers (see image above), again suggesting to the authors that the two work via a similar mechanism.
The authors repeated their experiments using Aβ dimers isolated from human brains. These made mouse neurons hyperactive at lower concentrations than did synthetic ones, suggesting the human dimers are more potent.
Together, the results suggest that Aβ dimers work in a vicious cycle to make active neurons even more active by interfering with glutamate uptake. However, Keith Vossel, University of Minnesota, Minneapolis, pointed out that the paper relied on circumstantial evidence. Direct proof that Aβ obstructs the diffusion of GLT-1 was lacking. Demonstrating that hippocampal slices need to have baseline activity before they are susceptible to Aβ-induced hyperexcitability, was important, he said.
Selkoe thought it promising that Aβ could interfere with diffusion of glutamate receptors. "A key next step will be to decipher at the biochemical level precisely how the oligomers block glutamate entry into local astrocytes and neurons,” he wrote in an accompanying editorial. He thinks that glutamine antagonists, or agents such as ceftriaxone, which boosts the number and function of glutamine transporters, could be clinically useful if given at the earliest stages of Alzheimer’s. Combined with therapies that reduce Aβ-oligomers, such as secretase inhibitors, they might work together to lower hyperactivity at the very earliest stages of the disease. The paper “highlights that efforts to identify and test anti-Aβ agents need to be actively pursued, even as attention appropriately turns to non–amyloid-based AD interventions,” he wrote.—Gwyneth Dickey Zakaib
- Soluble Aβ Takes Blame for Hyperactive Neurons in Mouse Brain
- Toxic Stew of Aβ Dimers Hides Out in Human Plaques
- 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. Neuron. 2009 Jun 25;62(6):788-801. PubMed.
- Li S, Jin M, Koeglsperger T, Shepardson NE, Shankar GM, Selkoe DJ. Soluble Aβ oligomers inhibit long-term potentiation through a mechanism involving excessive activation of extrasynaptic NR2B-containing NMDA receptors. J Neurosci. 2011 May 4;31(18):6627-38. PubMed.
- Busche MA, Chen X, Henning HA, Reichwald J, Staufenbiel M, Sakmann B, Konnerth A. Critical role of soluble amyloid-β for early hippocampal hyperactivity in a mouse model of Alzheimer's disease. Proc Natl Acad Sci U S A. 2012 May 29;109(22):8740-5. Epub 2012 May 16 PubMed.
- Busche MA, Eichhoff G, Adelsberger H, Abramowski D, Wiederhold KH, Haass C, Staufenbiel M, Konnerth A, Garaschuk O. Clusters of hyperactive neurons near amyloid plaques in a mouse model of Alzheimer's disease. Science. 2008 Sep 19;321(5896):1686-9. PubMed.
- Murphy-Royal C, Dupuis JP, Varela JA, Panatier A, Pinson B, Baufreton J, Groc L, Oliet SH. Surface diffusion of astrocytic glutamate transporters shapes synaptic transmission. Nat Neurosci. 2015 Feb;18(2):219-26. Epub 2015 Jan 12 PubMed.
- Zott B, Simon MM, Hong W, Unger F, Chen-Engerer HJ, Frosch MP, Sakmann B, Walsh DM, Konnerth A. A vicious cycle of β amyloid–dependent neuronal hyperactivation. Science, 09 Aug 2019
- Selkoe DJ. Early network dysfunction in Alzheimer's disease. Science, 09 Aug 2019