Copper mediated amyloid-\u03b2 binding to Transthyretin

William Shepard,Elisabetta Orlandini,Lidia Ciccone,Susanna Nencetti,Denis Servent,Enrico A Stura,Carole Fruchart-Gaillard,Gilles Mourier,Martin Savko

doi:10.1038/s41598-018-31808-5

Abstract

Transthyretin (TTR), a homotetrameric protein that transports thyroxine and retinol both in plasma and in cerebrospinal (CSF) fluid provides a natural protective response against Alzheimer’s disease (AD), modulates amyloid-β (Aβ) deposition by direct interaction and co-localizes with Aβ in plaques. TTR levels are lower in the CSF of AD patients. Zn2+, Mn2+ and Fe2+ transform TTR into a protease able to cleave Aβ. To explain these activities, monomer dissociation or conformational changes have been suggested. Here, we report that when TTR crystals are exposed to copper or iron salts, the tetramer undergoes a significant conformational change that alters the dimer-dimer interface and rearranges residues implicated in TTR’s ability to neutralize Aβ. We also describe the conformational changes in TTR upon the binding of the various metal ions. Furthermore, using bio-layer interferometry (BLI) with immobilized Aβ(1–28), we observe the binding of TTR only in the presence of copper. Such Cu2+-dependent binding suggests a recognition mechanism whereby Cu2+ modulates both the TTR conformation, induces a complementary Aβ structure and may participate in the interaction. Cu2+-soaked TTR crystals show a conformation different from that induced by Fe2+, and intriguingly, TTR crystals grown in presence of Aβ(1–28) show different positions for the copper sites from those grown its absence.

Highlights

Human transthyretin (TTR), a homotetrameric 127-residue protein, is the main carrier of thyroxine (T4) in celebrospinal fluid (CSF) and the second main carrier in blood[1]
In the CSF, TTR is recognized as the main Aβ binder, and, it is able to transport Aβ across the brain-blood barrier in the brain-to-blood direction[8]
The concentration of TTR is different in the blood and in the CSF: 3–4 μM and 0.1–0.4 μM, respectively[24]

Summary

Introduction

Human transthyretin (TTR), a homotetrameric 127-residue protein, is the main carrier of thyroxine (T4) in celebrospinal fluid (CSF) and the second main carrier in blood[1]. TTR binds soluble, oligomeric and Aβ fibrils[6,7] playing a role in Aβ clearance[8]. The precise mechanism by which TTR binds to Aβ remains unknown and is difficult to pinpoint because of the plethora of aggregated forms of Aβ. (but not the iron cations) can promote the formation of a TTR-amyloid complex in vitro, while chelators (EDTA or EGTA) can disrupt fibrils composed of aggregated TTR14. Zinc ions boost L55P-TTR fibril formation[15] and promote TTR metallopeptidase activity[16]. Such activity is induced by other ions, such as Co2+, Mn2+ and Fe2+, but not Cu2+ or Ni2+ 17. Apart from zinc, the binding of other metal ions to TTR has not yet been characterized crystallographically

Methods

Results

Conclusion