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Alzheimer's disease (AD) is a neurodegenerative disorder that is linked to the presence of amyloid beta-peptides that can form insoluble fibrils or soluble oligomeric assemblies. Soluble forms are present in the brains and tissues of Alzheimer's patients, and their presence correlates with disease progression. Long-lived soluble forms can be generated in vitro by using small amounts of aliphatic hydrocarbon chains of detergents or fatty acids in preparations of amyloid beta-peptides. Using NMR, we have characterized soluble oligomers of Abeta preglobulomer and globulomer that are stable and alter synaptic activity. The NMR data indicate that these soluble forms have a mixed parallel and antiparallel beta-sheet structure that is different from fibrils which contain only parallel beta-sheets. Using the structural data, we engineered a disulfide bond into the soluble Abeta globulomer to give a "new" soluble antigen that is stable, homogeneous, and binds with the same affinity to selective antibodies as the parent wt globulomer.

Abnormal accumulation of soluble oligomers of amyloid beta (Abeta) is believed to cause malfunctioning of neurons in Alzheimer's disease. It has been shown that Abeta oligomers impair synaptic plasticity, thereby altering the ability of the neuron to store information. We examined the underlying cellular mechanism of Abeta oligomer-induced synaptic modifications by using a recently described stable oligomeric Abeta preparation called "Abeta(1-42) globulomer." Synthetically prepared Abeta(1-42) globulomer has been shown to localize to neurons and impairs long-term potentiation (Barghorn et al., 2005). Here, we demonstrate that Abeta(1-42) globulomer does not affect intrinsic neuronal properties, as assessed by measuring input resistance and discharge characteristics, excluding an unspecific alteration of membrane properties. We provide evidence that Abeta(1-42) globulomer, at concentrations as low as 8 nM, specifically suppresses spontaneous synaptic activity resulting from a reduction of vesicular release at terminals of both GABAergic and glutamatergic synapses. EPSCs and IPSCs were primarily unaffected. A detailed search for the precise molecular target of Abeta(1-42) globulomer revealed a specific inhibition of presynaptic P/Q calcium currents, whereas other voltage-activated calcium currents remained unaltered. Because intact P/Q calcium currents are needed for synaptic plasticity, the disruption of such currents by Abeta(1-42) globulomer may cause deficits in cellular mechanisms of information storage in brains of Alzheimer's disease patients. The inhibitory effect of Abeta(1-42) globulomer on synaptic vesicle release could be reversed by roscovitine, a specific enhancer of P/Q currents. Selective enhancement of the P/Q calcium current may provide a promising strategy in the treatment of Alzheimer's disease.

Amyloid beta-peptide (Abeta)(1-42) oligomers have recently been discussed as intermediate toxic species in Alzheimer's disease (AD) pathology. Here we describe a new and highly stable Abeta(1-42) oligomer species which can easily be prepared in vitro and is present in the brains of patients with AD and Abeta(1-42)-overproducing transgenic mice. Physicochemical characterization reveals a pure, highly water-soluble globular 60-kDa oligomer which we named 'Abeta(1-42) globulomer'. Our data indicate that Abeta(1-42) globulomer is a persistent structural entity formed independently of the fibrillar aggregation pathway. It is a potent antigen in mice and rabbits eliciting generation of Abeta(1-42) globulomer-specific antibodies that do not cross-react with amyloid precursor protein, Abeta(1-40) and Abeta(1-42) monomers and Abeta fibrils. Abeta(1-42) globulomer binds specifically to dendritic processes of neurons but not glia in hippocampal cell cultures and completely blocks long-term potentiation in rat hippocampal slices. Our data suggest that Abeta(1-42) globulomer represents a basic pathogenic structural principle also present to a minor extent in previously described oligomer preparations and that its formation is an early pathological event in AD. Selective neutralization of the Abeta globulomer structure epitope is expected to have a high potential for treatment of AD.