Transthyretin (TTR)-derived amyloid fibrils: Immunoelectron microscopy of fibrils formedin vivoandin vitrofrom synthetic peptides and normal transthyretin

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Abstract
The fibril forming capacity of synthetic peptides representing parts of the human TTR sequence was investigated. The in vitro formed fibrils were studied with electron microscopy and with polarized light after staining with Congo red dye and also compared with fibrils formed in vitro from normal TTR and fibrils extracted from amyloid deposits from a patient with senile systemic amyloidosis. Most of the synthetic peptides that were fibrillogenic in this study represent parts of the TTR sequence that are mostly in the β-strands in the tertiary structure of TTR. A peptide corresponding to the α-helix was also strongly fibrillogenic in vitro as was a peptide representing the DE-loop. in senile systemic amyloidosis and in familial amyloidotic polyneuropathy the fibrils contain C-terminal fragments of TTR starting just before the D strand. The cleavage of the TTR molecule thus exposes a highly fibrillogenic sequence, possibly important for in vivo fibrillogenesis. The in vitro formed fibrils from synthetic peptides and normal TTR and fibrils extracted from amyloid deposits were immunolabelled with antisera raised against synthetic TTR peptides, amyloid TTR and against normal TTR. An antiserum against TTR(II.5-124) immunoreacted with amyloid fibrils formed in vivo and fibrils formed from normal TTR in vitro but not with purified normal TTR in Western blot, ELISA or with TTR in pancreatic islet cells in immunohistochemistry.