Activation of caspase-3 in single neurons and autophagic granules of granulovacuolar degeneration in Alzheimer's disease - Evidence for apoptotic cell death

Neuronal loss is prominent in Alzheimer's disease (AD), and its mechanisms remain unresolved. Apoptotic cell death has been implicated on the basis of studies demonstrating DNA fragmentation and an upregulation of proapoptoti c proteins in the AD brain. However, DNA fragmentation in neurons is too fr equent to account for the continuous neuronal loss in a degenerative diseas e extending over many years. Furthermore, the typical apoptotic morphology has not been convincingly documented in AD neurons with fragmented DNA. We report the detection of the activated form of caspase-3, the central effect or enzyme of the apoptotic cascade, in AD and Down's syndrome (DS) brain us ing an affinity-purified antiserum. In AD and DS, single neurons with apopt otic morphology showed cytoplasmic immunoreactivity for activated caspase-3 , whereas no neurons were labeled in age-matched controls. Apoptotic neuron s were identified at an approximate frequency of 1 in 1100 to 5000 neurons in the cases examined. Furthermore, caspase-3 immunoreactivity was detected in granules of granulovacuolar degeneration. Our results provide direct ev idence for apoptotic neuronal death in AD with a frequency compatible with the progression of neuronal degeneration in this chronic disease and identi fy autophagic vacuoles of granulovacuolar degeneration as possible means fo r the protective segregation of early apoptotic alterations in the neuronal cytoplasm.