Aη, or A-eta, made an initial splash when scientists reported that these fragments of amyloid precursor protein—products of a heretofore unknown cleavage—dampen neural activity. But how? The story went quiet. Nine years later, comes a possible answer. In the June 14 Neuron, the same researchers, led by Hélène Marie, Institut de Pharmacologie Moléculaire et Cellulaire, Valbonne, France, and Michael Willem at Ludwig-Maximillians-Universität, Munich, report that the peptides modulate NMDA glutamate receptors.

  • Aη peptides modulate NMDA receptors and neural activity.
  • They weaken long-term potentiation and strengthen long-term depression.
  • Mice need them for their memory to function properly.

Oddly, Aη both suppresses ion flux through these cell membrane channels and strengthens their non-ionotropic activity. No other modulator pulls off this dual action, according to the authors. In fact, no molecule has ever been found to activate the non-ionotropic signaling, they write. The upshot? Aη regulates synaptic plasticity by dialing down long-term potentiation and dialing up long-term depression. Willem now believes this might be one of APP’s main physiological functions.

“The work is both persuasive and believable,” Jesper Sjöström, McGill University, Montreal, told Alzforum. Sjöström studies NMDA receptor modulation and plasticity. He was surprised by these results, but thinks they make sense. “We were waiting to see this kind of pharmacology, something that would have differential action on the two modes of [NMDA] signaling,” he told Alzforum. “This makes us think about NMDA signaling and Alzheimer’s disease in different ways,” he noted.

To Better Modulate NMDA? Much like Aβ, Aη peptides get snipped out of APP by sequential proteolysis. A new paper proposes a function for the phenomenon. [Courtesy of Dunot et al., 2024.]

The sequential cleavage of APP’s ectodomain by η-secretase, followed by either α- or β-secretases, spawns Aη-α and the slightly shorter Aη-β (image above). Scientists have largely overlooked these products, focusing for decades on the infamous Aβ peptides that form amyloid plaques. In 2015, Willem, working in Christian Haass’ lab at LMU and collaborating with Marie’s group in Valbonne, reported that Aη peptides suppressed synaptic activity in the hippocampus (Aug 2015 news).

At the time, the finding did not catch on, in part, perhaps, because Willem had not yet figured out the mechanism. “It took 10 years,” he told Alzforum. “We needed the right techniques and the right models, and tour de force work by first authors Jade Dunot and Sebastien Moreno, but we’ve learned that these peptides modulate NMDA receptors in a way that might be important beyond AD, including for disorders such as schizophrenia and depression,” he said.

Dunot and Moreno, both in Valbonne, found that Aη suppresses synaptic activity by competing with the NMDA co-agonists glycine or D-serine. This throttles calcium entry through the channel—the ionotropic flux. In mouse brain slices, Aη reduced current through CA3-CA1 synapses of the hippocampus by about a third. Conductance through AMPA glutamate receptors was unaffected.

The Ionotropic Bit. Ten nM Aη suppressed NMDAR current in mouse hippocampal slices by 35 percent. [Courtesy of Dunot et al., 2024.]

NMDA activation can also lead to non-ionotropic signaling. This is often referred to as metabotropic signaling, but Marie prefers the former term in this case. “‘Metabotropic’ suggests G proteins are involved, and we don’t know if that is true,” she said.

Dunot and colleagues found that Aη caused a conformational change in the cytoplasmic domains of the NMDA receptors, which activated p38 inside the cells (image below). Previously, scientists in Karen Zito’s lab at the University of California, Davis, had discovered that this kinase mediates NMDAR-dependent shrinkage of synaptic spines, along with a weakening of synaptic strength. This resulted in long-term depression (LTD) of synaptic circuits (Stein et al., 2020). Zito collaborated with Willem and Marie on the current study.

 

The Non-Ionotropic Bit. Fluorescence resonance energy transfer (FRET) between cytoplasmic tails of NMDAR subunits (top) strengthens in the presence of Aη, suggesting a conformational change. This causes LTD. [Courtesy of Dunot et al., 2024.]

In keeping with the p38 induction, the scientists found that Aη promoted spine shrinkage and LTD in hippocampal slices. This happened even in the presence of the NMDAR ion channel blocker MK801, confirming that ion flux had nothing to do with this effect.

