β-amyloid immunotherapy prevents synaptic degeneration in a mouse model of Alzheimer's disease

Alzheimer's disease neuropathology is characterized by key features that include the deposition of the amyloid beta peptide (A beta) into plaques, the formation of neurofibrillary tangles, and the loss of neurons and synapses in specific brain regions. The loss of synapses, and particularly the associated presynaptic vesicle protein synaptophysin in the hippocampus and association cortices, has been widely reported to be one of the most robust correlates of Alzheimer's disease-associated cognitive decline. The beta-amyloid hypothesis supports the idea that A beta is the cause of these pathologies. However, the hypothesis is still controversial, in part because the direct role of A beta in synaptic degeneration awaits confirmation. In this study, we show that A beta reduction by active or passive A beta immunization protects against the progressive loss of synaptophysin in the hippocampal molecular layer and frontal neocortex of a transgenic mouse model of Alzheimer's disease. These results, substantiated by quantitative electron microscopic analysis of synaptic densities, strongly support a direct causative role of A beta in the synaptic degeneration seen in Alzheimer's disease and strengthen the potential of A beta immunotherapy as a treatment approach for this disease.