Shear-Induced Amyloid Formation in the Brain: III. The Roles of Shear Energy and Seeding in a Proposed Shear Model.

If cerebrospinal and interstitial fluids move through very narrow brain flow channels, these restrictive surroundings generate varying levels of fluid shear and different shear rates, and dissolved amyloid monomers absorb different shear energies. It is proposed that dissolved amyloid-β protein (Aβ) and other amyloid monomers undergo shear-induced conformational changes that ultimately lead to amyloid monomer aggregation even at very low brain flow and shear rates. Soluble Aβ oligomers taken from diseased brains initiate in vivo amyloid formation in non-diseased brains. The brain environment is apparently responsible for this result. A mechanism involving extensional shear is proposed for the formation of a seed Aβ monomer molecule that ultimately promotes templated conformational change of other Aβ molecules. Under non-quiescent, non-equilibrium conditions, gentle extensional shear within the brain parenchyma, and perhaps even during laboratory preparation of Aβ samples, may be sufficient to cause subtle conformational changes in these monomers. These result from brain processes that significantly lower the high activation energy predicted for the quiescent Aβ dimerization process. It is further suggested that changes in brain location and changes brought about by aging expose Aβ molecules to different shear rates, total shear, and types of shear, resulting in different conformational changes in these molecules. The consequences of such changes caused by variable shear energy are proposed to underlie formation of amyloid strains causing different amyloid diseases. Amyloid researchers are urged to undertake studies with amyloids, anti-amyloid drugs, and antibodies while all of these are under shear conditions similar to those in the brain.