The involvement of dityrosine crosslinks in lipofuscin accumulation in Alzheimer’s disease

Abstract

Lipofuscin is hydrophobic and insoluble yellow-brown pigment that accumulates in the nervous system of individuals and considered to be a biomarker of aging. However, it has been reported that amyloid-containing Alzheimer neurons contain a large amount of lipofuscin and lysosomal protease enzymes suggesting that the accumulation of Aβ may contribute to lipofuscin formation under oxidative stress conditions. Probing the contribution of oxidative stress using dityrosine cross-links as a marker will help to raise our understanding of the mechanism underlying the increased lipofuscin accumulation in Alzheimer (AD). In order to establish whether oxidised Aβ42 is found in lipofuscin pigments in AD brains, immunogold labelling for dityrosine and Aβ42 in lipofuscin of AD and control age matched brains was carried out. Single immunogold labelling of dityrosine was observed in the lipofuscin granules of control age-matched human brain. Interestingly, TEM immunogold labelling of dityrosine in lipofuscin of AD brain reveals two different labelling areas, low-and high-density dityrosine labelling. The quantification of immunogold particles shows significantly more dityrosine labelling in AD brain compared to age-matched controls. TEM immunogold co-labelling of dityrosine and Aβ in AD brain reveals some colocalisation within lipofuscin, although some areas showed low levels of Aβ labelling. These results may indicate that dityrosine cross-links could be generated from oxidation of various proteins that contain tyrosine residues. Our data show that dityrosine cross-links are increased in lipofuscin in AD brain, highlighting the important role played by dityrosine cross-links in the accumulation of lipofuscin in higher levels in AD compared to control brains.