Nanoscale morphological and structural analysis of round and donut oligomers formed by C-terminal domain of TDP-43.

Amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), Alzheimer's disease (AD), limbic predominant age-related TDP-43 encephalopathy (LATE), and Parkinson's disease are associated with an abrupt aggregation of TAR DNA-binding protein 43 (TDP-43). Although molecular mechanisms of this pathological aggregation remain unclear, accumulated evidence suggests that the C-terminus domain (C-terminal domain (CTD)) is the trigger of TDP-43 self-assembly into toxic oligomers and fibrils. While the secondary structure and morphology of protein fibrils have been well documented, very little is known about TDP-43 oligomers. This is primarily because of the transient nature and low concentrations of these protein species. In the current study, we utilize nano-infrared spectroscopy, also known as atomic force microscopy-infrared (AFM-IR) spectroscopy, to investigate the morphology and secondary structure of CTD of TDP-43 oligomers formed at the early and middle stages of protein aggregation. This innovative technique allows us to resolve both morphology and secondary structure of individual protein aggregates. We found that at the early stage of protein aggregation, CTD of TDP-43 formed two morphologically different protein aggregates: donut-like (DO) and round (RO) oligomers. DO yielded fibrillar species, while RO persisted throughout the entire course of CTD TDP-43 self-assembly.