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

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 beta-amyloid protein (A beta)/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 A beta 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 A beta/HSPG association. A beta 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 A beta/HSPG binding. These hydrophobic and ionic interactions led to the conversion of the random coil form of A beta into a beta-sheet structure which was the amyloidogenic folding. When HR was incubated with A beta, the A beta/HSPG association mechanism was enthalpy driven at all pH values. The affinity of A beta for HSPG decreased when TTR concentration increased due to the complexation of A beta with HR. Also, the decrease of the peak area with the increase of TTR concentration demonstrated that this A beta/TTR association led to the cleavage of A beta 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 HR concentration increased. This result confirmed that A beta was cleaved by HR in a part containing only the NT part. Our results demonstrated clearly that HR reversed the effect of acidic pH and thus played a protective role in AD. (C) 2014 Elsevier B.V. All rights reserved.