Dual glycation and oxidation of tau protein: impact of methylglyoxal and hydrogen peroxide on tau structure and fibril assembly.

Alzheimer's disease (AD) and type 2 diabetes mellitus (T2DM) indicate overlapping pathogenic mechanisms including, protein glycation and oxidative stress. Tau protein, a key player in AD pathology, undergoes various post-translational modifications, disturbing its physiological function and facilitating its aggregation/fibrillation. Nevertheless, the cumulative influence of glycation and oxidation on the structural integrity and fibrillation of tau remains inadequately elucidated. In this study, we examined the effects of methylglyoxal (MGO)-induced glycation and hydrogen peroxide (H2O2)-mediated oxidation, individually or in combination, on tau structure, fibrillation and cytotoxicity. Structural and morphological alterations were evaluated using SDS-PAGE, fluorescence spectroscopy, circular dichroism, dynamic light scattering, Fourier transform infrared spectroscopy, and atomic force microscopy. Fibrillation kinetics were monitored under two conditions: (i) pre-fibrillation modification and (ii) simultaneous modification and fibrillation. Our results indicated that the co-treatment with MGO and H2O2 synergistically altered tau structure. Moreover, the fibrillation kinetics of pre-modified tau samples with MGO indicated a reduction in fibrillation through the generation of oligomeric species. Conversely, the fibrillation kinetics of pre-modified tau samples with H2O2 and both compounds increased tau fibrillation. On the other hand, simultaneous modification and fibrillation of tau samples with MGO and H2O2 resulted in an increase in tau fibrillation and structural changes. Collectively, our results indicated that co-treatment with MGO and H2O2 synergistically enhanced tau fibrillation and produced more ordered fibril structures with increased cytotoxicity toward SH-SY5Y cells. These findings provide mechanistic insights into how glycation and oxidative stress cooperatively modulate tau fibrillation and offer a molecular basis for the pathological link between diabetes and AD.