Extracellular chaperones and amyloids

The pathology of more than 40 human degenerative diseases is associated with fibrillar proteinaceous deposits called amyloid. Collectively referred to as protein deposition diseases, many of these affect the brain and the central nervous system. In many cases the amyloid deposits are extracellular and are found associated with newly identified abundant extracellular chaperones (ECs). Evidence is discussed which suggests an important regulatory role for ECs in amyloid formation and disposal in vivo. This is emerging as an exciting field. A model is presented in which it is proposed that, under normal conditions, ECs stabilize extracellular misfolded proteins by binding to them, and then guide them to specific receptors for uptake and subsequent degradation. In this scenario, EC receptors are a critical part of a quality control system which protects the brain against dangerously hydrophobic proteins/peptides. However, it also appears possible that in the presence of a high molar excess of misfolded protein, such as might occur during disease, the limited amounts of ECs available may actually exacerabate pathology. Further advances in understanding of the mechanisms that control extracellular protein folding are likely to identify new strategies for effective disease therapies.