The nuclear pore complex and nuclear transport

Nucleoplasmic signals promote directed transmembrane protein import simultaneously via multiple channels of nuclear pores

Roughly 10% of eukaryotic transmembrane proteins are found on the nuclear membrane, yet how such proteins target and translocate to the nucleus remains in dispute. Most models propose transport through the nuclear pore complexes, but a central outstanding question is whether transit occurs through their central or peripheral channels. Using live-cell high-speed super-resolution single-molecule microscopy we could distinguish protein translocation through the central and peripheral channels, finding that most inner nuclear membrane proteins use only the peripheral channels, but some apparently extend intrinsically disordered domains containing nuclear localization signals into the central channel for directed nuclear transport. These nucleoplasmic signals are critical for central channel transport as their mutation blocks use of the central channels; however, the mutated proteins can still complete their translocation using only the peripheral channels, albeit at a reduced rate. Such proteins can still translocate using only the peripheral channels when central channel is blocked, but blocking the peripheral channels blocks translocation through both channels. This suggests that peripheral channel transport is the default mechanism that was adapted in evolution to include aspects of receptor-mediated central channel transport for directed trafficking of certain membrane proteins.

The SQSTM1-NUP214 fusion protein interacts with Crm1, activates Hoxa and Meis1 genes, and drives leukemogenesis in mice

The NUP98 and NUP214 nucleoporins (NUPs) are recurrently fused to heterologous proteins in leukemia. The resulting chimeric oncoproteins retain the phenylalanine-glycine (FG) repeat motifs of the NUP moiety that mediate interaction with the nuclear export receptor Crm1. NUP fusion leukemias are characterized by HOXA gene upregulation; however, their molecular pathogenesis remains poorly understood. To investigate the role of Crm1 in mediating the leukemogenic properties of NUP chimeric proteins, we took advantage of the Sequestosome-1 (SQSTM1)-NUP214 fusion. SQSTM1-NUP214 retains only a short C-terminal portion of NUP214 which contains FG motifs that mediate interaction with Crm1. We introduced point mutations targeting these FG motifs and found that the ability of the resulting SQSTM1-NUP214FGmut protein to interact with Crm1 was reduced by more than 50% compared with SQSTM1-NUP214. Mutation of FG motifs affected transforming potential: while SQSTM1-NUP214 impaired myeloid maturation and conferred robust colony formation to transduced hematopoietic progenitors in a serial replating assay, the effect of SQSTM1-NUP214FGmut was considerably diminished. Moreover, SQSTM1-NUP214 caused myeloid leukemia in all transplanted mice, whereas none of the SQSTM1-NUP214FGmut reconstituted mice developed leukemia. These oncogenic effects coincided with the ability of SQSTM1-NUP214 and SQSTM1-NUP214FGmut to upregulate the expression of Hoxa and Meis1 genes in hematopoietic progenitors. Indeed, chromatin immunoprecipitation assays demonstrated that impaired SQSTM1-NUP214 interaction with Crm1 correlated with impaired binding of the fusion protein to Hoxa and Meis1 genes. These findings highlight the importance of Crm1 in mediating the leukemogenic properties of SQSTM1-NUP214, and suggest a conserved role of Crm1 in recruiting oncoproteins to their effector genes.

Role of Nucleoporins and Transport Receptors in Cell Differentiation

Bidirectional molecular movements between the nucleus and cytoplasm take place through nuclear pore complexes (NPCs) embedded in the nuclear membrane. These macromolecular structures are composed of several nucleoporins, which form seven different subcomplexes based on their biochemical affinity. These nucleoporins are integral components of the complex, not only allowing passive transport but also interacting with importin, exportin, and other molecules that are required for transport of protein in various cellular processes. Transport of different proteins is carried out either dependently or independently on transport receptors. As well as facilitating nucleocytoplasmic transport, nucleoporins also play an important role in cell differentiation, possibly by their direct gene interaction. This review will cover the general role of nucleoporins (whether its dependent or independent) and nucleocytoplasmic transport receptors in cell differentiation.

