amyloid

Micrograph showing amyloid deposits (pink) in small bowel. H&E stain.

Amyloids are insoluble fibrous protein aggregates sharing specific structural traits. They arise from at least 18 inappropriately folded versions of proteins and polypeptides present naturally in the body.^[1] These misfolded structures alter their proper configuration such that they erroneously interact with one another or other cell components forming insoluble fibrils. They have been associated with the pathology of more than 20 serious human diseases in that, abnormal accumulation of amyloid fibrils in organs may lead to amyloidosis, and may play a role in various neurodegenerative disorders.

Definition

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 found that they were, in fact, deposits of proteinaceous material.^[2]

Micrograph showing amyloid deposition in small bowel. Congo red stain.

• The classical, histopathological definition of amyloid is an extracellular, proteinaceous deposit exhibiting beta sheet structure. Common to most cross-beta type structures they are generally identified by apple-green birefringence when stained with congo red and seen under polarized light. These deposits often recruit various sugars and other components such as Serum Amyloid P component, resulting in complex, and sometimes inhomogeneous structures.^[3] Recently this definition has come into question as some classic, amyloid species have been observed in distinctly intracellular locations.^[4]

• A more recent, biophysical definition is broader, including any polypeptide which polymerizes to form a cross-beta structure, in vivo, or in vitro. Some of these, although demonstrably cross-beta sheet, do not show some classic histopathological characteristics such as the Congo red birefringence. Microbiologists and biophysicists have largely adopted this definition,^[5][6] leading to some conflict in the biological community over an issue of language.

The remainder of this article will use the biophysical context.

Diseases featuring amyloids

The International Society of Amyloidosis classifies amyloid fibrils based upon associated proteins.^[17]

Non-disease and functional amyloids

• Native amyloids in organisms^[18]
  • Curli E. coli Protein (curlin)
  • Chaplins from Streptomyces coelicolor
  • Podospora Anserina Prion Het-s
  • Malarial coat protein
  • Spider silk (some but not all spiders)
  • Mammalian melanosomes (pMel)
  • Tissue-type plasminogen activator (tPA), a hemodynamic factor
• Proteins and peptides engineered to make amyloid
• Several yeast prions are based on an infectious amyloid, e.g. [PSI+] (Sup35p); [URE3] (Ure2p); [PIN+] (Rnq1p); [SWI1+] (Swi1p) and [OCT8+] (Cyc8p)

Amyloid biophysics

Amyloid is characterized by a cross-beta sheet quaternary structure. 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 contains cross-beta fibres is by placing a sample in an X-ray diffraction beam. The term "cross-beta" was based on the observation of two sets of diffraction lines, one longitudinal and one transverse, that form a characteristic "cross" pattern.^[19] There are two characteristic scattering diffraction signals produced at 4.7 and 10 Ångstroms (0.47 nm and 1.0 nm), corresponding to the interstrand and stacking distances in beta sheets.^[20] The "stacks" of beta sheet are short and traverse the breadth of the amyloid fibril; the length of the amyloid fibril is built by aligned strands.

Recent x-ray diffraction studies of microcrystals revealed atomistic details of core region of amyloid.^[21][22] In the crystallographic structure short stretches from amyloid prone region of amyloidogenic proteins run perpendicular to the filament axis, confirming the "cross-beta" model. In addition two layers of beta-sheet interdigitate to create compact dehydrated interface termed as steric-zipper interface. There are eight classes of steric-zipper interfaces, depending on types of beta-sheet (parallel and anti-parallel) and symmetry between two adjacent beta-sheets.

