An anti-Aβ (amyloid β) single-chain variable fragment prevents amyloid fibril formation and cytotoxicity by withdrawing Aβ oligomers from the amyloid pathway

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Abstract
Aβ (amyloid β) immunotherapy has been revealed as a possible tool in Alzheimer's disease treatment. In contrast with complete antibodies, the administration of scFvs (single-chain variable fragments) produces neither meningoencephalitis nor cerebral haemorrhage. In the present study, the recombinant expression of scFv-h3D6, a derivative of an antibody specific for Aβ oligomers, is presented, as well as the subsequent proof of its capability to recover the toxicity induced by the Aβ1–42 peptide in the SH-SY5Y neuroblastoma cell line. To gain insight into the conformational changes underlying the prevention of Aβ toxicity by this antibody fragment, the conformational landscape of scFv-h3D6 upon temperature perturbation is also described. Heating the native state does not lead to any extent of unfolding, but rather directly to a β-rich intermediate state which initiates an aggregation pathway. This aggregation pathway is not an amyloid fibril pathway, as is that followed by the Aβ peptide, but rather a worm-like fibril pathway which, noticeably, turns out to be non-toxic. On the other hand, this pathway is thermodynamically and kinetically favoured when the scFv-h3D6 and Aβ1–42 oligomers form a complex in native conditions, explaining how the scFv-h3D6 withdraws Aβ1–42 oligomers from the amyloid pathway. To our knowledge, this is the first description of a conformational mechanism by which a scFv prevents Aβ-oligomer cytotoxicity.

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