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

BackgroundThe absence of transthyretin (TTR) in AD mice decreases brain A 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 A, and TTR tetrameric stabilizers such as iododiflunisal ameliorate AD features in vivo. MethodsWe firstly investigated TTR folding status in human plasma measuring the resistance to urea denaturation. The importance of TTR stability on A internalization was studied in human cerebral microvascular endothelial (hCMEC/D3) and hepatoma cells (HepG2), by flow cytometry. To investigate the fate of A at the blood-brain barrier, A efflux from hCMEC/D3 cells seeded on transwells was measured using ELISA. Further, to assess A 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. ResultsWe showed that TTR stability is decreased in AD and that WT TTR and drug-stabilized L55P TTR are able to increase uptake of A.Furthermore, measurement of A efflux showed that stable or stabilized TTR increased A efflux from the basolateral to the apical side. Moreover, HepG2 cells incubated with A in the presence of WT TTR, but not L55P TTR, showed an increased number of lysosomes. Further, in the presence of WT TTR, A peptide colocalized with lysosomes, indicating that only stable TTR assists A internalization, leading to its degradation. Finally, we demonstrated that only stable TTR can increase LRP1 levels. ConclusionTTR stabilization exerts a positive effect on A 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.