Amyloid polymerization (aggregation or non-covalent 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 or non-coded alpha-aminoisobutyric acid.^[23] For example, humans produce amylin, an amyloidogenic peptide associated with type II diabetes, but in rats and mice prolines are substituted in critical locations and amyloidogenesis does not occur.^{[citation needed]}

There are two broad classes of amyloid-forming polypeptide sequences. Glutamine-rich polypeptides are important in the amyloidogenesis of Yeast and mammalian prions, as well as Trinucleotide repeat disorders including Huntington's disease. When peptides are in a beta-sheet conformation, particularly when the residues are parallel and in-register (causing alignment), glutamines can brace the structure by forming intrastrand hydrogen bonding between its amide carbonyls and nitrogens. In general, for this class of diseases, toxicity correlates with glutamine content.^{[citation needed]} This has been observed in studies of onset age for Huntington's disease (the longer the polyglutamine sequence, the sooner the symptoms appear), and has been confirmed in a C. elegans model system with engineered polyglutamine peptides.^{[24][citation needed]}

Other polypeptides and proteins such as amylin and the Alzheimer's beta protein do not have a simple consensus sequence and are thought to operate by hydrophobic association.^{[citation needed]} Among the hydrophobic residues, aromatic amino-acids are found to have the highest amyloidogenic propensity.^[25][26]

For these peptides, cross-polymerization (fibrils of one polypeptide sequence causing other fibrils of another sequence to form) is observed in vitro and possibly in vivo.^{[citation needed]} This phenomenon is important since it would explain interspecies prion propagation and differential rates of prion propagation, as well as a statistical link between Alzheimer's and type 2 diabetes.^{[citation needed]} In general, the more similar the peptide sequence the more efficient cross-polymerization is, though entirely dissimilar sequences can cross-polymerize and highly similar sequences can even be "blockers" which prevent polymerization.^{[citation needed]} Polypeptides will not cross-polymerize their mirror-image counterparts, indicating that the phenomenon involves specific binding and recognition events.^{[citation needed]}

Amyloid pathology

The reasons for amyloid association with disease are unclear. In some cases, the deposits physically disrupt tissue architecture, suggesting disruption of function by some bulk process. An emerging consensus implicates prefibrillar intermediates, rather than mature amyloid fibers, in causing cell death.^{[27] [9]}

Studies have shown that amyloid deposition is associated with mitochondrial dysfunction and a resulting generation of reactive oxygen species (ROS), which can initiate a signalling pathway leading to apoptosis.^[28]

There are reports that indicate amyloid polymers (such as those of huntigntin) can induce the polymerization of essential amyloidogenic proteins, which should be deleterious to cells. Also, interaction partners of these essential proteins can also be sequestered ^[29]

Histological staining

Clinically, amyloid diseases are typically identified by a change in the fluorescence intensity of planar aromatic dyes such as thioflavin T or congo red. Congo red positivity remains the gold standard for diagnosis of amyloidosis. This is generally attributed to the environmental change, as these dyes intercalate between beta-strands. Congophilic amyloid plaques generally cause apple-green birefringence when viewed through crossed polarimetric filters. To avoid nonspecific staining, other histology stains, such as the hematoxylin and eosin stain, are used to quench the dyes' activity in other places such as the nucleus where the dye might bind. Modern 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.

See also

• Proteopathy

References

External links

• Bacterial Inclusion Bodies Contain Amyloid-Like Structure at SciVee
• Amyloid Cascade Hypothesis
• Stanford University Amyloid Center
• Amyloid Treatment and Research Program at Boston University
• Amyloid: Journal of Protein Folding Disorders web page at InformaWorld
• Information, support and advice to anyone with Amyloidosis, particularly in Australia (www.amyloidosisaustralia.org)
• Amyloidosis Support Network at amyloidosis.org
• UK National Amyloidosis Centre - one of the largest amyloid diagnosis and research centres at ucl.ac.uk
• Engineering Amyloid for material at University of California, Berkeley
• Amyloidosis Foundation amyloidosisresearchfoundation.org
• National Kidney and Urologic Diseases Information Clearinghouse at National Institute of Health
• Role of anesthetics in Alzheimer's disease: Molecular details revealed
• Mini Review Amyloidosis Covering structure, mechanisms of action and kinetics of amyloid fibrils.
• Video of amyloid formation.