Passive diffusion through nuclear pore complexes regulates levels of the yeast SAGA and SLIK coactivator complexes

Nuclear pore complexes (NPCs) control gene expression by regulating the bi-directional exchange of proteins and RNAs between nuclear and cytoplasmic compartments, including access of transcriptional regulators to the nucleoplasm. Here, we show that the yeast (Saccharomyces cerevisiae) nucleoporin Nup170, in addition to binding and silencing subtelomeric genes, supports transcription of genes regulated by the SAGA transcriptional activator complex. Specifically, we show that a lower amount of SAGA complex is bound to target genes in the absence of Nup170. Consistent with this observation, levels of the SAGA complex are decreased in cells lacking Nup170, while those of the SAGA-related SLIK complexes are increased. This change in the ratio of SAGA to SLIK complexes is due to increased nuclear activity of Pep4, a protease responsible for production of the SLIK complex. Further analyses of various nucleoporin mutants revealed that the increased nuclear entry of Pep4 observed in the nup170Δ mutant likely occurs as the consequence of an increase in the sieving limits of the NPC diffusion channel. On the basis of these results, we propose that changes in passive diffusion rates represent a mechanism for regulating SAGA- and SLIK complex-mediated transcriptional events.

Adenovirus flow in host cell networks

Viruses are obligatory parasites that take advantage of intracellular niches to replicate. During infection, their genomes are carried in capsids across the membranes of host cells to sites of virion production by exploiting cellular behaviour and resources to guide and achieve all aspects of delivery and the downstream virus manufacturing process. Successful entry hinges on execution of a precisely tuned viral uncoating program where incoming capsids disassemble in consecutive steps to ensure that genomes are released at the right time, and in the right place for replication to occur. Each step of disassembly is cell-assisted, involving individual pathways that transmit signals to regulate discrete functions, but at the same time, these signalling pathways are organized into larger networks, which communicate back and forth in complex ways in response to the presence of virus. In this review, we consider the elegant strategy by which adenoviruses (AdVs) target and navigate cellular networks to initiate the production of progeny virions. There are many remarkable aspects about the AdV entry program; for example, the virus gains targeted control of a large well-defined local network neighbourhood by coupling several interacting processes (including endocytosis, autophagy and microtubule trafficking) around a collective reference state centred on the interactional topology and multifunctional nature of protein VI. Understanding the network targeting activity of protein VI, as well as other built-in mechanisms that allow AdV particles to be efficient at navigating the subsystems of the cell, can be used to improve viral vectors, but also has potential to be incorporated for use in entirely novel delivery systems.

Nucleocytoplasmic transport of intrinsically disordered proteins studied by high-speed super-resolution microscopy

Both natively folded and intrinsically disordered proteins (IDPs) destined for the nucleus need to transport through the nuclear pore complexes (NPCs) in eukaryotic cells. NPCs allow for passive diffusion of small folded proteins while barricading large ones, unless they are facilitated by nuclear transport receptors. However, whether nucleocytoplasmic transport of IDPs would follow these rules remains unknown. By using a high-speed super-resolution fluorescence microscopy, we have measured transport kinetics and 3D spatial locations of transport routes through native NPCs for various IDPs. Our data revealed that the rules executed for folded proteins are not well followed by the IDPs. Instead, both large and small IDPs can passively diffuse through the NPCs. Furthermore, their diffusion efficiencies and routes are differentiated by their content ratio of charged (Ch) and hydrophobic (Hy) amino acids. A Ch/Hy-ratio mechanism was finally suggested for nucleocytoplasmic transport of IDPs.

