Bitetto and Di Fonzo Translational Neurodegeneration (2020) 9:25 https://doi.org/10.1186/s40035-020-00205-2 REVIEW Open Access Nucleo–cytoplasmic transport defects and protein aggregates in neurodegeneration Giacomo Bitetto and Alessio Di Fonzo* Abstract In the ongoing process of uncovering molecular abnormalities in neurodegenerative diseases characterized by toxic protein aggregates, nucleo-cytoplasmic transport defects have an emerging role. Several pieces of evidence suggest a link between neuronal protein inclusions and nuclear pore complex (NPC) damage. These processes lead to oxidative stress, inefficient transcription, and aberrant DNA/RNA maintenance. The clinical and neuropathological spectrum of NPC defects is broad, ranging from physiological aging to a suite of neurodegenerative diseases. A better understanding of the shared pathways among these conditions may represent a significant step toward dissecting their underlying molecular mechanisms, opening the way to a real possibility of identifying common therapeutic targets. Keywords: Aging, Neurodegeneration, Neurodegenerative disease, Nucleo–cytoplasmic transport, Nuclear pore complex, Protein aggregate Background [5]. One essential NCT role is to activate regulatory ele- Neuronal protein aggregates are a characteristic neuro- ments necessary for adaptation and response to external pathological hallmark of neurodegenerative disorders stimuli, especially those implicated in neuronal plasticity such as amyotrophic lateral sclerosis (ALS), frontotem- [6]. Moreover, in non-dividing post-mitotic cells such as poral dementia (FTD), Alzheimer’s disease (AD), Hun- neurons, proteins involved in DNA maintenance and re- tington’s disease (HD), polyglutamine expansion related pair reach the nucleus through this mechanism [7, 8]. In ataxias, and Parkinson’s disease (PD) [1]. One possible addition, all ribonucleic proteins involved in processes of consequence of these aggregates is interference with the RNA maturation, stability, splicing, and export require proper functioning of the highly conserved nucleo–cyto- this transport for localization to the nucleus [9]. plasmic transport (NCT) mechanism in the cell, which The cell relies on an essential, conserved, and dynamic ensures the transport of nucleic acids and proteins structure called Nuclear Pore Complex (NPC) to accom- across the nuclear membrane. This pathway is funda- plish a correct NCT [5]. Small molecules (molecular mental for long-lived cells such as neurons [2, 3], and weight < 40 kDa, diameter < 5 nm) can relatively freely dif- emerging evidence links damage to the nuclear mem- fuse through the NPC [5]. In contrast, higher molecular- brane and nuclear pores to neurodegenerative diseases weight proteins depend on the highly regulated and very and physiological neuronal aging [4]. specific mechanisms of the NCT machinery [5]. A properly functional NCT is fundamental for neurons NPCs are large cellular structures that span the nu- to allow transcription factors to enter into the nucleus clear envelope [10] and control exchanges between the cytoplasm and nucleus. The NPC is constituted by pro- * Correspondence: [email protected] teins collectively referred to as nucleoporins (Nups) [11] IRCCS Foundation Ca’ Granda Ospedale Maggiore Policlinico, Dino Ferrari Center, Neuroscience Section, Department of Pathophysiology and that have different roles in regulating NPC functions Transplantation, University of Milan, Via Francesco Sforza 35, 20122 Milan, and transport of matter, energy, and information Italy © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Bitetto and Di Fonzo Translational Neurodegeneration (2020) 9:25 Page 2 of 16 between the nucleus and cytoplasm [12]. In addition, the associated with oxidative stress: reduced Ran GDP/GTP NPC is involved in cell cycle regulation, chromatin ratio, mislocalization of Nups, altered functions (binding, organization, and gene activation [6, 13]. docking) of importins/exportins, and decreased integrity Despite the relatively large size of NPCs, most proteins of the nuclear membrane [28]. and RNAs cannot freely diffuse through them. Indeed, RNA transport defects are an additional sign of neur- the passage through NPC is a highly coordinated and se- onal sensitivity to NCT impairment, and of course, the lective process, mediated by a family of soluble receptors export of RNAs from the nucleus to the cytoplasm is called karyopherins (Kaps), also known as importins/ key to gene expression [29]. Moreover, a tRNA retro- exportins [14]. The small Ras-related nuclear protein grade transport between cytoplasm and nucleus has re- (Ran) plays a central role in this transport and regulates cently been proposed as part of the cellular response to interactions between Kaps and their cargos on either oxidative stress [30]. Among neurons, proper RNA shut- side of the NPC [15]. Ran is the only known member of tling function appears even more relevant for cells with the Ras superfamily of small GTP-binding proteins that a high transcriptional activity such as Purkinje cells and is localized principally inside the nucleus [16, 17]. A gra- motor neurons [31–33]. All of these NCT defects that dient of RanGDP/GTP across the nuclear membrane is significantly affect DNA and RNA are related to aging essential in establishing the directionality of the trans- and are even more relevant in neurodegeneration [26]. port. To maintain the gradient, RanGTP is hydrolyzed to RNA-binding proteins mutated in specific neurodegen- RanGDP on the cytoplasmic side and RanGDP is con- erative disorders (TDP-43, FUS, and hnRNPA1) can verted to RanGTP in the nucleus [17]. alter the dynamics of membrane-less organelles such as The nuclei of human cells have several thousand NPCs, stress granules, or the more recently identified liquid with a structure that seems to have been highly conserved, droplets, and accelerate fibrillization in neurons, result- underlining their importance for cells [18]. Nevertheless, ing in the formation of pathological amyloid-like fibrils despite this high degree of structural conservation of the that deposit in the cell bodies and neuropil [34–36]. NPC complex, two crucial time points in evolution from Neurodegenerative disorders display a unique patho- prokaryotes to humans have been suggested in NPC de- logical hallmark arising from specific gene mutations. velopment, implying that the nuclear pore is still highly This hallmark is protein aggregates, which are known to adaptive and flexible at the sequence level [19]. damage NPCs [34, 37, 38], and evidence indicates that Neurons seem particularly sensitive to the damage of protein and RNA aggregates may interfere directly with these structures, as demonstrated by the exclusively neu- specific Nups [34, 37]. In some neurons, Nups mislocali- rodegenerative consequence of many diseases due to dam- zation and altered NPC function could lead to an even age of the NPC function and the NCT. So far, several greater increase in aggregate accumulation and genome reasons for this vulnerability have been studied. One clue stress due to chromatin and DNA/RNA impairment [39, is the inability of neurons to undergo mitosis, the usual 40]. Further complicating this picture, the pathogenic process of cellular renewal [20]. Through DNA replica- process includes aberrant transcript maturation, trans- tion, cells maintain physiological protection of the genome port, and translation [41] (Fig. 1). from exogenous damages. To preserve genomic integrity, The neuronal inclusions detected in neurodegenerative there are at least four active DNA repair pathways in ner- forms have been linked to deficient mechanisms of deg- vous system each corresponding to a particular type of radation, especially proteasomal and autophagy pro- DNA lesions [21]. Genome instability can appear when cesses [42]. the accumulation of DNA damage exceeds a neuron’sre- Whether the pathological cascade, starting from the pair capacity or the DNA repair machinery is defective formation of the aggregates and the damage of the NPC, [20]. Thus, transport across the nuclear membrane repre- to the enhanced fibrillation, may represent a common sents a unique possibility for neurons to guarantee DNA paradigm underlying different neurodegenerative dis- integrity, and genome instability is a major factor in neur- eases is still unknown. onal aging [22, 23]. For example, somatic single nucleotide In summary, several lines of evidences are emerging to variants have been