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