Alzheimer’s disease (AD) is a neurodegenerative disorder defined by the progressive formation and spread of fibrillar aggregates of Aβ peptide and tau protein. Polymorphic forms of these aggregates may contribute to disease in varying ways since different neuropathologies appear to be associated with different sets of fibrillar structures and follow distinct pathological trajectories that elicit characteristic clinical phenotypes. The molecular mechanisms underlying the spread of these aggregates in disease may include nucleation, replication, and migration all of which could vary with polymorphic form, stage of disease, and region of brain. Given the linkage between mechanisms of progression and distribution of polymorphs, mapping the distribution of fibrillar structures in situ has the potential to discriminate between mechanisms of progression. However, the means of carrying out this mapping are limited. Optical microscopy lacks the resolution to discriminate between polymorphs in situ, and higher resolution tools such as ssNMR and cryoEM require the isolation of fibrils from tissue, destroying relevant spatial information. Here, we demonstrate the use of scanning x-ray microdiffraction (XMD) to map the locations of fibrillar polymorphs of Aβ peptides and tau protein in histological thin sections of human brain tissue. Coordinated examination of serial sections by immunohistochemistry was used to aid in the interpretation of scattering patterns and to put the observations in a broader anatomical context. Scattering from lesions in tissue shown to be rich in Aβ fibrils by immunohistochemistry exhibited scattering patterns with a prototypical 4.7 Å cross-β peak, and overall intensity distribution that compared well with that predicted from high resolution structures. Scattering from lesions in tissue with extensive tau pathology also exhibited a 4.7 Å cross-β peak but with intensity distributions that were distinct from those seen in Aβ-rich regions. In summary, these observations demonstrate that XMD is a rich source of information on the distribution of fibrillar polymorphs in diseased human brain tissue. When used in coordination with neuropathological examination it has the potential to provide novel insights into the molecular mechanisms underlying disease.

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Small-angle x-ray scattering (SAXS) from fibrils embedded in a fixed, thin section of tissue includes contributions from the fibrils; the polymeric matrix surrounding the fibrils; other constituents of the tissue; and cross-terms due to the spatial correlation between fibrils and neighbouring molecules. This complex mixture severely limits the amount of information that can be extracted from scattering studies. However, availability of micro- and nano-beams has made possible measurement of scattering from very small volumes which, in some cases, may be dominated by a single fibrillar constituent. In those cases, information about the predominant species may be accessible. Nevertheless, even in these cases, the correlations between the positions of fibrils and other constituents have significant impact on the observed scattering. Here, we propose strategies to extract partial information about fibril structure and tissue organization on the basis of SAXS from samples of this type. We show that the spatial correlation function of the fibril in the direction perpendicular to the fibril axis can be computed and contains information about the predominant fibril structure and the organization of the surrounding tissue matrix. It has significant advantages over approaches based on techniques developed for x-ray solution scattering. We present examples of the calculation of correlation in different types of samples to demonstrate the kinds of information that can be obtained from these measurements.SynopsisThe availability of micro- and nano- x-ray beams is making possible measurement of scattering from very small volumes, opening possibilities for derivingin situstructural information on fibrillar constituents in complex materials and tissues. This work outlines a set of strategies for confronting the formidable technical obstacles to extracting useful structural information from scattering derived from these materials.