The amyloid peptides Ab 40 and Ab 42 of Alzheimer's disease are thought to contribute differentially to the disease process. Although Ab 42 seems more pathogenic than Ab 40 , the reason for this is not well understood. We show here that small alterations in the Ab 42 :Ab 40 ratio dramatically affect the biophysical and biological properties of the Ab mixtures reflected in their aggregation kinetics, the morphology of the resulting amyloid fibrils and synaptic function tested in vitro and in vivo. A minor increase in the Ab 42 :Ab 40 ratio stabilizes toxic oligomeric species with intermediate conformations. The initial toxic impact of these Ab species is synaptic in nature, but this can spread into the cells leading to neuronal cell death. The fact that the relative ratio of Ab peptides is more crucial than the absolute amounts of peptides for the induction of neurotoxic conformations has important implications for anti-amyloid therapy. Our work also suggests the dynamic nature of the equilibrium between toxic and non-toxic intermediates.

Interfacing neurons with micro-and nano-electronic devices has been a subject of intense study over the last decade. One of the major problems in assembling efficient neuroelectronic hybrid systems is the weak electrical coupling between the components. This is mainly attributed to the fundamental property of living cells to form and maintain an extracellular cleft between the plasma membrane and any substrate to which they adhere. This cleft shunts the current generated by propagating action potentials and thus reduces the signal-to-noise ratio. Reducing the cleft thickness, and thereby increasing the seal resistance formed between the neurons and the sensing surface, is thus a challenge and could improve the electrical coupling coefficient. Using electron microscopic analysis and field potential recordings, we examined here the use of gold micro-structures that mimic dendritic spines in their shape and dimensions to improve the adhesion and electrical coupling between neurons and micro-electronic devices. We found that neurons cultured on a gold-spine matrix, functionalized by a cysteine-terminated peptide with a number of RGD repeats, readily engulf the spines, forming tight apposition. The recorded field potentials of cultured Aplysia neurons are significantly larger using gold-spine electrodes in comparison with flat electrodes.

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