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Amyloid -- Amyloidosis

Amyloid describes various types of protein aggregations that share specific traits when examined microscopically. The name amyloid comes from the early mistaken identification of the substance as starch (amylum in Latin), based on crude iodine-staining techniques. For a period the scientific community debated whether or not amyloid deposits were fatty deposits or carbohydrate deposits until it was finally resolved that it was neither, rather a deposition of proteinaceous mass.

The phenotypes of genetically transmitted amyloid diseases are often inherited in an autosomal dominant fashion; (Huntington's disease is in fact in most genetics texts the canonical autosomal dominant disease). Sometimes, the difference between aggressive amyloid diseases and senescent amyloid diseases is due to a mutation which makes the protein more prone to aggregation. Most commonly seen are point mutations which affect the cohesiveness of the protein and promote misfolding; other mutations cause aggregation-prone pieces of the protein to be cleaved off from the rest of the protein.

Diseases featuring amyloid

It should be noted that in almost all of the organ-specific pathologies, there is significant debate as to whether the amyloid plaques are the causal agent of the disease or if they are instead a symptom downstream of a common ideopathic agent. The associated proteins are indicated in parentheses.

Non-disease amyloids

Amyloid biophysics

The amyloid fold is characterized by a cross-beta sheet quaternary structure, that is, a monomeric unit contributes a beta strand to a beta sheet which spans across more than one molecule. While amyloid is usually identified using fluorescent dyes, stain polarimetry, circular dichroism, or FTIR (all indirect measurements), the "gold-standard" test to see if a structure is amyloid is by placing a sample in an X-ray diffraction beam; there are two characteristic scattering bands produced at 4 and 10 angstroms (0.4 nm and 1.0 nm}, corresponding to the interstrand distances in the beta sheet structure.

Amyloid polymerization is generally sequence-sensitive, that is, causing mutations in the sequence can prevent self-assembly, especially if the mutation is a beta-sheet breaker, such as proline. For example, humans produce an amyloidogenic peptide associated with type II diabetes, but in rodentia, a proline is substituted in a critical location and amyloidogenesis does not occur. Cross-polymerization is a phenomenon observed in vitro, and this is a putative mechanism explaining a link between diabetes and Alzheimer's disease. This is also the mechanism of prion propagation.

Histological staining

Amyloid is typically identified by a change in the fluorescence intensity of planar aromatic dyes such as Thioflavin T or Congo Red. This is generally attributed to the environmental change as these dyes intercolate between beta-strands. Congophillic amyloid plaques generally cause apple-green birefringence, when viewed through crossed polarimetric filters. To avoid nonspecific staining histology stains such as haematoxylin and eosin are used to quench the dyes' activity in other places where the dye might bind, such as the nucleus. The dawn of antibody technology and immunohistochemistry has made specific staining easier, but often this can cause trouble because epitopes can be concealed in the amyloid fold; an amyloid protein structure is generally a different conformation from that which the antibody recognizes.

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