The Hitchhiker's Guide to Nucleocytoplasmic Trafficking in Neurodegeneration

The widespread nature of nucleocytoplasmic trafficking defects and protein accumulation suggests distinct yet overlapping mechanisms in a variety of neurodegenerative diseases. Detailed understanding of the cellular pathways involved in nucleocytoplasmic transport and its dysregulation are essential for elucidating neurodegenerative pathogenesis and pinpointing potential areas for therapeutic intervention. The transport of cargos from the nucleus to the cytoplasm is generally regulated by the structure and function of the nuclear pore as well as the karyopherin α/β, importin, exportin, and mRNA export mechanisms. The disruption of these crucial transport mechanisms has been extensively described in the context of neurodegenerative diseases. One common theme in neurodegeneration is the cytoplasmic aggregation of proteins, including nuclear RNA binding proteins, repeat expansion associated gene products, and tau. These cytoplasmic aggregations are partly a consequence of failed nucleocytoplasmic transport machinery, but can also further disrupt transport, creating cyclical feed-forward mechanisms that exacerbate neurodegeneration. Here we describe the canonical mechanisms that regulate nucleocytoplasmic trafficking as well as how these mechanisms falter in neurodegenerative diseases.

The Novel Nuclear Targeting and BFRF1-Interacting Domains of BFLF2 Are Essential for Efficient Epstein-Barr Virus Virion Release

Epstein-Barr virus (EBV) genomic DNA is replicated and packaged into procapsids in the nucleus to form nucleocapsids, which are then transported into the cytoplasm for tegumentation and final maturation. The process is facilitated by the coordination of the viral nuclear egress complex (NEC), which consists of BFLF2 and BFRF1. By expression alone, BFLF2 is distributed mainly in the nucleus. However, it colocalizes with BFRF1 at the nuclear rim and in cytoplasmic nuclear envelope-derived vesicles in coexpressing cells, suggesting temporal control of the interaction between BFLF2 and BFRF1 is critical for their proper function. The N-terminal sequence of BFLF2 is less conserved than that of alpha- and betaherpesvirus homologs. Here, we found that BFLF2 amino acids (aa) 2 to 102 are required for both nuclear targeting and its interaction with BFRF1. Coimmunoprecipitation and confocal analysis indicated that aa 82 to 106 of BFLF2 are important for its interaction with BFRF1. Three crucial amino acids (R47, K50, and R52) and several noncontinuous arginine and histidine residues within aa 59 to 80 function together as a noncanonical nuclear localization signal (NLS), which can be transferred onto yellow fluorescent protein (YFP)-LacZ for nuclear targeting in an importin β-dependent manner. Virion secretion is defective in 293 cells harboring a BFLF2 knockout EBV bacmid upon lytic induction and is restored by trans-complementation of wild-type BFLF2, but not NLS or BFRF1-interacting defective mutants. In addition, multiple domains of BFRF1 were found to bind BFLF2, suggesting multiple contact regions within BFRF1 and BFLF2 are required for proper nuclear egress of EBV nucleocapsids.IMPORTANCE Although Epstein-Barr virus (EBV) BFRF1 and BFLF2 are homologs of conserved viral nuclear egress complex (NEC) in all human herpesviruses, unique amino acid sequences and functions were identified in both proteins. In this study, the nuclear targeting and BFRF1-interacting domains were found within the N terminus of BFLF2. We showed that amino acids (aa) 82 to 106 are the major region required for BFLF2 to interact with BFRF1. However, the coimmunoprecipitation (Co-IP) data and glutathione transferase (GST) pulldown experiments revealed that multiple regions of both proteins contribute to reciprocal interactions. Different from the canonical nuclear localization signal (NLS) in other herpes viral homologs, BFLF2 contains a novel importin-dependent nuclear localization signal, including R47, K50, and R52 and several neighboring discontinuous arginine and histidine residues. Using a bacmid complementation system, we show that both the nuclear targeting and the novel nuclear localization signal within aa 82 to 106 of BFLF2 are required for virion secretion.

