Last Wednesday, Marc Tessier-Lavigne wowed the crowd at the Keystone Symposium, Neurodegenerative Diseases: New Molecular Mechanisms by revealing that a soluble piece of APP is a ligand for a cell surface receptor that sets off an apoptotic cascade (see ARF related Keystone story). Last Friday, it was Stephen Strittmatter’s turn to surprise attendees, when he showed that oligomers of Aβ can bind cellular prion protein (the non-toxic kind). What’s more, in the absence of the prion, Aβ oligomers no longer suppressed long-term potentiation—one of the best-characterized Aβ toxicities. These findings also appear in yesterday’s Nature. In an accompanying Nature News & Views article, Moustapha Cisse and Lennart Mucke of the Gladstone Institute of Neurological Disease, University of California, San Francisco, write that “…the discovery that PrPc may be a mediator in the development of Alzheimer’s disease is fascinating, not least from a therapeutic perspective.”

Finding Aβ oligomers bound to cellular prion (PrPc) was absolutely unexpected, Strittmatter told ARF by phone after the symposium. “We clearly think of Alzheimer’s and prion disease as being quite separate pathologically and physically. On the other hand, some similarities exist and we are looking forward to functional studies to look into them,” he said.

Strittmatter and colleagues, from Yale University, New Haven, Connecticut, made their discovery when screening for proteins that bind Aβ oligomers. As Strittmatter outlined in his Keystone presentation, the lab had previously shown that the Nogo receptor, known for limiting axon regeneration and repair, binds to the amyloid-β precursor protein (APP) and to Aβ, but not necessarily oligomers (see Park et al., 2006 and Park et al., 2006). To find proteins that specifically bind to oligomeric species of the peptide, which are now widely believed to be the most toxic Aβ entities, Juha Lauren and colleagues in the lab used biotin-labeled oligomers (prepared as per the ADDL method of Bill Klein’s group—see Chromy et al., 2003) to screen COS-7 cells expressing a mouse adult brain cDNA library. Screening 200,000 clones generated two strong hits—both expressing the prion protein. The researchers then tested two related proteins, doppel and SPRN, but neither bound to the Aβ oligomers. A screen of 352 transmembrane proteins (done individually) also failed to detect strong interactions. This method revealed that the APP homolog APLP1 and the transmembrane protein 30B (TMEM30B) bound the Aβ oligomers with low affinity.

To test whether this oligomer-prion interaction was real, the researchers took advantage of PrPc knockout mice (Prnp-/-), which seem to have normal synaptic plasticity as judged by LTP measurements. At Keystone, Strittmatter showed that after 15-20 days in vitro, cultured hippocampal neurons from wild-type mice bound Aβ oligomers. This timing corresponds to the emergence of PrPc expression in Map2-positive dendrites on the neurons, suggesting PrP is a major Aβ binding site in these cells. The Aβ oligomers and the PrPc colocalized, and while the researchers did detect Aβ oligomers on PrPc-negative neurons, binding was reduced by about 50 percent, again indicating PrPc as a major site for oligomers binding. That binding is not exclusive, though. “Multiple alternative sites, including APLP1, TMEM30A, TMEM30B, RAGE, and other unidentified proteins may explain Aβ42 binding to Prnp-/- neurons,” write the authors in the paper.

What might be the functional significance of this prion-Aβ interaction? To test this, the researchers measured long-term potentiation in the Schaeffer collateral pathway of the hippocampus. In hippocampal slices from normal mice, Aβ42 oligomers reduced LTP significantly, but in slices from PrPc-negative animals the oligomers did not. In addition, wild-type slices were protected from the toxic effects of Aβ oligomers if they were first treated with the PrPc antibody 6D11. “Thus, we conclude that PrPc exerts a receptor action acutely to mediate Aβ42-oligomer inhibition of synaptic plasticity in the hippocampal slice,” write the authors.

How PrPc mediates LTP suppression is unclear, said Strittmatter. One possibility is that the prion-Aβ complex directly interacts with glutamate receptors and causes their downregulation. The other is that the complex sets off a signal transduction cascade that culminates in glutamate receptor dysfunction. To address this question, the researchers expressed prion protein and glutamate receptors in Xenopus oocytes and then tested the receptor activity by the voltage clamp. “We saw no effect of Aβ oligomers on glutamate receptors, with or without PrP, so I don’t think it is a very direct interaction. I think instead, Aβ binding to PrP causes changes in calcium, kinases, and endocytosis, the net result of which is glutamate receptor dysfunction,” said Strittmatter.

How the two proteins bind is also unknown. These particular oligomers are made up of around 100 monomers and are about 500 kDa in size. One possibility is that the oligomers interact with PrPc through some intermediary. This is unlikely, however, because the researchers found that oligomers bound directly to a purified, immobilized prion chimera comprising the ectodomain of the prion fused to the Fc tail of immunoglobulin G.

