Foes, A. | bioRxiv Science

bioRxiv2023

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

Aggregation of amyloid beta (A{beta}) peptides into extracellular plaques is a hallmark of the molecular pathology of Alzheimers disease (AD). Amyloid aggregates have been extensively studied in-vitro, and it is well known that mature amyloid fibrils contain an ordered parallel {beta} structure. The structural evolution from unaggregated peptide to fibrils can be mediated through intermediate structures that deviate significantly from mature fibrils, such as antiparallel {beta}-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 {beta}-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 {beta}-sheet structure is a distinct structural facet of amyloid fibrils.

biophysics↗

bioRxiv2022

Structural heterogeneity of amyloid aggregates identified by spatially resolved nanoscale infrared spectroscopy

Amyloid plaques, composed of aggregates of the amyloid beta (A{beta}) protein, are one of the central manifestations of Alzheimers disease pathology. Aggregation of A{beta} from amorphous oligomeric species to mature fibrils has been extensively studied. However, significantly less in known about early-stage aggregates compared to fibrils. In particular, structural heterogeneities in prefibrillar species, and how that affects the structure of later stage aggregates are not yet well understood. Conventional spectroscopies cannot attribute structural facets to specific aggregates due to lack of spatial resolution, and hence aggregates at any stage of aggregation must be viewed as having the same average structure. The integration of infrared spectroscopy with Atomic Force Microscopy (AFM-IR) allows for identifying the signatures of individual nanoscale aggregates by spatially resolving spectra. In this report, we use AFM-IR to demonstrate that amyloid oligomers exhibit significant structural variations as evidenced in their infrared spectra, ranging from ordered beta structure to disordered conformations with predominant random coil and beta turns. This heterogeneity is transmitted to and retained in protofibrils and fibrils. We show for the first time that amyloid fibrils do not always conform to their putative ordered structure and structurally different domains can exist in the same fibril. We further show the implications of these results in amyloid plaques in Alzheimers tissue using infrared imaging, where these structural heterogeneities manifest themselves as lack of expected beta sheet structure.

biophysics↗