ALS-Causing Mutations Significantly Perturb the Self-Assembly and Interaction with Nucleic Acid of the Intrinsically Disordered Prion-Like Domain of TDP-43

Abstract

TAR-DNA-binding protein-43 (TDP-43) C-terminus encodes a prion-like domain widely presented in RNA-binding proteins, which functions to form dynamic oligomers and also, amazingly, hosts most amyotrophic lateral sclerosis (ALS)-causing mutations. Here, as facilitated by our previous discovery, by circular dichroism (CD), fluorescence and nuclear magnetic resonance (NMR) spectroscopy, we have successfully determined conformations, dynamics, and self-associations of the full-length prion-like domains of the wild type and three ALS-causing mutants (A315E, Q331K, and M337V) in both aqueous solutions and membrane environments. The study decodes the following: (1) The TDP-43 prion-like domain is intrinsically disordered only with some nascent secondary structures in aqueous solutions, but owns the capacity to assemble into dynamic oligomers rich in β-sheet structures. By contrast, despite having highly similar conformations, three mutants gained the ability to form amyloid oligomers. The wild type and three mutants all formed amyloid fibrils after incubation as imaged by electron microscopy. (2) The interaction with nucleic acid enhances the self-assembly for the wild type but triggers quick aggregation for three mutants. (3) A membrane-interacting subdomain has been identified over residues Met311-Gln343 indispensable for TDP-43 neurotoxicity, which transforms into a well-folded Ω-loop-helix structure in membrane environments. Furthermore, despite having very similar membrane-embedded conformations, three mutants will undergo further self-association in the membrane environment. Our study implies that the TDP-43 prion-like domain appears to have an energy landscape, which allows the assembly of the wild-type sequence into dynamic oligomers only under very limited condition sets, and ALS-causing point mutations are sufficient to remodel it to more favor the amyloid formation or irreversible aggregation, thus supporting the emerging view that the pathologic aggregation may occur via the exaggeration of functionally important assemblies. Furthermore, the coupled capacity of TDP-43 in aggregation and membrane interaction may critically account for its high neurotoxicity, and therefore its decoupling may represent a promising therapeutic strategy to treat TDP-43 causing neurodegenerative diseases.

The role of ubiquitinated TDP-43 in amyotrophic lateral sclerosis

“…The TDP-43 prionlike domain appears to have an energy landscape, which allows the assembly of the wild-type sequence into dynamic oligomers only under very limited conditions. ALS-causing point mutations are sufficient to remodel it into a more favorable formation of amyloid and its irreversible aggregation, thus supporting the emerging view that such pathologic aggregation may occur via the exaggeration of functionally important assemblies [24] . TDP-43 oligomers may further delay the release from each other [11] , resulting in the TDP-43 oligomerization in the nucleus, which is a possible mechanism of disruption of TDP-43 [24] .…”

“…ALS-causing point mutations are sufficient to remodel it into a more favorable formation of amyloid and its irreversible aggregation, thus supporting the emerging view that such pathologic aggregation may occur via the exaggeration of functionally important assemblies [24] . TDP-43 oligomers may further delay the release from each other [11] , resulting in the TDP-43 oligomerization in the nucleus, which is a possible mechanism of disruption of TDP-43 [24] . Aging or inhibition of protein degradation may increase the toxicity of TDP-43 in glial cells and cause neuropathological changes.…”

“…The assembly of the wild-type sequence into dynamic oligomers was only seen under very limited conditions; ALS-causing point mutations are sufficient to remodel it to favor the amyloid formation or irreversible aggregation, thus supporting the emerging view that pathologic aggregation may occur via the exaggeration of functionally important assemblies [24] . Furthermore, the coupled capacity of TDP-43 in aggregation and membrane interaction may critically account for its high neurotoxicity [27] .…”

“…This prion domain is present in most of the pathogenic mutations of TDP-43 [23,24] . The pathological C terminus is intrinsically disordered only with some nascent secondary structures in aqueous solutions, but processes the capacity to assemble into dynamic oligomers rich in b-sheet structures.…”

“…In addition, the pathogenic mutations caused by tardbp are concentrated in the glycine enriched region of the C terminus [23,26] . So far, over 60 mutations in tardbp have been found to cause fALS and FTD [24] ; they are listed in Table 1 as extracted from the ClinVar database. Tardbp mutations in the nucleus might disrupt the formation of alpha helices, or their ability to stabilize [11] .…”