23 June 2009. Though it remains a prime pathological hallmark of Alzheimer disease, Aβ amyloid has yet to convincingly prove itself as the major toxic species driving AD. Until such evidence emerges, some scientists have pondered the alternative theory that amyloids might help more than harm. A Science paper published online this week puts that provocative concept on firmer footing. Led by Roland Riek, formerly at the Salk Institute for Biological Studies in La Jolla, California, and now at the Swiss Federal Institute of Technology in Zurich, researchers report that endocrine hormones form amyloids when packaged in secretory granules. “The important thing they reported is that biology uses the amyloid structure for a purpose,” Jeff Kelly, Scripps Research Institute, San Diego, told ARF.
Several years ago, Kelly’s group published the first evidence of functional amyloid in mammals. The researchers found that the Pmel17 protein organizes into amyloid-like cross β-sheet structures within melanosomes and, as such, seemed to accelerate melanin production by serving as a scaffold for polymerization of its derivatives (Fowler et al., 2005 and ARF related news story; see also review, Fowler et al., 2007). Scientists have also identified biological roles for amyloids in bacteria, silkworms, and fungi. The current study adds to this list with evidence of physiological amyloid in the mammalian endocrine system.
Riek and colleagues came to this discovery through the back door, turning a nagging technical problem into productive inquiry. Years ago, the researchers were studying the structure of a stress hormone receptor, but their attempts at getting a nice complex kept getting foiled because the hormone formed amyloids in vitro. When they tried to identify the region responsible for the aggregation, they found something curious: the region was involved in neither binding to the receptor nor activating it. “We knew that nature tries to get rid of aggregation-prone regions, yet here was an example where nature didn’t do that,” Riek told ARF. “So we turned around and said maybe this amyloid has a function, because nature didn’t evolve away from it.”
Riek, lead author Samir Maji, who is now at the School of Bioscience and Bioengineering in Mumbai, India, and colleagues reasoned that it might make sense for certain hormones to form amyloid-like aggregates when packed at high concentrations within secretory granules. To address this idea, they tested whether peptide and protein hormones from numerous species and organs, and with varied three-dimensional structures, could form amyloids in vitro. In an initial experiment, 10 of 42 hormones showed this capability, determined using amyloid-specific dyes and several spectroscopic and microscopy techniques. The fraction of amyloidogenic hormones shot up when low-molecular-weight heparin was added to the reaction in lieu of glycosaminoglycans, which are believed to be necessary for formation of secretory granules and amyloid fibrils. In the presence of heparin, 31 of 42 hormones formed amyloid fibrils in vitro.
One feature that distinguished the hormone amyloids in this study from Aβ amyloids found in AD brains is that the former release monomeric hormone upon dilution. Aβ amyloids, on the other hand, are much more rigid and would not dissociate as monomers under the same conditions. Further tests showed that the released hormone monomers retained proper structure and function, consistent with the expected properties of secretory granules.
The researchers found that some hormone amyloids were toxic in the MTT (3-[4,5-dimethyl-2-thiazolyl]-2,5-diphenyl-2H-tetrazolium bromide) assay, though not as potent as Aβ1-42 fibrils. To Kelly, this finding was “largely irrelevant because hormone amyloids are packaged into secretory vesicles, and they never see the inside of a cell.”
Do the new findings have any relevance to the amyloid hypothesis of AD, which presumes that amyloid deposits drive pathogenesis? Hard to say, Kelly told ARF. “My feeling is that these hormones have not only evolved to make secretory structures, they’ve also evolved to come apart—in contrast to pathologic amyloid,” he said. “The key is, biology uses controlled endoproteolysis and vesicular compartmentalization most likely to control the untoward effects of making cross β-sheet structures.”—Esther Landhuis.
Maji SK, Perrin MH, Sawaya MR, Jessberger S, Vadodaria K, Rissman RA, Singru PS, Nilsson KPR, Simon R, Schubert D, Eisenberg D, Rivier J, Sawchenko P, Vale W, Riek R. Functional Amyloids as Natural Storage of Peptide Hormones in Pituitary Secretory Granules. 18 June 2009. Science. Abstract