There are, of course, smaller oligomers, such as Aβ*56 isolated by Karen Ashe’s group at the University of Minnesota in Minneapolis (see ARF related news story), oligomers Dominic Walsh first isolated from conditioned medium of APP-expressing CHO cells when he was at Dennis Selkoe’s lab at Brigham and Women’s Hospital, Boston (see ARF related news story), or even dimers that Ganesh Shankar, Walsh, Selkoe, and colleagues recently isolated from Alzheimer patients (see ARF related news story). Whether any of those bind to PrPc is not known at this point. Strittmatter said that it would be good to test all types of Aβ for prion binding. His lab will look for the functional effects of PrP in mouse models of Alzheimer disease, by crossing PrP nulls with APP transgenic mice and by treating the same transgenic mice with anti-prion antibodies.

Curiously, the region in PrPc, amino acids 95-110, that interacts with Aβ oligomers also causes profound neurodegeneration when deleted from the prion protein, said Strittmatter (see Baumann et al., 2007 and Li et al., 2007). “One conclusion from that is there is a natural function for cellular prion, which when perturbed can lead to neurodegeneration,” said Strittmatter. “One way to perturb the natural function of prion protein is with toxic prions; a second way is with oligomeric Aβ. It is, in some sense, a receptor for both of these toxic species.”

“Lauren and colleagues’ observations create fertile ground for future investigations,” write Cisse and Mucke. They note that human tau forms complexes with PrPc, which may link Aβ toxicity to tau, and they observed that the same PrPc region that binds Aβ is also cleaved by α-secretase (see Vincent et al., 2008), which is essential for non-amyloidogenic processing of APP. “So one way to prevent both Aβ production and the activation of downstream mediators by PrPc might be to increase α-secretase activity,” they write. Array tomography, recently used to localized Aβ oligomers to dendritic spines in postmortem tissue samples, might also prove a useful model for measuring Aβ-prion interactions in vivo.—Tom Fagan.

References:
Lauren J, Gimbel DA, Nygaard HB, Gilbert JW, Strittmatter SM. Cellular prion protein mediates impairment of synaptic plasticity by amyloid-beta oligomers. Nature. 2009, February 26;457:1128-1132. Abstract

Cisse M, Mucke L. A prion protein connection. Nature. 2009, February 26; 457: 1090-1091. Abstract

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  1. This paper reports the surprising identification by expression cloning of the prion protein (PrP) as a receptor for Aβ oligomers and the synaptotoxic consequences of this interaction. There is extensive evidence in support of these findings, and if confirmed by other labs, PrP could be an important new target for AD therapeutics...so long as such compounds do not induce PrP into an aggregation-prone form called PrPsc, which causes devastating spongiform encephalopathies (e.g., CJD in humans). That would be a side effect that would be worse than the disease that one is trying to treat!

    View all comments by Michael Wolfe
  2. Lauren et al. report that Aβ oligomers bind to PrPc and that the detrimental effect of Aβ on hippocampal LTP is not observed in PrPc knockout mice; PrPc presumably mediates this detrimental effect not by direct modulation of glutamate receptors but in an indirect way. There are earlier studies hinting at an association of Aβ with PrPc (e.g., Brown, 2000; Schwarze-Eicker et al., 2005) but the demonstration of a specific Aβ-binding site on PrPc opens up possibilities of exploring the role of PrPc in Alzheimer disease and the role of Aβ in prion diseases; since a high-affinity PrPc binding site for Aβ should not be accidental, it might also indicate a physiological role for Aβ. With picomolar concentrations of Aβ monomers and oligomers stimulating synaptic activity (Puzzo et al., 2008), certain species of Aβ oligomers should not be toxic under physiological conditions and their binding to PrPc may contribute to normal synaptic activity.

    It has been proposed that some effects of PrPc involve an interaction of PrPc with a surface receptor and that the binding site of PrPc for this receptor overlaps segment 105-125 of PrPc (review Westergard et al., 2007). In their discussion, Lauren et al. suppose that "a putative PrPc-associated transmembrane co-receptor is likely to have a central role in Alzheimer’s disease-mediated neurodegeneration." As several publications indicate that the neurotrophin receptor p75 is essential for Alzheimer-like degeneration (e.g., review Capsoni and Cattaneo, 2006; Sotthibundhu et al., 2008), it is a candidate for such a co-receptor.

