Amyloid

Amyloids are aggregates of proteins characterised by a fibrillar morphology of typically 7–13 nm in diameter, a β-sheet secondary structure (known as cross-β) and ability to be stained by particular dyes, such as Congo red.^[1] In the human body, amyloids have been linked to the development of various diseases.^[2] Pathogenic amyloids form when previously healthy proteins lose their normal structure and physiological functions (misfolding) and form fibrous deposits within and around cells. These protein misfolding and deposition processes disrupt the healthy function of tissues and organs.

Such amyloids have been associated with (but not necessarily as the cause of) more than 50^[2][3] human diseases, known as amyloidosis, and may play a role in some neurodegenerative diseases.^[2][4] Some of these diseases are mainly sporadic and only a few cases are familial. Others are only familial. Some result from medical treatment. Prions are an infectious form of amyloids that can act as a template to convert other non-infectious forms.^[5] Amyloids may also have normal biological functions; for example, in the formation of fimbriae in some genera of bacteria, transmission of epigenetic traits in fungi, as well as pigment deposition and hormone release in humans.^[6]

Amyloids have been known to arise from many different proteins.^[2][7] These polypeptide chains generally form β-sheet structures that aggregate into long fibers; however, identical polypeptides can fold into multiple distinct amyloid conformations.^[8] The diversity of the conformations may have led to different forms of the prion diseases.^[6]

An unusual secondary structure named α sheet has been proposed as the toxic constituent of amyloid precursor proteins,^[9] but this idea is not widely accepted at present.