Transthyretin stability is critical in assisting beta amyloid clearance- Relevance of transthyretin stabilization in Alzheimer's disease.

ABSTRACT

BACKGROUND: The absence of transthyretin (TTR) in AD mice decreases brain Aß clearance and reduces the low-density lipoprotein receptor-related protein 1 (LRP1). It is possible that neuroprotection by TTR is dependent on its tetramer structural stability, as studies using TTR mutants showed that unstable L55P TTR has low affinity for Aß, and TTR tetrameric stabilizers such as iododiflunisal ameliorate AD features in vivo. METHODS: We firstly investigated TTR folding status in human plasma measuring the resistance to urea denaturation. The importance of TTR stability on Aß internalization was studied in human cerebral microvascular endothelial (hCMEC/D3) and hepatoma cells (HepG2), by flow cytometry. To investigate the fate of Aß at the blood-brain barrier, Aß efflux from hCMEC/D3 cells seeded on transwells was measured using ELISA. Further, to assess Aß colocalization with lysosomes, Lysotracker was used. Moreover, levels of LRP1 were assessed in the liver and plasma of mice with different TTR backgrounds or treated with iododiflunisal. RESULTS: We showed that TTR stability is decreased in AD and that WT TTR and drug-stabilized L55P TTR are able to increase uptake of Aß. Furthermore, measurement of Aß efflux showed that stable or stabilized TTR increased Aß efflux from the basolateral to the apical side. Moreover, HepG2 cells incubated with Aß in the presence of WT TTR, but not L55P TTR, showed an increased number of lysosomes. Further, in the presence of WT TTR, Aß peptide colocalized with lysosomes, indicating that only stable TTR assists Aß internalization, leading to its degradation. Finally, we demonstrated that only stable TTR can increase LRP1 levels. CONCLUSION: TTR stabilization exerts a positive effect on Aß clearance and LRP1 levels, suggesting that TTR protective role in AD is dependent on its stability. These results provide relevant information for the design of TTR-based therapeutic strategies for AD.