Alzheimer's disease (AD) is characterized by decreased synapse density in hippocampus and neocortex, and synapse loss is the strongest anatomical correlate of the degree of clinical impairment. Although considerable evidence supports a causal role for the amyloid-␤ protein (A␤) in AD, a direct link between a specific form of A␤ and synapse loss has not been established. We demonstrate that physiological concentrations of naturally secreted A␤ dimers and trimers, but not monomers, induce progressive loss of hippocampal synapses. Pyramidal neurons in rat organotypic slices had markedly decreased density of dendritic spines and numbers of electrophysiologically active synapses after exposure to picomolar levels of soluble oligomers. Spine loss was reversible and was prevented by A␤-specific antibodies or a small-molecule modulator of A␤ aggregation. Mechanistically, A␤-mediated spine loss required activity of NMDA-type glutamate receptors (NMDARs) and occurred through a pathway involving cofilin and calcineurin. Furthermore, NMDARmediated calcium influx into active spines was reduced by A␤ oligomers. Partial blockade of NMDARs by pharmacological antagonists was sufficient to trigger spine loss. We conclude that soluble, low-n oligomers of human A␤ trigger synapse loss that can be reversed by therapeutic agents. Our approach provides a quantitative cellular model for elucidating the molecular basis of A␤-induced neuronal dysfunction.

The accumulation of amyloid β-protein (Aβ) in brain regions serving memory and cognition is a central pathogenic feature of Alzheimer's disease (AD). We have shown that small soluble oligomers of human Aβ that are naturally secreted by cultured cells inhibit hippocampal long-term potentiation (LTP) in vitro and in vivo and transiently impair the recall of a complex learned behaviour in rats. These results support the hypothesis that diffusible oligomers of Aβ initiate a synaptic dysfunction that may be an early event in AD. We now report detailed electrophysiological analyses that define conditions under which acute application of soluble Aβ inhibits hippocampal synaptic plasticity in wild-type mice. To ascertain which Aβ assemblies contribute to the impairment of LTP, we fractionated oligomers by size-exclusion chromatography and found that Aβ trimers fully inhibit LTP, whereas dimers and tetramers have an intermediate potency. Natural Aβ oligomers are sensitive to heat denaturation, primarily inhibit the induction phase of LTP, and cause a sustained impairment of LTP even after extensive washout. We observed no effects of Aβ oligomers on presynaptic vesicle release. LTP in juvenile mice is resistant to the effects of Aβ oligomers, as is brain-derived-neurotrophic-factor-induced LTP in adult hippocampus. We conclude that specific assemblies, particularly timers, of naturally secreted Aβ oligomers are potent and selective inhibitors of certain forms of hippocampal LTP.

A microarray technology suitable for analyzing the expression of microRNAs and of other small RNAs was used to determine the microRNA expression profile during mouse-brain development and observed a temporal wave of gene expression of sequential classes of microRNAs.