Poor old pores-The challenge of making and maintaining nuclear pore complexes in aging

The nuclear pore complex (NPC) is the sole gateway to the nuclear interior, and its function is essential to all eukaryotic life. Controlling the functionality of NPCs is a tremendous challenge for cells. Firstly, NPCs are large structures, and their complex assembly does occasionally go awry. Secondly, once assembled, some components of the NPC persist for an extremely long time and, as a result, are susceptible to accumulate damage. Lastly, a significant proportion of the NPC is composed of intrinsically disordered proteins that are prone to aggregation. In this review, we summarize how the quality of NPCs is guarded in young cells and discuss the current knowledge on the fate of NPCs during normal aging in different tissues and organisms. We discuss the extent to which current data supports a hypothesis that NPCs are poorly maintained during aging of nondividing cells, while in dividing cells the main challenge is related to the assembly of new NPCs. Our survey of current knowledge points toward NPC quality control as an important node in aging of both dividing and nondividing cells. Here, the loss of protein homeostasis during aging is central and the NPC appears to both be impacted by, and to drive, this process.

Deletion of low-density lipoprotein-related receptor 5 inhibits liver Cancer cell proliferation via destabilizing Nucleoporin 37

Background

LRP5/6 are co-receptors in Wnt/β-catenin pathway. Recently, we discovered multiple β-catenin independent functions of LRP5/6 in tumor cells and in the diseased heart. Nucleoporin 37 (NUP37) is an important component of the nuclear pore complex (NPC), whose elevated expression is associated with worsened prognosis in liver cancer. Previous studies have shown that NUP37 interacted with YAP and activated YAP/TEAD signaling in liver cancer. Our preliminary findings showed a nuclear location of LRP5. We thus tested the hypothesis that LRP5 may act as a genuine regulator of YAP/TEAD signaling via modulating NUP37 in a β-catenin-independent way.

Methods

We performed siRNA knockdown of LRP5, LRP6, or β-catenin in liver cancer HepG2 cells to determine the effect on tumor cell proliferation. Protein expressions and interaction between LRP5 and NUP37 were determined using immunoprecipitation and western blot analyses.

Results

HepG2 cell proliferation was markedly inhibited by knockdown of LRP5 but not LRP6 or β-catenin, suggesting that LRP5 has a specific, β-catenin-independent role in inhibiting HepG2 cell proliferation. Knockdown of NUP37 by siRNA inhibited the proliferation of HepG2 cells, whereas overexpression of NUP37 reversed the decrease in cell proliferation induced by LRP5 knockdown. Immunoprecipitation assays confirmed that LRP5 bound to NUP37. Furthermore, LRP5 overexpression restored NUP37 knockdown-induced downregulation of YAP/TEAD pathway.

Conclusions

LRP5 deletion attenuates cell proliferation via destabilization of NUP37, in a β-catenin-independent manner. LRP5 therefore acts as a genuine regulator of YAP/TEAD signaling via maintaining the integrity of the NPC, and implicates a therapeutic strategy in targeting LRP5 for inhibiting liver cancer cell proliferation.

Biallelic Variants in the Nuclear Pore Complex Protein NUP93 Are Associated with Non-progressive Congenital Ataxia

Nuclear pore complexes (NPCs) are the gateways of the nuclear envelope mediating transport between cytoplasm and nucleus. They form huge complexes of 125 MDa in vertebrates and consist of about 30 different nucleoporins present in multiple copies in each complex. Here, we describe pathogenic variants in the nucleoporin 93 (NUP93) associated with an autosomal recessive form of congenital ataxia. Two rare compound heterozygous variants of NUP93 were identified by whole exome sequencing in two brothers with isolated cerebellar atrophy: one missense variant (p.R537W) results in a protein which does not localize to NPCs and cannot functionally replace the wild type protein, whereas the variant (p.F699L) apparently supports NPC assembly. In addition to its recently described pathological role in steroid-resistant nephrotic syndrome, our work identifies NUP93 as a candidate gene for non-progressive congenital ataxia.

