Intermediate Antiparallel ß Structure in Amyloid ß Plaques Revealed by Infrared Spectroscopic Imaging.

Aggregation of amyloid ß (Aß) peptides into extracellular plaques is a hallmark of the molecular pathology of Alzheimer's disease (AD). Amyloid aggregates have been extensively studied in vitro, and it is well-known that mature amyloid fibrils contain an ordered parallel ß structure. The structural evolution from unaggregated peptide to fibrils can be mediated through intermediate structures that deviate significantly from mature fibrils, such as antiparallel ß-sheets. However, it is currently unknown if these intermediate structures exist in plaques, which limits the translation of findings from in vitro structural characterizations of amyloid aggregates to AD. This arises from the inability to extend common structural biology techniques to ex vivo tissue measurements. Here we report the use of infrared (IR) imaging, wherein we can spatially localize plaques and probe their protein structural distributions with the molecular sensitivity of IR spectroscopy. Analyzing individual plaques in AD tissues, we demonstrate that fibrillar amyloid plaques exhibit antiparallel ß-sheet signatures, thus providing a direct connection between in vitro structures and amyloid aggregates in the AD brain. We further validate results with IR imaging of in vitro aggregates and show that the antiparallel ß-sheet structure is a distinct structural facet of amyloid fibrils.

Intermediate antiparallel beta structure in amyloid plaques revealed by infrared spectroscopic imaging.

Aggregation of amyloid beta (Aß) peptides into extracellular plaques is a hallmark of the molecular pathology of Alzheimer's disease (AD). Amyloid aggregates have been extensively studied in-vitro, and it is well known that mature amyloid fibrils contain an ordered parallel ß structure. The structural evolution from unaggregated peptide to fibrils can be mediated through intermediate structures that deviate significantly from mature fibrils, such as antiparallel ß-sheets. However, it is currently unknown if these intermediate structures exist in plaques, which limits the translation of findings from in-vitro structural characterizations of amyloid aggregates to AD. This arises from the inability to extend common structural biology techniques to ex-vivo tissue measurements. Here we report the use of infrared (IR) imaging, wherein we can spatially localize plaques and probe their protein structural distributions with the molecular sensitivity of IR spectroscopy. Analyzing individual plaques in AD tissues, we demonstrate that fibrillar amyloid plaques exhibit antiparallel ß-sheet signatures, thus providing a direct connection between in-vitro structures and amyloid aggregates in AD brain. We further validate results with IR imaging of in-vitro aggregates and show that antiparallel ß-sheet structure is a distinct structural facet of amyloid fibrils.

Nanoscale Infrared Spectroscopy Identifies Structural Heterogeneity in Individual Amyloid Fibrils and Prefibrillar Aggregates.

Amyloid plaques are one of the central manifestations of Alzheimer's disease pathology. Aggregation of the amyloid beta (Aß) protein from amorphous oligomeric species to mature fibrils has been extensively studied. However, structural heterogeneities in prefibrillar species, and how that affects the structure of later-stage aggregates are not yet well understood. The integration of infrared spectroscopy with atomic force microscopy (AFM-IR) allows for identifying the signatures of individual nanoscale aggregates by spatially resolving spectra. We use AFM-IR to demonstrate that amyloid oligomers exhibit significant structural variations as evidenced in their infrared spectra. This heterogeneity is transmitted to and retained in protofibrils and fibrils. We show that amyloid fibrils do not always conform to their putative ordered structure and structurally different domains exist in the same fibril. We further demonstrate that these structural heterogeneities manifest themselves as a lack of ß sheet structure in amyloid plaques in Alzheimer's tissue using infrared imaging.

Label-Free Infrared Spectroscopic Imaging Reveals Heterogeneity of ß-Sheet Aggregates in Alzheimer's Disease.

The aggregation of the amyloid beta (Aß) protein into plaques is a pathological feature of Alzheimer's disease (AD). While amyloid aggregates have been extensively studied in vitro, their structural aspects and associated chemistry in the brain are not fully understood. In this report, we demonstrate, using infrared spectroscopic imaging, that Aß plaques exhibit significant heterogeneities in terms of their secondary structure and phospholipid content. We show that the capabilities of discrete frequency infrared imaging (DFIR) are ideally suited for characterization of amyloid deposits in brain tissues and employ DFIR to identify nonplaque ß-sheet aggregates distributed throughout brain tissues. We further demonstrate that phospholipid-rich ß-sheet deposits exist outside of plaques in all diseased tissues, indicating their potential clinical significance. This is the very first application of DFIR toward a characterization of protein aggregates in an AD brain and provides a rapid, label-free approach that allows us to uncover ß-sheet heterogeneities in the AD, which may be significant for targeted therapeutic strategies in the future.