FORCED VL DIMERIZATION PROMOTES THE WITHDRAWAL FROM AMYLOID TO WORM-LIKE PATHWAY

2018 edition

gabriel serra

Mechanisms of amyloidogenesis remain poorly understood. Amyloidoses are incurable life threatening diseases, which encompass a heterogeneous group of proteins. Amyloid proteins aggregate as insoluble deposits either intracellularly or extracellularly, leading to cell death and subsequently to tissue impairment. One of the most destructive amyloidosis, due to its arduous diagnostic and the lack of curative treatments, is light chain amyloidosis. Likewise, extremely high sequence variability exists within the same disease, conferring exclusive biochemical and structural properties to each protein. AL-09 is a light chain variable domain obtained from a patient with cardiac affectation that differs by seven amino acids from its germline, called ╬║I O18/O8. Moreover, Qin and co-workers reported a protective effect of dimerization; i.e. monomers were more prone to fibril formation than dimers. Nevertheless, AL-09 presents an altered dimer interface twisted 90┬║ relative to the canonical dimer interface adopted by ╬║I O18/O8. It has been postulated that such an altered interface is the main responsible for the enhanced amyloidogenicity of AL-09. Here, we recombinantly produced VVK08 and VVA09, two single chain variable fragments (scFv) composed of two identical VL domains from both proteins mentioned above. Firstly, we evaluated secondary and tertiary structure and their change upon thermal denaturation. Then, we attempted to determine their aggregation pathways and the effect of the dimerization provided by the single chain format on misfolding. By Far-UV circular dichroism (CD), intrinsic Trp fluorescence, and attenuated total reflectance (ATR) we determined that both chimeric proteins show the canonical immunoglobulin fold in native conditions, although VVK08 is far more compact that VVA09. On the other hand, VVK08 showed greater thermal stability than VVA09, as expected. We also found, using deconvoluted ATR spectra at 60┬║C and transmission electron microscopy images, that VVK08 and VVA09 aggregate as worm-like fibrils rather than amyloid fibrils, in contrast to the parental VL domains. In addition, VVA09 aggregation pathway is featured by the presence of a metastable intermediate that drives to the formation of well-organized fibrils. This is not the case for the scFv derived from the germline and, in consonance; worm-like fibrils formed by this chimera are less organized. As a conclusion, dimerization provided by the single chain format withdraws both VVK08 and VVA09 from the amyloid pathway to the WL aggregation pathway. Despite further studies are required, our results suggest a protective effect of dimerization against amyloid deposition in light chain amyloidosis.