Accumulation of cerebral amyloid-beta peptide (Abeta) is essential for developing synaptic and cognitive deficits in Alzheimer's disease. However, the physiological functions of Abeta, as well as the primary mechanisms that initiate early Abeta-mediated synaptic dysfunctions, remain largely unknown. Here we examine the acute effects of endogenously released Abeta peptides on synaptic transfer at single presynaptic terminals and synaptic connections in rodent hippocampal cultures and slices. Increasing extracellular Abeta by inhibiting its degradation enhanced release probability, boosting ongoing activity in the hippocampal network. Presynaptic enhancement mediated by Abeta was found to depend on the history of synaptic activation, with lower impact at higher firing rates. Notably, both elevation and reduction in Abeta levels attenuated short-term synaptic facilitation during bursts in excitatory synaptic connections. These observations suggest that endogenous Abeta peptides have a crucial role in activity-dependent regulation of synaptic vesicle release and might point to the primary pathological events that lead to compensatory synapse loss in Alzheimer's disease.

The amyloid-␤ (A␤) peptide, a major pathological hallmark of Alzheimer's disease (AD), undergoes a cascade of interactions resulting in the formation of soluble aggregates and their conversion in the brain to insoluble deposits and mature senile plaques. Furthermore, the apoE4 isoform of apolipoprotein E (apoE), which is the major genetic risk factor of AD, is associated with increased A␤ deposition. It is not known how the different A␤ aggregates in the amyloid cascade are formed, contribute to the pathogenesis of AD, or are affected by apoE4. To investigate the initial aggregation stages underlying the amyloid cascade in vivo and how apoE affects them, we examined the effects of prolonged inhibition and subsequent reactivation of the A␤-degrading protease neprilysin on deposition, disaggregation, and fibrillization of A␤ in apoEtransgenic and control mice. In control mice, intracerebroventricular infusion of thiorphan, which inhibits neprilysin, induced A␤42 and A␤40 deposition and fibrillization. On termination of thiorphan treatment, the number of A␤ deposits decreased, whereas the fibrillar A␤ deposits were unaffected. Similar treatments in apoE-deficient mice and mice transgenic for human apoE4 or apoE3 revealed that apoE4 enhances specifically the nucleation and aggregation of immunopositive A␤ deposits and that reversible disaggregation of these deposits and their irreversible conversion to fibrillar deposits are stimulated similarly by the different apoE isoforms. Deposition of A␤ and its enhancement by apoE4 were accompanied by increased astrogliosis both far from and near the A␤ deposits, suggesting that astrogliosis might be triggered by both insoluble and soluble A␤ aggregates.C onverging genetic and histopathological observations led to the formulation of the amyloid hypothesis, which proposes that accumulation of amyloid-␤ (A␤) peptide, a major constituent of the brain plaques characteristic of Alzheimer's disease (AD), is the primary event in AD pathogenesis (1, 2). Recent findings suggest that brains of individuals with AD also contain soluble and neurotoxic A␤ oligomers, which seem to be an intermediate state in the A␤-aggregation cascade, whose downstream product is the senile plaque (3-10). There is poor correlation between the severity of dementia and the density of amyloid plaques in AD (11-13); furthermore, in mice transgenic for the human A␤ precursor protein (APP), synaptic degeneration and cognitive decline are observed even before amyloid is deposited (14-16). This finding suggests that the pathological effects of A␤ in AD are mediated by A␤ aggregates that precede the formation of the senile plaque. The relative contributions of the different soluble and insoluble A␤ aggregates to the pathology of AD and the mechanisms underlying their formation in vivo are not yet known.Apolipoprotein E (apoE), the major brain lipid-binding protein, is expressed in humans as three isoforms (apoE2, apoE3, and apoE4), which differ in one or two amino acids (17). The apoE4 genotype is the major genetic ...

Accumulated genetic evidence suggests that attenuation of the ratio between cerebral amyloid-β Aβ40 and Aβ42 isoforms is central to familial Alzheimer's disease (FAD) pathogenesis. However, FAD mutations account for only 1-2% of Alzheimer's disease cases, leaving the experience-dependent mechanisms regulating Aβ40/42 an enigma. Here we explored regulation of Aβ40/42 ratio by temporal spiking patterns in the rodent hippocampus. Spike bursts boosted Aβ40/42 through a conformational change in presenilin1 (PS1), the catalytic subunit of γ-secretase, and subsequent increase in Aβ40 production. Conversely, single spikes did not alter basal PS1 conformation and Aβ40/42. Burst-induced PS1 conformational shift was mediated by means of Ca(2+)-dependent synaptic vesicle exocytosis. Presynaptic inhibition in vitro and visual deprivation in vivo augmented synaptic and Aβ40/42 facilitation by bursts in the hippocampus. Thus, burst probability and transfer properties of synapses represent fundamental features regulating Aβ40/42 by experience and may contribute to the initiation of the common, sporadic Alzheimer's disease.

Presynaptic GABA(B) receptor (GABA(B)R) heterodimers are composed of GB(1a)/GB(2) subunits and critically influence synaptic and cognitive functions. Here, we explored local GABA(B)R activation by integrating optical tools for monitoring receptor conformation and synaptic vesicle release at individual presynaptic boutons of hippocampal neurons. Utilizing fluorescence resonance energy transfer (FRET) spectroscopy, we detected a wide range of FRET values for CFP/YFP-tagged GB(1a)/GB(2) receptors that negatively correlated with release probabilities at single synapses. High FRET of GABA(B)Rs associated with low release probability. Notably, pharmacological manipulations that either reduced or increased basal receptor activation decreased intersynapse variability of GB(1a)/GB(2) receptor conformation. Despite variability along axons, presynaptic GABA(B)R tone was dendrite specific, having a greater impact on synapses at highly innervated proximal branches. Prolonged neuronal inactivity reduced basal receptor activation, leading to homeostatic augmentation of release probability. Our findings suggest that local variations in basal GABA concentration are a major determinant of GB(1a)/GB(2) conformational variability, which contributes to heterogeneity of neurotransmitter release at hippocampal synapses.

The allele E4 of apolipoprotein E4 (apoE4), which is the most prevalent genetic risk factor of Alzheimer's disease (AD), inhibits synaptogenesis and neurogenesis and stimulates apoptosis in brains of apoE4 transgenic mice that have been exposed to an enriched environment. In the present study, we investigated the hypothesis that the brain activity-dependent impairments in neuronal plasticity, induced by apoE4, are mediated via the amyloid cascade. Importantly, we found that exposure of mice transgenic for either apoE4, or the Alzheimer's disease benign allele apoE3, to an enriched environment elevates similarly the hippocampal levels of amyloid-b peptide (Ab) and apoE of these mice, but that the degree of aggregation and spatial distribution of Ab in these mice are markedly affected by the apoE genotype. Accordingly, environmental stimulation triggered the formation of extracellular plaque-like Ab deposits and the accumulation of intra-neuronal oligomerized Ab specifically in brains of apoE4 mice. Further experiments revealed that hippocampal dentate gyrus neurons are particularly susceptible to apoE4 and environmental stimulation and that these neurons are specifically enriched in both oligomerized Ab and apoE. These findings show that the impairments in neuroplasticity which are induced by apoE4 following environmental stimulation are associated with the accumulation of intraneuronal Ab and suggest that oligomerized Ab mediates the synergistic pathological effects of apoE4 and environmental stimulation.