The dual functions of the extreme N-terminus of TDP-43 in regulating its biological activity and inclusion formation

Abstract

TAR DNA-binding protein-43 (TDP-43) is the principal component of ubiquitinated inclusions in amyotrophic lateral sclerosis (ALS) and the most common pathological subtype of frontotemporal dementia—frontotemporal lobar degeneration with TDP-43-positive inclusions (FTLD-TDP). To date, the C-terminus of TDP-43, which is aggregation-prone and contains almost all ALS-associated mutations, has garnered much attention while the functions of the N-terminus of TDP-43 remain largely unknown. To bridge this gap in our knowledge, we utilized novel cell culture and computer-assisted models to evaluate which region(s) of TDP-43 regulate its folding, self-interaction, biological activity and aggregation. We determined that the extreme N-terminus of TDP-43, specifically the first 10 residues, regulates folding of TDP-43 monomers necessary for proper homodimerization and TDP-43-regulated splicing. Despite such beneficial functions, we discovered an interesting dichotomy: full-length TDP-43 aggregation, which is believed to be a pathogenic process, also requires the extreme N-terminus of TDP-43. As such, we provide new insight into the structural basis for TDP-43 function and aggregation, and we suggest that stabilization of TDP-43 homodimers, the physiologically active form of TDP-43, may be a promising therapeutic strategy for ALS and FTLD-TDP.

TDP-43 N terminus encodes a novel ubiquitin-like fold and its unfolded form in equilibrium that can be shifted by binding to ssDNA

“…Mysteriously, the TDP-43 N terminus appears to be not only indispensable for its splicing activity, but also for its ability in forming pathological inclusions associated with impaired neurite outgrowth (15). Now our results provide a molecular mechanism for this unique dichotomy (Fig.…”

“…2E). In the structure, the first nine residues, which were previously shown to be critical for normal functions and formation of TDP-43 inclusion (15), adopt the first β-strand to have extensive contacts with β-strands 2 and 5 (Fig. 2B).…”

“…S3B). Further analysis of both 15 N-and 13 C-edited NOESY spectra indicated that only several d αN(i, i+2) but no long-range nuclear Overhauser effects (NOEs) could be found for the unfolded form (Fig. S2A).…”

“…However, previous research attention has mostly been directed to the recognized domains including two RRMs and prion-like domains. Only a very recent study defined the TDP-43 extreme N terminus to be a central player in both physiology and pathogenesis (15). Unfortunately despite intense efforts, the structure of the TDP-43 N terminus remains unknown due to its severe aggregation (16).…”

“…Only very recently has it been identified that the extreme N terminus (residues 1-75) functions as a doubleedged sword: it not only regulates normal TDP-43 functions, but also drives neurodegeneration in TDP-43 proteinopathies (15). More specifically, the deletion of the N terminus, or even the first nine residues, is sufficient to abolish the TDP-43-regulated RNA splicing, as well as the aggregation of the full-length TDP-43, which is required for impaired neurite outgrowth (15). In this regard, the elucidation of the structure of the TDP-43 N terminus would provide valuable insights into the molecular mechanism underlying this intriguing dichotomy.…”