Originally, vaccination with amyloid-β (Aβ) peptides aimed to fight Alzheimer disease by enlisting the immune system to clear aggregated amyloid plaques from the brain. But there is mounting evidence that soluble forms of Aβ, and particularly toxic multimers, cause considerable neuronal damage long before the amyloid deposits form. Soluble multimers of Aβ are neurotoxic, and interfere with synaptic function and memory storage by inhibiting hippocampal long-term potentiation (LTP) (see Lambert et al., 1998 and ARF related news story). Accordingly, immunotherapy should block the early toxic action of soluble Aβ, to have any chance of slowing or reversing the cognitive and memory deficits that occur even before plaque deposition.
A new study from the labs of Michael Rowan of Trinity College in Dublin and Dennis Selkoe at Harvard Medical School shows that Aβ-targeted immunotherapy can block the action of soluble Aβ in vivo. Their paper, which appeared online in Nature Medicine on April 17, shows that treatment with antibodies or a vaccine prevents inhibition of LTP in living animals after intraventricular injection of soluble Aβ. Their demonstration that antibodies can neutralize the short-term effects of soluble Aβ in vivo may explain how immunotherapy can rapidly improve cognitive function in AD, even before significant changes in plaque deposition are seen (see ARF related news story).
The Selkoe and Rowan groups teamed up 3 years ago to investigate the effects of naturally produced soluble Aβ oligomers on LTP in rat brain. They showed that intraventricular injection of conditioned medium from CHO cells transfected with the V717F mutant form of amyloid precursor protein (APP) inhibited LTP measured in living rats (see ARF related news story). In the current report, the researchers, led this time by first author Igor Klyubin, used the same experimental system to investigate whether antibodies to Aβ could block the effect. First, they showed that two commercially available monoclonal antibodies to Aβ (6E10 or 4G8) prevented the inhibition of LTP when injected along with Aβ-containing conditioned medium. Reasoning that the antibodies probably neutralized Aβ before the mixture was administered to the rats, they tried giving the animals Aβ first, followed 10 minutes later by the antibody. This gave the same results, showing that the antibodies could neutralize Aβ in vivo.
The conditioned medium Klyubin et al. used contained several multimeric forms of Aβ that were recognized by the antibodies, including mostly monomeric Aβ, but also dimers and trimers. To determine which fraction was responsible for the inhibition of LTP, they used exclusion chromatography to separate the different size oligomers. Only the fraction enriched in dimers and trimers inhibited LTP, and its activity was blocked by the 6E10 antibody.
To see if naturally produced antibodies could provide resistance to Aβ, the researchers immunized rats with a synthetic, preaggregated mix of Aβ40/42. The immunized animals were classified as responders if they had detectable plasma antibodies reactive with soluble Aβ, or nonresponders if they had little or no detectable Aβ immunoreactivity. Both immunized and nonimmunized animals showed normal LTP. But immunized responders did not show as much inhibition of LTP after intraventricular Aβ injection as did nonresponders or nonimmunized rats. The resistance to Aβ seemed to correlate with plasma antibody titers, because the strongest responders showed the highest levels of LTP after Aβ injection. However, the protective effect of immunization was only partial and appeared transient. LTP was restored only about halfway in the strong responders, and inhibition by Aβ increased over the 3-hour test period. The authors speculate that the partial and transient effects may be due to low serum titer of antibody, or the antibody failing to enter the hippocampus in high enough levels to sustain the response.
To date, the emphasis in immunotherapy has been on clearing of excess Aβ from the brain, as measured by the disappearance of amyloid plaques. The current results suggest that immunotherapy could also be effective at blocking immediate early effects of soluble Aβ toxicity, particularly those that lead to the memory impairment seen early in AD.—Pat McCaffrey
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- Lambert MP, Barlow AK, Chromy BA, Edwards C, Freed R, Liosatos M, Morgan TE, Rozovsky I, Trommer B, Viola KL, Wals P, Zhang C, Finch CE, Krafft GA, Klein WL. Diffusible, nonfibrillar ligands derived from Abeta1-42 are potent central nervous system neurotoxins. Proc Natl Acad Sci U S A. 1998 May 26;95(11):6448-53. PubMed.
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- Klyubin I, Walsh DM, Lemere CA, Cullen WK, Shankar GM, Betts V, Spooner ET, Jiang L, Anwyl R, Selkoe DJ, Rowan MJ. Amyloid beta protein immunotherapy neutralizes Abeta oligomers that disrupt synaptic plasticity in vivo. Nat Med. 2005 May;11(5):556-61. PubMed.