Formation of Apoptosis‐Inducing Amyloid Fibrils by Tryptophan

Abstract

Many major degenerative disorders are associated with the formation of amyloid fibrils by proteins and peptides. Recent studies have extended the repertoire of amyloidogenic building blocks to non‐proteinaceous entities, including amino acids and nucleobases. Here, based on the high propensity of tryptophan‐containing proteins and peptides to form amyloid fibrils, we explored the self‐assembly profile of this amino acid. We discovered that tryptophan forms fibrillary assemblies with a diameter of 15–75 nm. These fibrils bind the thioflavin T amyloid‐specific dye and show a typical spectrum of amyloid proteins upon binding. Furthermore, the assemblies show significant cytotoxicity triggered by an apoptosis mechanism, similar to that known for amyloids. As a control, the non‐amyloidogenic amino acid alanine was used under the same conditions and did not show any toxicity. Molecular dynamics simulations were used to explore the possible growth mechanism, molecular organization, and stability of tryptophan amyloidal fibrils. Taken together, we provide further extension to the amyloid hypothesis and additional indication for a knwon mechanism of toxicity for both amyloid‐associated and metabolic disorders.

Amino Acid Based Self‐assembled Nanostructures: Complex Structures from Remarkably Simple Building Blocks

“…[14d] As in the case of Phe fibrils,[28] 18C6 inhibited the fiber formation of Gly by generating flake-like morphology (Figure 5C, D), whereas 15C5 showed no effect. Gly fibers are related with numerous genetic disorders, like nonketotic hyperglycinemia, D-glyceria aciduria, and iminoglycinuria, [14b] emphasizing the significance of these findings.…”

“…[14a] At a concentration of 4.0–5.5 mM, fibers with a cross-section of a few microns were observed, whereas a decrease in concentration (~ 2.8 mM) provided nanoribbon strip-like nanostructures. At a further lower concentration (0.8 mM), small particles appeared at the end of the fibers.…”

“…[14b] These fibers also exhibited cytotoxicity and induced apoptotic cell death. This work further extended the hypotheses of amyloid formation by building blocks simpler than proteins and peptides.…”

“…(A–C) TEM micrographs of Tyr assemblies at ~ 4 mM at various time points: (A) 6 days, (B) 14 days, (C) 22 days. [14a] (D–F) Amyloid nature of Tyr fibers. (D) TEM image (scale bar 500 nm), (E) ThT microscopy (scale bar 20 μm) and (F) fluorescence spectrum of Tyr fibers.…”

“…[13] Follow-up studies had confirmed the ability of other amino acids, as well as additional metabolites, to form ordered supramolecular fibrillar assemblies. [14] X-ray crystallography of Phe in its zwitterionic state, under conditions allowing its fibrillization, confirmed the organization of the amino acid into an ordered β-sheet-like layered organization, which was stabilized by a network of hydrogen bonds and aromatic interactions. [15]…”