The protease activity of transthyretin reverses the effect of pH on the amyloid-β protein/heparan sulfate proteoglycan interaction: A biochromatographic study

Abstract

Patients suffering of Alzheimer's disease (AD) are characterized by a low transthyretin (TTR) level in the brain. The effect of pH and TTR concentration in the medium on the β-amyloid protein (Aβ)/heparan sulfate proteoglycan (HSPG) association mechanism were studied using a biochromatographic approach. For this purpose, HSPG was immobilized via amino groups onto the amino propyl silica pre-packed column, activated with glutaraldehyde, by using the Schiff base method. Using an equilibrium perturbation method, it was clearly shown that Aβ can be bound with HSPG. This approach allowed the determination of the thermodynamic data of this binding mechanism. The role of the pH was also analyzed. Results from enthalpy–entropy compensation and the plot of the number of protons exchanged versus pH showed that the binding mechanism was dependent on pH with a critical value at pH=6.5. This value agreed with a histidine protonation as an imidazolium cation. Moreover, the corresponding thermodynamical data showed that at pH>6.5, van der Waals and hydrogen bonds due to aromatic amino acids as tyrosine or phenylalanine present in the N-terminal (NT) part governed the Aβ/HSPG association. Aβ remained in its physiological structure in a random coil form (i.e. the non-amyloidogenic structure) because van der Waals interactions and hydrogen bonds were preponderant. At acidic pH (pH<6.5), ionic and hydrophobic interactions, created by histidine protonation and hydrophobic amino acids, appeared in the Aβ/HSPG binding. These hydrophobic and ionic interactions led to the conversion of the random coil form of Aβ into a β-sheet structure which was the amyloidogenic folding. When TTR was incubated with Aβ, the Aβ/HSPG association mechanism was enthalpy driven at all pH values. The affinity of Aβ for HSPG decreased when TTR concentration increased due to the complexation of Aβ with TTR. Also, the decrease of the peak area with the increase of TTR concentration demonstrated that this Aβ/TTR association led to the cleavage of Aβ full length to a smaller fragment. For acidic pH (pH<6.5), it was shown that the importance of the hydrophobic and ionic interactions decreased when TTR concentration increased. This result confirmed that Aβ was cleaved by TTR in a part containing only the NT part. Our results demonstrated clearly that TTR reversed the effect of acidic pH and thus played a protective role in AD.