Cryo-EM Structures of Four Polymorphic TDP-43 Amyloid Cores

Summary ParagraphTDP-43 is an essential DNA/RNA processing protein that undergoes both functional and pathogenic aggregation. Functional TDP-43 aggregates are reversible, forming transient species such as nuclear bodies, stress granules, and myo-granules1-3. In contrast pathogenic TDP-43 aggregates are irreversible, forming stable intracellular amyloid-like inclusions4,5. These inclusions are the primary pathology of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with TDP-43 inclusions (FTLD-TDP)6. Disease-associated, hereditary mutations in TDP-43 are known to accelerate the deposition of irreversible aggregates in the cytoplasm7. Reversible TDP-43 aggregation has been shown to precede the formation of irreversible amyloid fibrils similar to the behavior of proteins hnRNPA1 and FUS8-10. Still unknown, however, are the structural features of TDP-43 fibrils that confer both reversibility and irreversibility and how hereditary mutations can impose irreversible aggregation. Here, we determined the structures of amyloid fibrils formed by two segments previously reported to be the pathogenic cores of TDP-43 aggregation7,11,12; these are termed SegA (residues 311-360) and SegB A315E (residues 286-331 containing the ALS hereditary mutation A315E). SegA forms three polymorphs, all with dagger-shaped folds. SegB forms R-shaped folds. All four polymorphs have folds confined to two dimensions, and are stabilized by hydrophobic cores and peripheral hydrogen bonds. Energetic analysis suggests that the dagger-shaped polymorphs are examples of the irreversible fibril structures of TDP-43, whereas the SegB polymorph may participate in both reversible and irreversible fibril structure. Our structure suggests how the A315E mutation may convert this polymorph to the irreversible type and lead to mutation-enhanced pathology.