Visualization of polymorphism in apolipoprotein C-II amyloid fibrils

The misfolding and self-assembly of proteins into amyloid fibrils, which occur in several debilitating and age-related diseases, are affected by common components of amyloid deposits, notably lipids and lipid complexes. Previously, the effects of phospholipids on amyloid fibril formation by apolipoprotein (apo) C-II have been examined, where low concentrations of micellar phospholipids and lipid bilayers induce a new, straight rod-like morphology for apoC-II fibrils. This fibril appearance is distinct from the twisted-ribbon morphology observed when apoC-II fibrils are formed in the absence of lipids. We used total internal reflection fluorescence microscopy (TIRFM) to visualize the described polymorphism of apoC-II amyloid fibrils. The spontaneous assembly of apoC-II into either twisted-ribbon fibrils in the absence of lipids or into fibrils of straight rod-like morphology when lipids are present was captured by TIRFM. The latter was found to be better suited for visualization using TIRFM. The difference between seeding of apoC-II straight fibrils on microscopic quartz slide and in test tube suggested a role for the effects of incubation surface on fibril formation. Seed-dependent growth of apoC-II straight fibrils was probed further by using a dual-labelling construct, giving insights into the straight fibril growth pattern.