Structure and assembly-disassembly properties of wild-type transthyretin amyloid protofibrils observed with atomic force microscopy.

Transthyretin (TTR) is an important human transport protein present in the serum and the cerebrospinal fluid. Aggregation of TTR in the form of amyloid fibrils is associated with neurodegeneration, but the mechanisms of cytotoxicity are likely to stem from the presence of intermediate assembly states. Characterization of these intermediate species is therefore essential to understand the etiology and pathogenesis of TTR-related amyloidoses. In the present work we used atomic force microscopy to investigate the morphological features of wild-type (WT) TTR amyloid protofibrils that appear in the early stages of aggregation. TTR protofibrils obtained by mild acidification appeared as flexible filaments with variable length and were able to bind amyloid markers (thioflavin T and Congo red). Surface topology and contour-length distribution displayed a periodic pattern of ∼ 15 nm, suggesting that the protofibrils assemble via an end-binding oligomer fusion mechanism. The average height and periodic substructure found in protofibrils is compatible with the double-helical model of the TTR amyloid protofilament. Over time protofibrils aggregated into bundles and did not form mature amyloid-like fibrils. Unlike amyloid fibrils that are typically stable under physiological conditions, the bundles dissociated into component protofibrils with axially compacted and radially dilated structure when exposed to phosphate-buffered saline solution. Thus, WT TTR can form metastable filamentous aggregates that may represent an important transient state along the pathway towards the formation of cytotoxic TTR species.