Importin α/β and the tug of war to keep TDP-43 in solution: quo vadis?

Abstract

The transactivation response-DNA binding protein of 43 kDa (TDP-43) is an aggregation-prone nucleic acid-binding protein linked to the etiology of Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Lobar Degeneration (FTLD). These conditions feature the accumulation of insoluble TDP-43 aggregates in the neuronal cytoplasm that lead to cell death. The dynamics between cytoplasmic and nuclear TDP-43 are altered in the disease state where TDP-43 mislocalizes to the cytoplasm, disrupting Nuclear Pore Complexes (NPCs), and ultimately forming large fibrils stabilized by the C-terminal prion-like domain. Here, we review three emerging and poorly understood aspects of TDP-43 biology linked to its aggregation. First, how post-translational modifications in the proximity of TDP-43 N-terminal domain (NTD) promote aggregation. Second, how TDP-43 engages FG-nucleoporins in the NPC, disrupting the pore permeability and function. Third, how the importin α/β heterodimer prevents TDP-43 aggregation, serving both as a nuclear import transporter and a cytoplasmic chaperone.

Keywords: FG-nucleoporins; NTD; TDP-43; neurodegeneration; protein aggregation; importin α/β.

Figure 1.. TDP-43 topology and domain structure.

(A) Schematic diagram of full-length (FL) TDP-43 showing the NLS sequence, with key basic residues in red. (B) Cartoon representation of the TDP-43 domains, informed by the SAXS structure of the detergent-solubilized FL-TDP-43 ^[10] and published structures of each domain, namely, PDBs: 6T4B (NTD), 7N9H (NLS), RRM1 (4IUF), RRM2 (3D2W), 7PY2 (CTD). TDP-43 is shown as a dimer, with one protomer color-coded as in panel A and the second protomer in gray. The CTD is shown in the double-spiral-shaped fold described by Arseni et al. (PDB: 7PY2). (C) Zoom-in of the dimeric NTD with two protomers engaged in a head-over-tail dimerization interface (in magenta and red, respectively). (D) Magnified view of TDP-43 NLS (PDB: 7N9H).

Figure 2.. The FG Repeats of the TDP-43 CTD.

(A) Amino acid sequence of TDP-43 with the CTD colored in red. Underlined is the CTD region visualized by Arseni et al. (PDB: 7PY2). All FG- and FG-like repeats are highlighted in yellow. (B) Structure of the aggregated TDP-43 CTD, residues 282–360 (PDB: 7PY2). All FG-like repeats are shown as yellow spheres.

Figure 3.. TDP-43 minor NLS site is a hot spot.

(A) A model of ΔIBB-Imp α1 bound to TDP-43 NTD-NLS-RRM1 ^[7]. (B) Zoom-in view of the TDP-43 NLS that illustrates all PTMs (left) and point mutations (right) that promote loss of importins-binding and aggregation.

Figure 4.. Role of importins in TDP-43 aggregation pathway.

(A) Monomeric TDP-43 forms a dimer at low concentrations that regulates pre-mRNA splicing within the nucleus. The dimerization interface allows for ‘head-over-tail’ stacking of TDP-43 monomers. (B) Imp α1, and potentially other isoforms of importin α ^[56,61], part of the Imp α/β heterodimer functions like a prybar binding the NLS with high affinity and disrupting NTD dimerization and possibly higher order assemblies. (C) Under the pressure of PTMs, mutations, TDP-43 phase separates, CTDs come together to generate a double spiral-shaped fiber (PDB: 7PY2) that leads over time to (D) amyloid-like filaments (reproduced from ^[62] with permission).