Differentiating Abeta40/42 in plaques with small molecule fluorescent probes

Massachusetts General Hospital, Boston MA

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Abstract

Amyloid beta (Aβ) plaques constitute one of the most distinctive morphological hallmarks of Alzheimer’s disease (AD). In the past decades, the contribution of Aβ plaques to the overall cognitive decline in AD has been debated extensively. Postmortem studies suggest that plaque abundance does not correlate strongly with the severity of sporadic AD. Conversely, preclinical studies provide strong evidence that plaques are clear sites of pathology and are associated with dystrophic neurites, the loss of dendritic spines and rapid neuron cell death in their surroundings. In the plaques, A40 and A42 peptides are the major constituents. Nonetheless, unlike the role of plaque, there is nearly no argument that A42 has a much higher neurotoxicity than A40 does. Conceivably, differentiating A40 and A42 can considerably clarify the role of plaque in AD pathology. Differentiating A40 and A42 has long been considered as an impossible mission with small-molecule probes, due to the small difference in the amino acid sequence of the peptides. It is obvious that the C-terminal of A peptide is the key for designing small-molecule probes to distinguish them, because the only difference of two amino acids (isoleucine-alanine) is within the C-terminals of A40/42 peptides. There is no prior knowledge to teach us how to design such probes. Nonetheless, it has been routinely performed with anti-A42 antibodies to determine the contents of A42 in cell media and brain extracts. These anti-A42 antibodies were designed based on the epitope of the C-terminal of the peptide. This fact has bolstered us to believe that the properties of the C-terminal can be relied on to design our small molecule probes. In the past, X-ray structures of full-length As were rare. However, recently the advanced Cryo-EM technology has impressively facilitated A structure studies. In our preliminary results, based on the unique environment of the C-terminal, we designed small- molecule fluorescence probe ICTAD-1 that has the capacity to spectrally differentiate A40/42 in vitro and brain slides. In this proposal, we plan to design new probes and validate their capacity for differentiating A40/42.