Peptides or proteins convert under some conditions from their soluble forms into highly ordered fibrillar aggregates. Such transitions can give rise to pathological conditions ranging from neurodegenerative disorders to systemic amyloidoses. In this review, we identify the diseases known to be associated with formation of fibrillar aggregates and the specific peptides and proteins involved in each case. We describe, in addition, that living organisms can take advantage of the inherent ability of proteins to form such structures to generate novel and diverse biological functions. We review recent advances toward the elucidation of the structures of amyloid fibrils and the mechanisms of their formation at a molecular level. Finally, we discuss the relative importance of the common main-chain and side-chain interactions in determining the propensities of proteins to aggregate and describe some of the evidence that the oligomeric fibril precursors are the primary origins of pathological behavior.

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Protein Misfolding, Functional Amyloid, and Human Disease

The binding of thioflavin-T to amyloid fibrils: localisation and implications.

Because understanding amyloid fibrillation in molecular detail is essential for development of strategies to control amyloid formation and overcome neurodegenerative disorders, increased understanding of present molecular probes as well as development of new probes are of utmost importance. To date, the binding modes of these molecular probes to amyloid fibrils are by no means adequately described or understood, and the large number of studies on Thioflavin T (ThT) and Congo Red (CR) binding have resulted in models that are incomplete and conflicting. Different types of binding sites are likely to be present in amyloid fibrils with differences in binding modes. ThT may bind in channels running parallel to the long axis of the fibril. In the channels, ThT may bind in either a monomeric or dimeric form of which the molecular conformation is likely to be planar. CR may bind in grooves formed along the β-sheets as a planar molecule in either a monomeric or supramolecular form.

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Binding mode of Thioflavin T and other molecular probes in the context of amyloid fibrils—current status

Synchrotron-based infrared and X-ray imaging shows focalized accumulation of Cu and Zn co-localized with beta-amyloid deposits in Alzheimer's disease.

Congo red and protein aggregation in neurodegenerative diseases.

New advances in polarized light microscopy were used to image Congo red-stained cerebral amyloidosis in sharp relief. The rotating-polarizer method was used to separate the optical effects of transmission, linear birefringence, extinction, linear dichroism, and orientation of the electric dipole transition moments and to display them as false-color maps. These effects are typically convolved in an ordinary polarized light microscope. In this way, we show that the amyloid deposits in Alzheimer's disease plaques contain structurally disordered centers, providing clues to mechanisms of crystallization of amyloid in vivo. Comparisons are made with plaques from tissues of subjects having Down's syndrome and a prion disease. In plaques characteristic of each disease, the Congo red molecules are oriented radially. The optical orientation in amyloid deposited in blood vessels from subjects having cerebral amyloid angiopathy was 90° out of phase from that in the plaques, suggesting that the fibrils run tangentially with respect to the circumference of the blood vessels. Our result supports an early model in which Congo red molecules are aligned along the long fiber axis and is in contrast to the most recent binding models that are based on computation. This investigation illustrates that the latest methods for the optical analysis of heterogeneous substances are useful for in situ study of amyloid.

/pdf/imaging-linear-birefringence-and-dichroism-in-cerebral-892h8zcha6.pdf

Imaging linear birefringence and dichroism in cerebral amyloid pathologies

Polarimetric imaging of amyloid

New advances in polarized light microscopy have been employed to image Congo red stained amyloid plaques in sharp relief. The principle diagnostic criterion of amyloidosis, established by Divry and Florkin in 1927, is the detection with a polarizing optical microscope of so-called “apple green” birefringence from CR stained tissue sections. The chapter presents a newly developed imaging system to separate the optical transmission, refractive index anisotropy, and optical extinction that are otherwise convolved to produce the ill-defined apple green birefringence. The polarizing microscope, a prototype of the MetriPol System now available from Oxford Cryosystems, is adapted with a stepper motor driven rotating polarizer, circular analyzer consisting of a linear analyzer and quarter wave plate aligned at 45°, an 8-bit monochrome CCD digital camera, and a PC. The chapter shows that the system can be used to determine the absorption anisotropy and orientation of the electric dipole transition moment.

Werner Kaminksy

Papers

Imaging linear birefringence and dichroism in cerebral amyloid pathologies

Polarimetric imaging of amyloid

Status of Congo Red Stained Amyloid in Polarized Light