Inside out: the role of nucleocytoplasmic transport in ALS and FTLD

Abstract

Neurodegenerative diseases are characterized by the presence of protein inclusions with a different protein content depending on the type of disease. Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are no exceptions to this common theme. In most ALS and FTLD cases, the predominant pathological species are RNA-binding proteins. Interestingly, these proteins are both depleted from their normal nuclear localization and aggregated in the cytoplasm. This key pathological feature has suggested a potential dual mechanism with both nuclear loss of function and cytoplasmic gain of function being at play. Yet, why and how this pathological cascade is initiated in most patients, and especially sporadic cases, is currently unresolved. Recent breakthroughs in C9orf72 ALS/FTLD disease models point at a pivotal role for the nuclear transport system in toxicity. To address whether defects in nuclear transport are indeed implicated in the disease, we reviewed two decades of ALS/FTLD literature and combined this with bioinformatic analyses. We find that both RNA-binding proteins and nuclear transport factors are key players in ALS/FTLD pathology. Moreover, our analyses suggest that disturbances in nucleocytoplasmic transport play a crucial initiating role in the disease, by bridging both nuclear loss and cytoplasmic gain of functions. These findings highlight this process as a novel and promising therapeutic target for ALS and FTLD.

Keywords: Aggregation; Exportin; Importin; Neurodegeneration; Nuclear pore; Nuclear transport; Ran-GTP cycle; TDP-43.

Fig. 1

RNA-binding proteins are strongly implicated in ALS/FTLD pathology. a Schematic overview of protein mislocalization and aggregation in ALS/FTLD, as determined in post-mortem brain or spinal cord (muscle for MSP data). Patients are divided into different categories based on their pathology: SOD1, TDP-43, FUS, mutant FUS, Tau or DPR. DPR pathology is concurrent with RNA foci and TDP pathology. ALS/FTLD disease genes are boxed. Proteins can be grouped according to their function: ‘RNA metabolism’, ‘Nuclear transport’, ‘Cytoskeleton’ and ‘Proteostasis’. Misregulated RNA-binding proteins can be further divided into three classes based on their misregulation: I (red), II (orange) and III (yellow). Asterisk denotes debated findings; hash denotes occurrence in several neurodegenerative diseases; double hash denotes occurrence in all protein aggregation diseases. AD Alzheimer’s disease, HD Huntington’s disease, MSP multisystem proteinopathy, SCA2 spinocerebellar ataxia type 2, aMND atypical motor neuron disease. b Network analysis using GeneMANIA indicates that all pathological proteins are highly interactive. Physical interactions are depicted in red, genetic in green, and colocalization in blue. Ubiquitin was not included in this network. c Word cloud depicting significantly overrepresented terms, as analyzed by ingenuity pathway analysis (IPA). Functional terms are green, disease and pathology terms black. Terms are scaled to the −log10(p value). References: TDP-43 [86], FUS [52, 87, 117], EWS [21, 87], TAF15 [22, 87], Matrin-3 [47], hnRNPA1 [42, 53, 88], hnRNPA2B1 [53], hnRNPA3 [81], TIA-1 [35, 70, 124], RBM45 [19], HuR [77], NONO [108], SMN [44, 122], PDCD7 [44], Coilin [44], Gemin-8 [122], PABP-1 [35, 75], ATX2 [29, 31], G3BP-1 [124], TTP [124], eIF3b [70], rpS6 [35], eIF4g [28], Drosha [96], XRN1 [125], Staufen-1 [125], C9orf72 [52, 133], Nup62 [58, 84], Nup88 [58], Nup107 [144], Nup153 [58], Nup205 [144], Importin α-1 [92], Importin β-1 [58, 84], Transportin-1 [91], Ran [126], Rangap-1 [133, 144], Tau [109, 137], NF-H [52, 131], β-actin [131], Peripherin [52, 131], KAP-3 [116], TTBK1 [66], TTBK2 [66], RGNEF [52], SOD1 [52, 128], Optineurin [16], RNF19A [41], Ubiquilin-2 [12], p62 [52], ubiquitin [101], PDI [6] and unc-119 [74]

Fig. 2

ALS/FTLD-related RNA-binding proteins undergo liquid–liquid phase transitions. a All RNA-binding proteins misregulated in ALS/FTLD are part of endogenous membrane-less organelles. b Several of these RNA-binding proteins are part of stress granules in vivo [7] or precipitate with β-isox in vitro [51] (green illustrates percent of total). c Scheme depicting RNA-binding protein phase transitions and the role of nuclear transport in this process. These phase transitions strongly depend on the local concentrations of the involved RNA-binding proteins. Disaggregases, such as VCP, and post-translational modifications (PTMs) also play a role in this process. Due to defects in the proper regulation of this process, e.g., aging and disease mutations, liquid-like stress granules can probably seed pathological aggregation

Fig. 3

Nuclear transport cargoes are strongly implicated in ALS/FTLD and related pathways. We analyzed transportin-1 and exportin-1 cargo lists, which were experimentally validated. a Scheme of experimental setup for determining transportin-1 cargoes [55]. b These cargo proteins form a dense interaction network. Two subnetworks, the ribosome and hnRNP family, are highlighted. Red depicts physical interactions, green genetic, and blue colocalization. c Word cloud of significantly overrepresented terms, as analyzed by ingenuity pathway analysis (IPA). Functional terms are black, and disease terms red. Terms are scaled to the −log10 (p value)

Fig. 4

Roadmap to neuronal death in ALS/FTLD. Hypothetical scheme showing occurrence of different pathogenic events in RNA-binding protein-related ALS/FTLD. Orange boxes depict natural processes associated with aging. Blue boxes show where disease mutants enhance pathological pathways. See Supplementary Data for disease gene functions. 1 Numerous disease mutations make RNA-binding proteins more aggregation-prone or affect proteins involved in proteostasis [68, 98]. 2 C9orf72 repeat expansions induce nucleocytoplasmic transport defects [8, 32, 48, 144]. 3 NLS mutants of both FUS [28] and Angiogenin (encoded by ANG) [93] have been reported, interfering with their proper nuclear targeting. 4 Mutations in GLE1 perturb RNA export [50], and mutant TDP-43 granules show altered axonal transport [2]. 5 3′UTR mutants of FUS [147] and TDP-43 [14] interfere with their autoregulation resulting in overexpression of these proteins. 6 TDP-43 is an important genetic regulator of nuclear transport [106, 126]. 7 Various protein aggregates are known to sequester transport factors [91, 120, 130]