Transthyretin (TTR), an indispensable transporter protein of thyroxine and retinol molecules in humans, constitutes a homo-tetrameric complex in its native conformation. However, this protein is also well known for its aggregation-prone propensity, which causes amyloidopathic diseases, such as senile systemic amyloidosis and familial amyloid polyneuropathy/cardiomyopathy. It is generally accepted that reduced stability of the tetrameric state of TTR and subsequent dissociation into the monomeric state is the primary limiting step of TTR aggregation. Monomeric TTR appears to have unstable tertiary conformation, thus being followed by misfolding and aggregation into amyloid fibrils [1,2]. Several factors can compromise TTR’s tetrameric stability, such as genetic mutation, low pH, increased temperature, and proteolytic cleavage. Here, we propose that lipids may also work as an important contributor for TTR aggregation. Although the interaction between TTR and lipid molecules was previously reported in a couple of studies, its molecular mechanisms and possible effects on TTR aggregation have yet to be elucidated.
To this end, we employed solution NMR spectroscopy and various biochemical approaches to investigate the interaction between TTR and lipid molecules. We found that a lipid micelle selectively interacts with the aggregation-competent state of TTR and stabilize a highly-dynamic monomeric state. Circular dichroism analysis indicated that a significant portion of the lipid-induced monomeric state forms an α-helical structure, in contrast to the β-sheet-rich structure in its native tetrameric conformation. Moreover, we observed that aggregation-prone species of TTR significantly reduced the structural integrity of a liposome membrane, suggesting that the cytotoxicity of TTR aggregates may come from this interaction between monomeric TTR and lipid molecules.
Taken together, we revealed the detailed interaction mechanism of how TTR interacts with lipids in its aggregation pathway. In addition, our work provided direct evidence of how TTR exhibits cytotoxicity during its aggregation pathway, which may help to develop novel therapeutic strategies to treat or prevent TTR amyloidosis.