Intracellular and Extracellular Localization of Transthyretin and Its Association With Amyloid‐β in Alzheimer's Disease Brains

ABSTRACT Transthyretin (TTR) can bind to Aβ and prevent the formation of Aβ fibrils in vitro; it is thus a highly interesting molecule in the field of Alzheimer's disease (AD) research. However, the distribution of TTR and its affinity to Aβ in both healthy human brains and those of AD patients remain unclear. We therefore examined TTR in human brains using postmortem brain samples. Paraffin sections and extracted protein samples were prepared from AD and control (non‐AD) brains. Immunohistochemistry was performed to detect TTR expression patterns, and immunofluorescent staining was used to reveal the relationships between the intracellular and extracellular localizations of TTR and Aβ. We also performed western blotting for TTR using brain extracts. In immunohistochemical staining of the human brain, TTR signal was detected not only in extracellular tissue but also in the cytoplasm of neurons and glia. The TTR‐positive area was significantly greater in AD brains than in non‐AD brains. However, expression of TTR transcripts did not differ between AD and non‐AD brains in our previously obtained RNA‐sequencing and microarray data. Immunofluorescent staining with multiple antibodies revealed that TTR was co‐localized with Aβ in the cytoplasm of neurons. In extracellular Aβ plaques, TTR presented in the same region but was not co‐localized with dense Aβ fibrils. Together, our results indicate that TTR is widely expressed in the human brain rather than being limited to the choroid plexus and that TTR is more abundant in AD brains. Our results also suggest that the affinity between TTR and Aβ changes depending on the structure of Aβ. Our data will be valuable for the future development of TTR‐related AD preventative methods and medications.