A nifty instrument for mapping the 3-D shape of macromolecular surfaces, the atomic force microscope (AFM) over the last five years has become an established method of measuring how environmental conditions affect Aβ fibril formation as well as oligomerization of other proteins implicated in neurodegeneration (see ARF related news story, selected citations in right margin). Of a current spate of studies using this desktop machine (see Liu et al, 2004, Moreno-Herrero et al., 2004, Gibson et al., 2004), one published in the January 23 Journal of Molecular Biology has employed it to characterize how Aβ antibodies interfere with fibrillization. One of these antibodies is thought to improve memory performance in a mouse model by attacking soluble Ab oligomers.
Justin Legleiter, working with Tomasz Kowalewski at Carnegie Mellon University in Pittsburgh and colleagues elsewhere, incubated Aβ42 with monoclonal antibodies to both the N-terminal end and the central domain of the peptide. They then used the AFM to measure the amount and length of fibrils formed. The m266 antibody is being investigated in passive vaccination paradigms (see ARF related news story; also see DeMattos et al. in ARF related news story), and the m3D6 has been studied with multiphoton microscopy for its ability to clear amyloid deposits in brain, (see Bacskai et al., 2002. For further information on these and other Ab antibodies, see Alzforum antibody directory).
The researchers found that the N-terminal antibody (m3D6) slowed down formation of Aβ fibrils, while the central domain antibody (m266.2) almost completely prevented this process. After incubating the proteins for five days, about three fibrils per square micrometer were detected in the absence of antibody. This density shrank to less than 0.5 fibrils when m3D6 was in the mix, and to almost zero in the presence of m266.2. A similar trend was apparent when Legleiter and colleagues examined fibril length. In the absence of antibody, the longest fibril recorded was 800 nanometers. m3D6 decreased fibril length to about 400, m266.2 to 50 nanometers.
The authors conclude that the differences are best explained by the antigen specificities of the antibodies, writing that m266.2 is better at preventing aggregation because it binds close to the hydrophobic core of the peptide, which is most involved in fibril formation (see ARF related news story). However, the relative affinities of the two antibodies are not discussed.
As for the atomic force microscope itself, its capacity to measure the contours and lengths of single Aβ fibrils indicates that it could be a useful tool for estimating the efficacy of potential fibril busters.—Tom Fagan
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