Elongation of the C-terminal domain of an anti-amyloid β single-chain variable fragment increases its thermodynamic stability and decreases its aggregation tendency

Amyloid (A) immunotherapy is considered a promising approach to Alzheimer disease treatment. In contrast to the use of complete antibodies, administration of single-chain variable fragments (scFv) has not been associated with either meningoencephalitis or cerebral hemorrhage. ScFv-h3D6 is known to preclude cytotoxicity of the A(1-42) peptide by removing its oligomers from the amyloid pathway. As is the case for other scFv molecules, the recombinant production of scFv-h3D6 is limited by its folding and stability properties. Here, we show that its urea-induced unfolding pathway is characterized by the presence of an intermediate state composed of the unfolded V-L domain and the folded V-H domain, which suggests the V-L domain as a target for thermodynamic stability redesign. The modeling of the 3D structure revealed that the V-L domain, located at the C-terminal of the molecule, was ending before its latest -strand was completed. Three elongation mutants, beyond V-L-K107, showed increased thermodynamic stability and lower aggregation tendency, as determined from urea denaturation experiments and Fourier-transform infrared spectroscopy, respectively. Because the mutants maintained the capability of removing A-oligomers from the amyloid pathway, we expect these traits to increase the half-life of scFv-h3D6 in vivo and, consequently, to decrease the effective doses. Our results led to the improvement of a potential Alzheimer disease treatment and may be extrapolated to other class-I scFv molecules of therapeutic interest.