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Transthyretin (TTR) is an amyloidogenic protein associated with hereditary and nonhereditary transthyretin amyloidoses (ATTR). Dissociation of the tetramer of TTR to the monomer induces TTR misfolding, which leads to amyloid fibril formation and triggers the onset of ATTR amyloidosis. Stabilizers of tetrameric TTR have been accepted as an effective ATTR amyloidosis treatment while effect is limited and they are too expensive. The aim of our study was to find more effective and cheep natural compound to suppress TTR amyloid formation. Glabridin, a prenylated isoflavan isolated from Glycyrrhiza glabra L., stabilized the TTR tetramer in vitro. The effects of licorice-derived flavonoid oil—Glavonoid, a natural substance that includes glabridin and several polyphenols—on stabilizing the TTR tetramer must still be elucidated. To examine plasma TTR stabilization by Glavonoid in vitro, we investigated the feasibility of utilizing glabridin plus Glavonoid to prevent TTR amyloid fibril formation. Glavonoid mixed with human plasma samples at 24 h incubation in vitro increased the tetramer level (P < 0.05) and reduced the monomer level (P < 0.01) and the monomer/tetramer ratio (P < 0.05) of TTR compared to those without Glavonoid by immunoblot analysis, such effect could not observe in the presence of glabridin. Oral Glavonoid (300 mg for 12 weeks) in 7 healthy volunteers effectively increased the plasma glabridin concentration. Glavonoid increased the TTR tetramer level and reduced the monomer/tetramer ratio of TTR (P < 0.05) in plasma at 12 weeks in healthy volunteers compared to those of age matched control subjects without the supplement. Thus, oral Glavonoid may effectively prevent TTR amyloid fibril formation via TTR tetramer stabilization. Glavonoid may become a promising supplement before onset of ATTR amyloidosis.

In the future, humans may live in space because of global pollution and weather fluctuations. In microgravity, convection does not occur, which may change the amyloidogenicity of proteins. However, the effect of gravity on amyloid fibril formation is unclear and remains to be elucidated. Here, we analyzed the effect of microgravity on amyloid fibril formation of amyloidogenic proteins including insulin, amyloid β42 (Aβ42), and transthyretin (TTR). We produced microgravity (10−3 g) by using the gravity controller Gravite. Human insulin, Aβ42, and human wild-type TTR (TTRwt) were incubated at pH 3.0, 7.0, and 3.5 at 37 °C, respectively, in 1 g on the ground or in microgravity. We measured amyloidogenicity via the thioflavin T (ThT) method and cell-based 1-fluoro-2,5-bis[(E)-3-carboxy-4-hydroxystyryl]benzene (FSB) assay. ThT fluorescence intensity and cell-based FSB assay results for human insulin samples were decreased in microgravity compared with results in 1 g. Aβ42 samples did not differ in ThT fluorescence intensity in microgravity and in 1 g on the ground. However, in the cell-based FSB assay, the staining intensity was reduced in microgravity compared with that on 1 g. Human TTRwt tended to form fewer amyloid fibrils in ThT fluorescence intensity and cell-based FSB assays in microgravity than in 1 g. Human insulin and Aβ42 showed decreased amyloid fibril formation in microgravity compared with that in 1 g. Human TTRwt tended to form fewer amyloid fibrils in microgravity. Our experiments suggest that the earth's gravity may be an accelerating factor for amyloid fibril formation.