Core Components of the Nuclear Pore Bind Distinct States of Chromatin and Contribute to Polycomb Repression

Interactions between the genome and the nuclear pore complex (NPC) have been implicated in multiple gene regulatory processes, but the underlying logic of these interactions remains poorly defined. Here, we report high-resolution chromatin binding maps of two core components of the NPC, Nup107 and Nup93, in Drosophila cells. Our investigation uncovered differential binding of these NPC subunits, where Nup107 preferentially targets active genes while Nup93 associates primarily with Polycomb-silenced regions. Comparison to Lamin-associated domains (LADs) revealed that NPC binding sites can be found within LADs, demonstrating a linear binding of the genome along the nuclear envelope. Importantly, we identified a functional role of Nup93 in silencing of Polycomb target genes and in spatial folding of Polycomb domains. Our findings lend to a model where different nuclear pores bind different types of chromatin via interactions with specific NPC sub-complexes, and a subset of Polycomb domains is stabilized by interactions with Nup93.

Nanocompartmentalization of the Nuclear Pore Lumen

The nuclear pore complex (NPC) employs the intrinsically disordered regions (IDRs) from a family of phenylalanine-glycine-rich nucleoporins (FG-Nups) to control nucleocytoplasmic transport. It has been a long-standing mystery how the IDR-mediated mass exchange can be rapid yet selective. Here, we use a computational microscope to show that nanocompartmentalization of IDR subdomains leads to a remarkably elaborate gating structure as programmed by the amino acid sequences. In particular, we reveal a heterogeneous permeability barrier that combines an inner ring barrier with two vestibular condensates. Throughout the NPC, we find a polarized electrostatic potential and a diffuse thermoreversible FG network featuring mosaic FG territories with low FG-FG pairing fraction. Our theoretical anatomy of the central transporter sheds light into the sequence-structure-function relationship of the FG-Nups and provides a picture of nucleocytoplasmic mass exchange that allows a reconciliation of transport efficiency and specificity.

The Non-Canonical Aspects of MicroRNAs: Many Roads to Gene Regulation

MicroRNAs (miRNAs) are critical regulators of gene expression. As miRNAs are frequently deregulated in many human diseases, including cancer and immunological disorders, it is important to understand their biological functions. Typically, miRNA-encoding genes are transcribed by RNA Polymerase II and generate primary transcripts that are processed by RNase III-endonucleases DROSHA and DICER into small RNAs of approximately 21 nucleotides. All miRNAs are loaded into Argonaute proteins in the RNA-induced silencing complex (RISC) and act as post-transcriptional regulators by binding to the 3'- untranslated region (UTR) of mRNAs. This seed-dependent miRNA binding inhibits the translation and/or promotes the degradation of mRNA targets. Surprisingly, recent data presents evidence for a target-mediated decay mechanism that controls the level of specific miRNAs. In addition, several non-canonical miRNA-containing genes have been recently described and unexpected functions of miRNAs have been identified. For instance, several miRNAs are located in the nucleus, where they are involved in the transcriptional activation or silencing of target genes. These epigenetic modifiers are recruited by RISC and guided by miRNAs to specific loci in the genome. Here, we will review non-canonical aspects of miRNA biology, including novel regulators of miRNA expression and functions of miRNAs in the nucleus.

Nucleocytoplasmic Shuttling of STATs. A Target for Intervention?

Signal transducer and activator of transcription (STAT) proteins are transcription factors that in the latent state are located predominantly in the cytoplasm. Activation of STATs through phosphorylation of a single tyrosine residue results in nuclear translocation. The requirement of tyrosine phosphorylation for nuclear accumulation is shared by all STAT family members but mechanisms of nuclear translocation vary between different STATs. These differences offer opportunities for specific intervention. To achieve this, the molecular mechanisms of nucleocytoplasmic shuttling of STATs need to be understood in more detail. In this review we will give an overview on the various aspects of nucleocytoplasmic shuttling of latent and activated STATs with a special focus on STAT3 and STAT5. Potential targets for cancer treatment will be identified and discussed.

