Apoptosis of astrocytes with enhanced lysosomal activity and oligodendrocytes in white matter lesions in Alzheimer's disease

Cerebral white matter lesions in Alzheimer's disease (AD) consist of subcortical degeneration and ischaemic-hypoxic changes. Glial changes are intimately associated with the white matter lesions, and regressive changes in astrocytes and loss of oligodendroglial cells have been reported. We quantitatively compared glial changes including apoptosis and enhanced lysosomal activity in the frontal and temporal white matter by using terminal dUTP nick end labelling (TUNEL) and immunohistochemistry for glial markers, lysosomes and apoptosis-regulating proteins in non-familial AD brains. The degree of myelin pallor and axonal loss varied considerably in both the frontal and temporal white matter but fibrillary gliosis in demyelinated lesions tended to be less prominent in the temporal white matter in AD cases. A morphometric study with planimetric methods for cross-sectional areas of frontal and temporal white matter revealed that the white matter of AD cases manifested atrophy with significant reduction in frontal (11.9%) and temporal (29.4%) white matter compared to normal controls. Double immunolabelling for glial fibrillary acidic protein (GFAP) and KP1 (CD68) revealed KP1-positive fragmented structures within the weakly GFAP-labelled astrocytes. These KP1-positive structures correspond to process fragmentation and cytoplasmic vacuoles, which in turn indicate enhanced lysosomal activity during regressive changes in astrocytes. The KP1-modified astrocytes were not found in Pick's disease and corticobasal degeneration. The density of apoptotic glial cells, largely oligodendroglial, was significantly higher in the temporal than in the frontal white matter, and most GFAP-positive astrocytes with regressive changes were apoptotic. GFAP-positive astrocyte density was statistically the same in the frontal and temporal white matter, but the density of KP1-modified astrocytes was higher in the temporal than in the frontal white matter. The rate of white matter shrinkage was significantly correlated with the density of apoptotic glial cells and the density of KP1-modified astrocytes in the temporal lobe in AD cases. An increase in apoptotic glial cell density was found to contribute to GFAP-positive astrocytes with regressive changes in temporal white matter, while apoptosis of vascular smooth muscle cells did not show topographical accentuation. Astrocytes labelled with beta amyloid protein were not apoptotic, and the density of apoptotic cells labelled with CD95 and caspase-3 was too low in both types of white matter to be statistically evaluated. Our results imply that regressive changes in astrocytes and glial apoptosis are, to some extent, associated with white matter lesions, particularly of the temporal lobe in AD brains. The presence of apoptotic astrocytes with evidence of regressive change could therefore be a histological hallmark for white matter degeneration in AD.