The amyloidogenic core in the TAR DNA-binding protein (TDP-43) C-terminal fragment has been characterized with its chemical, biochemical, and structural properties delineated. Various properties of the core sequence, including membrane impairment ability and the seeding effect, have also been studied.

TAR DNA-binding protein (TDP-43) was identified as the major ubiquitinated component deposited in the inclusion bodies in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U) in 2006. Later on, numerous ALS-related mutations were found in either the glycine or glutamine/asparagine-rich region on the TDP-43 C-terminus, which hinted on the importance of mutations on the disease pathogenesis. However, how the structural conversion was influenced by the mutations and the biological significance of these peptides remains unclear. In this work, various peptides bearing pathogenic or de novo designed mutations were synthesized and displayed their ability to form twisted amyloid fibers, cause liposome leakage, and mediate cellular toxicity as confirmed by transmission electron microscopy (TEM), circular dichroism (CD), Thioflavin T (ThT) assay, Raman spectroscopy, calcein leakage assay, and cell viability assay. We have also shown that replacing glycines with prolines, known to obstruct β-sheet formation, at the different positions in these peptides may influence the amyloidogenesis process and neurotoxicity. In these cases, GGG308PPP mutant was not able to form beta-amyloid, cause liposome leakage, nor jeopardized cell survival, which hinted on the importance of the glycines (308–310) during amyloidogenesis.

We report the incorporation of salicylaldehyde derivatives onto the hydrazine-tagged amyloidogenic peptides by forming photoisomerizable hydrazones. These hydrazones with positive photochromism are photostable under physiological conditions and enable photoswitching without the addition of external reductants or high-power irradiation. By applying superresolution microscopy, we were able to distinguish polymorphic nanoscopic structures of the hydrazone-incorporated peptides in vitro under different buffer conditions. Moreover, the additive-free condition in our platform allows the exploration of detailed amyloid aggregate morphologies in live cells.