Nuclear Envelope Permeability Barrier as a Fast-Response Intracellular Mechanostat

The nuclear envelope is an undisputed component of the intracellular mechanotransduction cascades which collect, process, and respond to mechanical stimuli from the environment. At the same time, the nuclear envelope performs the function of a selective barrier between the nuclear and cytoplasmic compartments. Although the mechanosensing and the barrier functions of the nuclear envelope have both been subjects of intense research, a possible reciprocal relationship between them is only beginning to emerge. In this report, the role of the nucleocytoplasmic permeability barrier is evaluated in nuclear mechanics. Using a combination of atomic force and confocal microscopy, the functional state of the nucleocytoplasmic permeability barrier and the nuclear mechanics is monitored. By modulating the stringency of the barrier and simulating the active transport imbalance across the nuclear envelope, the decisive impact of these parameters on nuclear mechanics is demonstrated. It is concluded that the nucleocytoplasmic barrier is the second essential component of the intracellular mechanostat function performed by the nuclear envelope.

Drive, filter, and stick: A protein sorting conspiracy in photoreceptors

The sorting of proteins into different functional compartments is a fundamental cellular task. In this issue, Maza et al. (2019. J. Cell Biol https://doi.org/10.1083/jcb.201906024) demonstrate that distinct protein populations are dynamically generated in specialized regions of photoreceptors via an interplay of protein-membrane affinity, impeded diffusion, and driven transport.

Mechanical regulation of nucleocytoplasmic translocation in mesenchymal stem cells: characterization and methods for investigation

Mesenchymal stem cells (MSCs) have immune-modulatory and tissue-regenerative properties that make them a suitable and promising tool for cell-based therapy application. Since the bio-chemo-mechanical environment influences MSC fate and behavior, the understanding of the mechanosensors involved in the transduction of mechanical inputs into chemical signals could be pivotal. In this context, the nuclear pore complex is a molecular machinery that is believed to have a key role in force transmission and in nucleocytoplasmic shuttling regulation. To fully understand the nuclear pore complex role and the nucleocytoplasmic transport dynamics, recent advancements in fluorescence microscopy provided the possibility to study passive and facilitated nuclear transports also in mechanically stimulated cell culture conditions. Here, we review the current available methods for the investigation of nucleocytoplasmic shuttling, including photo-perturbation-based approaches, fluorescence correlation spectroscopy, and single-particle tracking techniques. For each method, we analyze the advantages, disadvantages, and technical limitations. Finally, we summarize the recent knowledge on mechanical regulation of nucleocytoplasmic translocation in MSC, the relevant progresses made so far, and the future perspectives in the field.

Mechanisms of nuclear mRNA export: A structural perspective

Export of mRNA from the nucleus to the cytoplasm is a critical process for all eukaryotic gene expression. As mRNA is synthesized, it is packaged with a myriad of RNA-binding proteins to form ribonucleoprotein particles (mRNPs). For each step in the processes of maturation and export, mRNPs must have the correct complement of proteins. Much of the mRNA export pathway revolves around the heterodimeric export receptor yeast Mex67•Mtr2/human NXF1•NXT1, which is recruited to signal the completion of nuclear mRNP assembly, mediates mRNP targeting/translocation through the nuclear pore complex (NPC), and is displaced at the cytoplasmic side of the NPC to release the mRNP into the cytoplasm. Directionality of the transport is governed by at least two DEAD-box ATPases, yeast Sub2/human UAP56 in the nucleus and yeast Dbp5/human DDX19 at the cytoplasmic side of the NPC, which respectively mediate the association and dissociation of Mex67•Mtr2/NXF1•NXT1 onto the mRNP. Here we review recent progress from structural studies of key constituents in different steps of nuclear mRNA export. These findings have laid the foundation for further studies to obtain a comprehensive mechanistic view of the mRNA export pathway.

