Synaptic and sub-synaptic localization of amyloid-β protein precursor in the rat hippocampus

Amyloid-β protein precursor (AβPP) is a large transmembrane protein highly expressed in the central nervous system and cleavage of it can produce amyloid-β peptides (Aβ) involved in synaptic dysfunction and loss associated with cognitive impairment in Alzheimer's disease (AD). Surprisingly, little is known about the synaptic and sub-synaptic distribution of AβPP in different types of nerve terminals. We used total, synaptic, sub-synaptic, and astrocytic membrane preparations obtained from the hippocampus of adult rats to define the localization of AβPP, using two different antibodies against different AβPP epitopes. Western blot analysis revealed that AβPP was not significantly enriched in synaptosomal as compared to total membranes. Within synapses, AβPP immunoreactivity was more abundant in pre- (60 ± 4%) than post- (30 ± 5%) or extra-synaptic fractions (10 ± 2%). Immunocytochemical analysis of purified nerve terminals indicated that AβPP was more frequently associated with glutamatergic (present in 31 ± 4% of glutamatergic terminals) rather than with GABAergic (16 ± 3%) or cholinergic terminals (4 ± 1%, n = 4). We also observed a general lack of co-localization of AβPP and GFAP immunoreactivities in the hippocampus of sections of adult rat brain, albeit we could detect the presence of AβPP in gliosomes (vesicular specializations of astrocytic membranes), suggesting that AβPP has a heterogeneous localization restricted to certain regions of astrocytes. These results provide the first direct demonstration that AβPP is mostly distributed among glutamatergic rather than GABAergic or cholinergic terminals of the adult rat hippocampus, in remarkable agreement with the particular susceptibility to dysfunction and degeneration of glutamatergic synapses in early AD.