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.

Nuclear import receptors are recruited by FG-nucleoporins to rescue hallmarks of TDP-43 proteinopathy

“…9a-c,f-h). While we and others speculated that FG-Nups might associate with the FG-like repeats that are present in the TDP-43 PrLD [71], here we show that TDP-CTF variants where phenylalanine residues in the PrLD were replaced with glycine, alanine, or tyrosine residues still co-aggregate with Nup62 (Supplementary Fig. 9a).…”

“…1). While a recent study reported that binding of the importin-α/KPNB1 complex to the NLS prevents TDP-43 dimerization as the foundation of disaggregation, this in vitro structural analysis was limited to a recombinant soluble fragment of TDP-43 spanning residues aa1-177 that lacked the C-terminal PrLD, which was identified in the present study as the key domain for KPNB1-mediated disaggregation in the context of living cells [71,72]. The existence of a potential additional NLS-independent mechanism to suppress phase transition of RBPs is also supported by the observation that under certain conditions TNPO1 can dissolve truncated FUS lacking the PY-NLS [19,73] and that other NIRs, including KPNB1, can bind the RGG motifs of FUS and reduce its fibrillization [74].…”

Nuclear-import receptors as gatekeepers of pathological phase transitions in ALS/FTD

“…By contrast, importin-α/β1 complex was ineffective against fibrils formed by TDP-43 variants lacking the classical NLS and failed to prevent and reverse the self-assembly of proteins containing a PY-NLS [ 40 ]. The interaction of importin-α/β1 with TDP-43 sterically interferes with NTD-mediated TDP-43 oligomerization, which may contribute to the observed inhibition of TDP-43 self-assembly in presence of the importin-α/β1 heterodimer [ 13 , 38 ]. Similar to FUS, introducing an acetylation mimic in the NLS (K84Q) increases TDP-43 cytoplasmic mislocalization, presumably by disrupting interactions with importin-α-associated KPNB1.…”

Looking for answers far away from the soma—the (un)known axonal functions of TDP-43, and their contribution to early NMJ disruption in ALS

“…One hypothesis is that key proteins of the nuclear pore complex, Importin-α and Importin-β, bind the TDP-43 nuclear-localization-signal and prevent most of its cytoplasmic localization [ 39 ]. Interestingly, recent discoveries indicate that phosphorylation of TDP-43 in proximity to its nuclear-localization-signal prevents binding of Importins [ 39 ] and therefore can promote the cytoplasmic accumulation of the protein, thus suggesting that TDP-43 phosphorylation might be a key process that facilitates redistribution of the protein from the nucleus to the cytoplasm [ 40 ]. Additionally, a recent article by Khalil et al, indicates a new NLS-independent mechanism for TDP-43 cytoplasmic mislocalization, where the activity of karyopherin-β1 (KPNB1/Importin-β), increase the solubility of TDP-43 upon their recruitment to nucleoporin rich pTDP-43 condensates.…”

“…Alternatively, a recent report by Patel et al, has identified that the mRNAs of several RNA-binding proteins, such as Khsrp, are localized to peripheral nerve axons and are locally translated in response to injury [ 57 ]. TDP-43 is known to localize into axons and to carry mRNAs, including its own [ 39 , 40 , 53 ]. Additionally, detailed analysis of axonal transcriptome data from motor neurons revealed that TDP-43 mRNA is abundant in axons [ 58 ].…”