Functional amyloids signal their arrival

Abstract

Amyloids have traditionally been associated with misfolded protein aggregates and debilitating neurodegenerative diseases. However, a growing number of functional amyloids have now been described that demonstrate that amyloid formation can be an integral part of normal cellular physiology. Functional amyloid production is highly regulated, and the resulting fibers serve a variety of roles for the cells that produce them. A new role for amyloid as storage reservoirs for peptide hormones within mammalian secretory granules has been discovered. More than 30 different peptide hormones have been found to form amyloids in vitro, and both rats and mice have been shown to store hormone amyloid deposits in secretory granules. Thus, the emerging evidence adds to the diverse roles of amyloid and raises intriguing questions for both the peptide hormone and the functional amyloid fields.

Fig. 1

Amyloid storage of peptide hormones. Maji et al. found that 31 out of 42 peptide hormones fold into an amyloid configuration in vitro. From an in vivo perspective, secretory granules purified from AtT20 cells and rat pituitary contained peptide hormones in an amyloid-like structure. Moreover, immunostaining of mouse pituitary with several peptide hormones found that the peptide signal colocalized with the amyloid-specific dye thioflavin-S. Their results suggest a model where peptide hormones are stored in the secretory granules as amyloid fibers. Some peptide hormones form amyloid fibrils spontaneously (A), whereas other peptides form amyloid fibrils when coincubated with another peptide hormone (B) or with GAGs (C). Upon hormone release, the amyloid fibers are broken down by an unknown mechanism to soluble peptides, which are then secreted.