    In this context, the demonstrated binding site of Aβ oligomers on PrPc (around 95-110) might support the Aβ-crosslinker-hypothesis (see Current Hypotheses), which suggests an Aβ-binding site within PrPc segment 91-123 and describes possible physiological and pathological effects of an Aβ-mediated interaction between the neurotrophin receptor p75 and PrPc, APP, and α-synuclein; the recently found Aβ-binding site within the stalk and transmembrane domain of p75 (see my recent hypothesis) would be crucial to such interactions and link Aβ-related diseases. Aggregate species of Aβ can activate p75, and available or newly formed short Aβ oligomers may crosslink p75 and PrPc. The cooperation of stimulated p75 and PrPc would activate sphingomyelinase and NADPH oxidase in a synergistic feed-forward process, and p75-Aβ-PrPc complexes could provide reactive oxygen species and elevated intracellular calcium required for components of p75 signaling. A "rapid inhibitory effect of p75(NTR) on NMDA-R currents that antagonizes TrkB-mediated NMDA-R potentiation" (Sandoval et al., 2007) should be increased by excess p75-activating Aβ and might be negatively influenced by a lack of PrPc. Excess Aβ might also induce oxidative stress and/or disturb cellular calcium homeostasis through disproportionate PrPc-receptor (and perhaps also PrPc-PrPc) crosslinking.

    References:

    . PrPSc-like prion protein peptide inhibits the function of cellular prion protein. Biochem J. 2000 Dec 1;352 Pt 2:511-8. PubMed.

    . On the molecular basis linking Nerve Growth Factor (NGF) to Alzheimer's disease. Cell Mol Neurobiol. 2006 Jul-Aug;26(4-6):619-33. PubMed.

    . Picomolar amyloid-beta positively modulates synaptic plasticity and memory in hippocampus. J Neurosci. 2008 Dec 31;28(53):14537-45. PubMed.

    . Antagonistic effects of TrkB and p75(NTR) on NMDA receptor currents in post-synaptic densities transplanted into Xenopus oocytes. J Neurochem. 2007 Jun;101(6):1672-84. PubMed.

    . Prion protein (PrPc) promotes beta-amyloid plaque formation. Neurobiol Aging. 2005 Aug-Sep;26(8):1177-82. PubMed.

    . Beta-amyloid(1-42) induces neuronal death through the p75 neurotrophin receptor. J Neurosci. 2008 Apr 9;28(15):3941-6. PubMed.

    . The cellular prion protein (PrP(C)): its physiological function and role in disease. Biochim Biophys Acta. 2007 Jun;1772(6):629-44. Epub 2007 Mar 2 PubMed.

  3. This is outstanding work that makes a strong link for alterations in PrPc for synaptic and neuronal dysfunction. Several investigators have shown that PrPc participates in cellular signaling (see review by Linden et al., 2008); it is likely that some of these pathways may be altered/disturbed or overactivated by Aβ oligomers.

    References:

    . Physiology of the prion protein. Physiol Rev. 2008 Apr;88(2):673-728. PubMed.

References

News Citations

  1. Keystone: Death Receptor Ligand—New Role for APP, New Model for AD?
  2. Aβ Star is Born? Memory Loss in APP Mice Blamed on Oligomer
  3. “Natural” Aβ Oligomers Cause Transitory Cognitive Disruptions
  4. Paper Alert: Patient Aβ Dimers Impair Plasticity, Memory

Paper Citations

  1. . Alzheimer precursor protein interaction with the Nogo-66 receptor reduces amyloid-beta plaque deposition. J Neurosci. 2006 Feb 1;26(5):1386-95. PubMed.
  2. . Subcutaneous Nogo receptor removes brain amyloid-beta and improves spatial memory in Alzheimer's transgenic mice. J Neurosci. 2006 Dec 20;26(51):13279-86. PubMed.
  3. . Self-assembly of Abeta(1-42) into globular neurotoxins. Biochemistry. 2003 Nov 11;42(44):12749-60. PubMed.
  4. . Lethal recessive myelin toxicity of prion protein lacking its central domain. EMBO J. 2007 Jan 24;26(2):538-47. PubMed.
  5. . Neonatal lethality in transgenic mice expressing prion protein with a deletion of residues 105-125. EMBO J. 2007 Jan 24;26(2):548-58. PubMed.
  6. . Regulation of betaAPP and PrPc cleavage by alpha-secretase: mechanistic and therapeutic perspectives. Curr Alzheimer Res. 2008 Apr;5(2):202-11. PubMed.
  7. . Cellular prion protein mediates impairment of synaptic plasticity by amyloid-beta oligomers. Nature. 2009 Feb 26;457(7233):1128-32. PubMed.
  8. . Alzheimer's disease: A prion protein connection. Nature. 2009 Feb 26;457(7233):1090-1. PubMed.

Further Reading

Papers

  1. . Cellular prion protein mediates impairment of synaptic plasticity by amyloid-beta oligomers. Nature. 2009 Feb 26;457(7233):1128-32. PubMed.
  2. . Alzheimer's disease: A prion protein connection. Nature. 2009 Feb 26;457(7233):1090-1. PubMed.

Primary Papers

  1. . Cellular prion protein mediates impairment of synaptic plasticity by amyloid-beta oligomers. Nature. 2009 Feb 26;457(7233):1128-32. PubMed.
  2. . Alzheimer's disease: A prion protein connection. Nature. 2009 Feb 26;457(7233):1090-1. PubMed.