Selective regional loss of exocytotic presynaptic vesicle proteins in Alzheimer's disease brains.

We tested whether regional or selective alterations in presynaptic proteins occur in Alzheimer's disease (AD) and correlate with tests of cognitive function. We measured the levels of seven presynaptic proteins (synaptobrevin, synaptotagmin, SNAP-25, syntaxin, SV2, Rab3a, and synapsin I) by immunoblotting in postmortem tissue from four brain regions (hippocampus, entorhinal cortex, caudate nucleus, and occipital cortex). Three subject groups were studied: AD, possible/early AD (p-AD), and age-matched controls. Synaptobrevin and synaptotagmin were significantly reduced (29%, P<0.08; 38%, P<0. 07) in hippocampus in p-AD compared to controls. In definite AD compared to controls, selective regional reductions in vesicle proteins were found: synaptobrevin (46%, P<0.05), synaptotagmin (52%, P<0.01), and Rab3a (30%, P<0.05) in hippocampus; synaptobrevin (31%, P<0.01), synaptotagmin (15%, P<0.05), and Rab3a (44%, P<0.05) in entorhinal cortex. In contrast, the levels of two vesicle proteins (synapsin I and SV2) and two presynaptic membrane proteins (syntaxin and SNAP-25) were similar to controls. Synaptobrevin was the only vesicle protein reduced in AD in all four brain regions (occipital cortex 37%, P<0.05; caudate nucleus 31%, P<0.05). By univariate analysis of all cases, Mini-Mental State Examination, Blessed (BIMC) and Free Recall scores were strongly correlated with reduced levels of synaptic vesicle proteins synaptobrevin, synaptotagmin, and Rab3a in hippocampus and entorhinal cortex. These results suggest that there are selective and early defects in presynaptic vesicle proteins, but not synaptic plasma membrane proteins in AD and that defects correlate with cognitive dysfunction in this disease.