To check if this synaptic modulation can happen without adding exogenous Aη, Dunot and colleagues treated mice with the BACE inhibitor LY2811376. Blocking BACE gives η-secretase, which is as yet unidentified, a chance to cleave APP, since both secretases vie for the same substrate. Indeed, these mice had twice the normal levels of Aη peptides. In hippocampal slices from them, LTD was more pronounced even without added Aη. Bringing genetics to bear, the scientists found that knocking out the η-secretase cleavage site did the opposite. Post-synaptic currents in the hippocampus came at higher frequencies, and LTD could not be induced without adding Aη.

All told, the authors conclude that Aη’s modulation of NMDAR is a physiological phenomenon. Indeed, when they chemically activated neurons in the prefrontal cortices of mice, Aη levels there quadrupled. This suggests that not only do the peptides control synaptic activity, but the reverse is true as well.

Is all this relevant to function? This question is not fully explored, but mice lacking the η-secretase site in APP froze less often than did wild-type controls when threatened with a foot shock. This type of contextual fear memory is known to rely on NMDA receptors.

Could NMDA receptor modulation by Aη even be the raison d’être for APP? Willem thinks so. Despite APP’s established importance in Alzheimer’s development, understanding its main function has largely eluded the field. In their original 2015 paper the scientists reported that Aη accumulates over the course of the disease, possibly compromising NMDAR and synaptic spines and contributing to memory loss.

Since then, scientists led by Javier Sáez-Valero at Universidad Miguel Hernández, Alicante, Spain, reported that hC-terminal fragments of APP tick up in the CSF of people with sporadic and familial AD and in Down’s syndrome, suggesting elevated secretase processing (Garía-Ayllón et al., 2017). “To get a whole view of APP processing in AD would be challenging, because of the necessary inclusion in the equation of α- and β-secretase, and even γ-secretase activity,” Sáez-Valero and colleagues wrote to Alzforum. “Additionally, it is plausible that different APP fragments can modulate NMDAR in opposite ways, directly, or indirectly, by competing with each other, but also by blocking each other.” 

Indeed, unknowns abound. For one: How do Aη peptides pull off this dual, ionotropic/non-ionotropic modulation of NMDA receptors? “That’s difficult to say,” Willem acknowledged. These 22kDa peptides are huge compared to the co-agonist single amino acids, raising steric questions. “They are obviously covering the co-agonist site, and we’d love to know if smaller peptides would be sufficient,” Willem added. He thinks a crystal structure might be necessary to figure this out.

There’s also some mystery surrounding the η-secretase(s) themselves. The matrix metalloprotease MT5-MMP cleaves at the η site on APP, but Willem said that other enzymes do, too. “Which of these might be physiologically regulated, or upregulated in AD, needs to be investigated,” he said. Synaptic activity boosts η-secretase processing, but Willem said he does not know how this happens. There could be more enzyme, more brought to the cell surface, or simply more activity because some inhibitor has been removed, he suggested. Previously, crossing MT5-MMP nulls with 5xFAD mice improved LTP, but curiously reduced amyloid levels as well (Baranger et al., 2016).

All told, Willem hopes these findings will rally interest around these Aη peptides. For her part, Marie thinks the new data may push the NMDA field in new directions. One major consideration for the AD field is the effect of BACE inhibition, which shifts the balance of APP peptides away from Aβ and toward Aη. Some think this might explain the cognitive decline seen in BACE inhibitor trials, which suppressed BACE by about 70 percent. “It will be enlightening to see if low-dose BACE inhibition, i.e., decreasing BACE1 by only 30 percent, as has been proposed for the revival of BACE inhibitors, has any deleterious effects on Aη-mediated LTD (and LTP) changes,” wrote Justyna Zakaria, Northwestern University, Chicago.—Tom Fagan

News Citations

  1. Enter Aη: Alternative APP Cleavage Creates Synaptotoxic Peptide

Paper Citations

  1. .
    Molecular Mechanisms of Non-ionotropic NMDA Receptor Signaling in Dendritic Spine Shrinkage.
    J Neurosci. 2020 May 6;40(19):3741-3750. Epub 2020 Apr 22
    PubMed.
  2. .
    C-terminal fragments of the amyloid precursor protein in cerebrospinal fluid as potential biomarkers for Alzheimer disease.
    Sci Rep. 2017 May 30;7(1):2477.
    PubMed.
  3. .
    MT5-MMP is a new pro-amyloidogenic proteinase that promotes amyloid pathology and cognitive decline in a transgenic mouse model of Alzheimer’s disease.
    Cell Mol Life Sci. 2016 Jan;73(1):217-36. Epub 2015 Jul 23
    PubMed.

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