Lost in Transportation: Nucleocytoplasmic Transport Defects in ALS and Other Neurodegenerative Diseases

Abstract

Amyotrophic lateral sclerosis (ALS) is a progressive, fatal neurodegenerative disease characterized by degeneration of upper and lower motor neurons in the brain and spinal cord. The hallmark pathological feature in most cases of ALS is nuclear depletion and cytoplasmic accumulation of the protein TDP-43 in degenerating neurons. Consistent with this pattern of intracellular protein redistribution, impaired nucleocytoplasmic trafficking has emerged as a mechanism contributing to ALS pathology. Dysfunction in nucleocytoplasmic transport is also an emerging theme in physiological aging and other related neurodegenerative diseases, such as Huntington's and Alzheimer's diseases. Here we review transport through the nuclear pore complex, pointing out vulnerabilities that may underlie ALS and potentially contribute to this and other age-related neurodegenerative diseases.

Keywords: ALS; C9ORF72; FTD; aging; dipeptide repeats; neurodegenerative diseases; nuclear pore; nucleocytoplasmic transport.

Figure 1. Spatial organization of nucleoporins in the nuclear pore complex

(A) Field emission scanning electron micrograph of an isolated Xenopus laevis oocyte nuclear envelope showing the outer surface the outer nuclear membrane and the NPCs. (B) Stimulated emission depletion (STED) image of nuclear pores in X. laevis cells. The protein gp210 (red) is arranged symmetrically around the central pore channel. In green, various FG repeat nucleoporins were labeled with a pan-specific antibody. (C) A schematic cut-away view showing half of an NPC embedded in the nuclear envelope. Nups that modify C9ORF72-mediated phenotypes in G₄C₂-repeat expanded flies and yeast are represented by color-coded text: components in which loss of function (LOF) suppresses or in which gain of function (GOF) enhances degeneration are labeled green, components in which LOF enhances or GOF suppresses degeneration are labeled red. *Different studies have observed opposite modifying effects of Nup50. Images reproduced with permission from (A) Martin W. Goldberg and (B) Stefan W. Hell.

Figure 2. Schematic model for the structural assembly of FG-Nups in the NPC channel

(A) According to the selective phase model, highly cohesive FG domains in FG-Nups interact multivalently with each other to form a sieve-like hydrogel. The mesh size sets an upper size limit for free passage of cargos. Nuclear transport receptors (importins for proteins and NXF1-NXT1 for mRNPs) not only bind FG motifs but also disengage repeat-repeat interactions. This transiently opens meshes in the immediate vicinity of nuclear transport receptors and allows a nuclear transport receptor with its cargo to pass through the FG hydrogel. (B) Left panel: Metal-shadowed electron microscopy image of the nuclear envelope of the newt Notophthalmus viridescens. Image courtesy of Joseph G. Gall. Right panel: Image of the nuclear envelope of X. laevis as obtained by super-resolution stimulated emission depletion microscopy. Nuclear pore outer ring complexes are visualized by staining for gp210 (green); central channels of the nuclear pores are shown by staining for poly(PR)20 (red). Image courtesy of Zehra F. Nizami.

Figure 3. Schematics of protein movement through the nuclear pore complex

(A) The Ran gradient arises due to asymmetric distribution of Ran regulators, which dictate whether GTP or GDP is bound to Ran. Nuclear RanGEF (RCC1) promotes the dissociation of GDP from Ran and allows the binding of GTP. Ran in the nucleus is predominantly in the GTP-bound form. When RanGTP leaves the nucleus, the cytosolic RanGAP induces GTP hydrolysis by Ran in cooperation with RanBP. In the cytoplasm, the RanGTP concentration is low and the RanGDP concentration is high. Pi, inorganic phosphate. (B) In the cytoplasm, importin-β binds cargo via the adaptor protein importin-α, forming a trimeric complex. Importin-β then carries cargo through the NPC. Some importin-βs directly bind cargo, forming a dimeric complex that transports cargo into the nucleoplasm. In the nucleus, binding of RanGTP induces a conformational change in the importin, causing it to dissociate from its cargo. This process results in two complexes: a trimeric complex composed of importin-α, its nuclear export factor CAS, and RanGTP, and a dimeric complex composed of importin-β and RanGTP. These complexes translocate into the cytoplasm, where RanGTP is hydrolyzed. Importin-α and importin-β are released from CAS-RanGDP and RanGDP, respectively, and are available to transport the next cargo. (C) Exportin recruits its cargo when bound to RanGTP in the nucleus. This ternary export complex crosses the NPC to the cytoplasm, where GTP undergoes hydrolysis, triggering the release of the cargo. Proteins known to modify C9ORF72-mediated phenotypes in G₄C₂-repeat expanded flies and yeast are represented by color-coded text as follows: components in which LOF suppresses or in which GOF enhances degeneration are colored green, components in which LOF enhances or GOF suppresses degeneration are colored red. *Different studies have observed opposite modifying effects of RCC1.

Figure 4. Export of mRNPs through the nuclear pore complex

A single pre-mRNA is bound to multiple RNA-modifying proteins, including the cap-binding complex (CBC), serine–arginine-rich (SR) proteins, and the exon-junction complex (EJC). An mRNA containing an error in 3′ end processing, splicing, or packaging into mRNPs is directed to the exosome complex by the exosome cofactor complexes TRAMP and NEXT. The TREX and TREX2 complexes are recruited to a mature mRNP that passes quality control. NXF1 and NXT1 are then recruited to the mRNP, and the cargo mRNP is transferred to the nuclear basket, which initiates the translocation of the cargo mRNP through the NPC. In the cytoplasm, GLE1 and InsP₆ activate DBP5, which binds to the mRNA and alters the structure of the mRNP, thereby removing the NXF1–NXT1 complex in an ATP-dependent manner.

Figure 5. Cellular aging, disease, and nucleocytoplasmic transport

The dysfunction of nucleocytoplasmic transport is observed during aging and neurodegenerative diseases. Age-related transcriptome changes, in particular the downregulation of nuclear transport factors (e.g., karyopherins and RanBP1), lead to dysfunction of nucleocytoplasmic transport that results in the accumulation and aggregation of proteins in the cytoplasm. Cytosolic aggregates, in turn, cause structural abnormalities in the NPCs and further compromise nuclear import and export of proteins and mRNAs. Disease-associated proteins take the same route as aging to target and perturb the function of the NPC.