Although the formation of -amyloid (A) deposits in the brain is a hallmark of Alzheimer disease (AD), the soluble oligomers rather than the mature amyloid fibrils most likely contribute to A toxicity and neurodegeneration. Thus, the discovery of agents targeting soluble A oligomers is highly desirable for early diagnosis prior to the manifestation of a clinical AD phenotype and also more effective therapies. We have previously reported that a novel 15-amino acid peptide (15-mer), isolated via phage display screening, targeted A and attenuated its neurotoxicity (Taddei, K., Laws, S. M., Verdile, G., Munns, S., D'Costa, K., Harvey, A. R., Martins, I. J., Hill, F., Levy, E., Shaw, J. E., and Martins, R. N. (2010) Neurobiol. Aging 31, 203-214). The aim of the current study was to generate and biochemically characterize analogues of this peptide with improved stability and therapeutic potential. We demonstrated that a stable analogue of the 15-amino acid peptide (15M S.A.) retained the activity and potency of the parent peptide and demonstrated improved proteolytic resistance in vitro (stable to t ؍ 300 min, c.f. t ؍ 30 min for the parent peptide). This candidate reduced the formation of soluble A42 oligomers, with the concurrent generation of non-toxic, insoluble aggregates measuring up to 25-30 nm diameter as determined by atomic force microscopy. The 15M S.A. candidate directly interacted with oligomeric A42, as shown by coimmunoprecipitation and surface plasmon resonance/Biacore analysis, with an affinity in the low micromolar range. Furthermore, this peptide bound fibrillar A42 and also stained plaques ex vivo in brain tissue from AD model mice. Given its multifaceted ability to target monomeric and aggregated A42 species, this candidate holds promise for novel preclinical AD imaging and therapeutic strategies.
Alzheimer disease (AD)4 is a progressive neurodegenerative disorder, accounting for 50 -70% of late-onset dementia cases (2). The major biochemical hallmarks of AD are extracellular amyloid plaques and intracellular neurofibrillary tangles. The predominant species present in amyloid plaques is -amyloid (A), a 4.5-kDa peptide generated through amyloidogenic processing (sequential cleavage by -and ␥-secretases) of the parent amyloid precursor protein (APP). This produces two forms of A, comprising either 40 or 42 amino acids, where the relative amount of the longer 42-amino acid form (A42) is especially critical for AD progression, given its higher propensity to aggregate and form neurotoxic species (3, 4).In AD brain, the monomeric A42 peptide is known to aggregate and form various ordered assemblies, which precede plaque formation. These include low-n oligomers (dimers to octamers, reviewed in Ref. 5), high molecular weight oligomers such as A-derived diffusible ligands (6) and globulomers (7), protofibrils (8), and fibrils (9). Much evidence has indicated that soluble A42 oligomers, rather than mature amyloid plaques, correlate with disease severity (10, 11) and contribute to synapti...