Oligomers in AD: Too Much of a Bad Thing?
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Neuritic plaques are classic, obvious markers of Alzheimer’s disease, but it is a puzzle why their density is a poor predictor of disease symptoms. Recently, scientists have come to suspect that soluble oligomeric, not fibrillary, Aβ, may be the culprit in synaptic degeneration in AD. In the August 18 online PNAS, another piece of evidence implicating oligomers appeared in the latest edition of PNAS. Part of this paper was previously presented at the Society for Neuroscience meeting (see ARF related news story). Using an antibody specific for Aβ oligomers, Yuesong Gong and colleagues in William Klein’s group at Northwestern University, Chicago uncovered their presence in regions of AD brain that typically suffer the most degeneration, and also showed that they cluster on neuronal surfaces, binding with ligand-like specificity.
First, Gong et al. raised synthetic antibodies against Aβ oligomers and tested for immunoreactivity in tissue samples taken from the frontal cortex of AD and age-matched control brains. Consistent with results from other groups (see ARF related news story), AD cortex reacted robustly to the antibody, whereas control cortex showed little response. In some individual comparisons, AD tissue had as much as 70 times higher reactivity to oligomers than did control tissue.
Next, the authors visually examined the binding pattern of oligomers on cultured neurons, looking at cultured rat hippocampal neurons incubated with synthetic Aβ oligomers (or ADDLs), AD brain extract, or control tissue extract. Under the immunofluorescent microscope, distinct clusters of binding sites lit up on the surfaces of neurons treated with synthetic ADDL or AD brain extract. The authors note that the punctate distribution of binding sites is reminiscent of clusters of membrane rafts or synaptic terminals.
Finally, ligand overlay assays suggest that oligomers could perhaps act as ligands for neural membrane proteins. The authors tested synthetic oligomers with rat membrane proteins, and found that oligomers bound with high affinity to a small number of proteins in hippocampal and cortical, but not cerebellar, membranes. In a separate test, different concentrations of ADDL were applied to cortical and cerebellar cultures, revealing that ADDL at micromolar doses was selectively toxic to cortical neurons, as evidenced by a decrease in the level of MTT reduction. The authors suggest that the difference in oligomer binding seen in the different brain areas reflects differences in their intrinsic vulnerability to oligomer toxicity.
For hippocampal and cortical membrane preparations, synthetic ADDL binding was observed to proteins around 140 and 260 and, less so, 100 kDA. Binding in the p140 and p260 bands increased when the membrane protein mixture was enriched with rafts, suggesting that the binding sites may be concentrated in these membrane domains.
Human AD brain-derived extract yielded slightly different binding patterns from those of synthetic oligomers. The same three bands (p100, p140, and p260) showed up in the overlay pattern, but in this case, binding in p100 was the most robust. In contrast, the antibody did not bind detectably to control brain extract of any region.
P140 and p260 binding was also present in assays of synthetic oligomer and human membrane protein preparations. However, binding in both bands was significantly lower in AD brain than in control brain, which the authors say is consistent with reduction or degeneration of cells containing oligomer binding proteins in AD.
The authors suggest that, taken together, oligomers act as ligands to a small number of membrane proteins located in cortical and hippocampal neurons, and that these proteins may be disrupted or reduced in AD. It is not clear from this finding that the site of oligomer binding is actually in the synapses or in the rafts, although it’s been shown that oligomers have detrimental effects on LTP, thought to underlie synaptic plasticity (see ARF related news story). Nonetheless, the authors say, the finding of elevated levels of oligomers and their high binding affinity in brain areas most vulnerable to degeneration in AD further builds the case for focusing on oligomers in therapeutic strategies.—I-han Chou
I-han Chou is a science writer based in Japan.
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Primary Papers
- Gong Y, Chang L, Viola KL, Lacor PN, Lambert MP, Finch CE, Krafft GA, Klein WL. Alzheimer's disease-affected brain: presence of oligomeric A beta ligands (ADDLs) suggests a molecular basis for reversible memory loss. Proc Natl Acad Sci U S A. 2003 Sep 2;100(18):10417-22. PubMed.
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Yale University School of Medicine
Small, soluble aggregates of the Aβ1-42 peptide are now believed to be the toxic factor that is responsible for synaptic dysfunction and eventual neuronal degeneration in Alzheimer's disease. This work adds to this consensus by showing that soluble extracts of brains from five cases of AD react with an antiserum specific for the Aβ1-42 peptide. Samples of both naturally derived Aβ1-42 peptides and synthetic versions were also found to bind in a punctate fashion to the external surfaces of culture hippocampal neurons. The authors suggest that Aβ oligomers might be binding to sites of signaling specializations, possibly related to synaptic terminals. To rule out non-specific binding, the Aβ oligomers were incubated with specific antibodies before adding them to the cultured cells. This blocked the binding of the added Aβ peptides to the cells, but this step in effect removed the oligomers from contact with the neurons and did not address whether oligomers that were available to the cells could bind non-specifically. It was also found, using the SDS gel overlay technique, that synthetic Aβ peptides bind to a number if unidentified proteins of hippocampal membrane fractions. While these preliminary observations are provocative, more direct evidence is needed to support the claim that oligomeric Aβ ligands bind in a biologically meaningful way to specific neuronal proteins that mediate signal transduction and that they are directly involved in reversible memory loss.
View all comments by Vincent MarchesiNorthwestern University Institute for Neuroscience
Response to comment by Vincent Marchesi
Dr. Marchesi provides a thoughtful summary of our study and calls attention to an issue that's of central concern to us—specificity. AD is so memory-specific, especially early on, that one would hope to identify molecular pathogens capable of explaining this key feature of the disease. It's turning out that the property of specificity is a most intriguing aspect of ADDL nerve cell biology.
As Dr. Pascale Lacor will show in her poster at SFN-New Orleans, those hot spots of ADDL binding are neither random nor nonspecific. They actually are just what the doctor ordered—synapses. And when ADDLs get lodged in those synapses, they disrupt particular molecular mechanisms essential for memory. (Since Dr. Lacor's study is out for review, I won't comment further on its details.) The bottom line is that the specific manner in which ADDLs attack neurons can provide a synaptically localized mechanism to account for memory loss in AD.
However, even with these further interesting findings, we would, of course, agree that as always, more evidence is desirable.
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