Enhanced Nucleocytoplasmic Transport due to Competition for Elastic Binding Sites

Nuclear pore complexes (NPCs) control all traffic into and out of the cell nucleus. NPCs are molecular machines that simultaneously achieve high selectivity and high transport rates. The biophysical details of how cargoes rapidly traverse the pore remain unclear but are known to be mediated by interactions between cargo-binding chaperone proteins and natively unstructured nucleoporin proteins containing many phenylalanine-glycine repeats (FG nups) that line the pore's central channel. Here, we propose a specific and detailed physical mechanism for the high speed of nuclear import based on the elasticity of FG nups and on competition between individual chaperone proteins for FG nup binding. We develop a mathematical model to support our proposed mechanism. We suggest that the recycling of nuclear import factors back to the cytoplasm is important for driving high-speed import and predict the existence of an optimal cytoplasmic concentration of cargo for enhancing the rate of import over a purely diffusive rate.

Decoding inflammation, its causes, genomic responses, and emerging countermeasures

Inflammation is the mechanism of diseases caused by microbial, autoimmune, allergic, metabolic and physical insults that produce distinct types of inflammatory responses. This aetiologic view of inflammation informs its classification based on a cause‐dependent mechanism as well as a cause‐directed therapy and prevention. The genomic era ushered in a new understanding of inflammation by highlighting the cell's nucleus as the centre of the inflammatory response. Exogenous or endogenous inflammatory insults evoke genomic responses in immune and non‐immune cells. These genomic responses depend on transcription factors, which switch on and off a myriad of inflammatory genes through their regulatory networks. We discuss the transcriptional paradigm of inflammation based on denying transcription factors’ access to the nucleus. We present two approaches that control proinflammatory signalling to the nucleus. The first approach constitutes a novel intracellular protein therapy with bioengineered physiologic suppressors of cytokine signalling. The second approach entails control of proinflammatory transcriptional cascades by targeting nuclear transport with a cell‐penetrating peptide that inhibits the expression of 23 out of the 26 mediators of inflammation along with the nine genes required for metabolic responses. We compare these emerging anti‐inflammatory countermeasures to current therapies. The transcriptional paradigm of inflammation offers nucleocentric strategies for microbial, autoimmune, metabolic, physical and other types of inflammation afflicting millions of people worldwide.

Imaging within single NPCs reveals NXF1's role in mRNA export on the cytoplasmic side of the pore

Translocation of mRNA through the nuclear pore complex (NPC) requires interactions with different NPC regions. To determine the interactions that are crucial for effective mRNA export in living cells, we examined mRNA export within individual pores by applying various types of mRNA export blocks that stalled mRNPs at different stages of transition. Focusing on the major mRNA export factor NXF1, we found that initial mRNP binding to the NPC did not require NXF1 in the NPC, whereas release into the cytoplasm did. NXF1 localization in the NPC did not require RNA or RNA binding. Superresolution microscopy showed that NXF1 consistently occupied positions on the cytoplasmic side of the NPC. Interactions with specific nucleoporins were pinpointed using FLIM-FRET for measuring protein-protein interactions inside single NPCs, showing that Dbp5 helicase activity of mRNA release is conserved in yeast and humans. Altogether, we find that specific interactions on the cytoplasmic side of the NPC are fundamental for the directional flow of mRNA export.

Programmable On-Chip Artificial Cell Producing Post-Translationally Modified Ubiquitinated Protein

In nature, intracellular microcompartments have evolved to allow the simultaneous execution of tightly regulated complex processes within a controlled environment. This architecture serves as the blueprint for the construction of a wide array of artificial cells. However, such systems are inadequate in their ability to confine and sequentially control multiple central dogma activities (transcription, translation, and post-translational modifications) resulting in a limited production of complex biomolecules. Here, an artificial cell-on-a-chip comprising hierarchical compartments allowing the processing and transport of products from transcription, translation, and post-translational modifications through connecting channels is designed and fabricated. This platform generates a tightly controlled system, yielding directly a purified modified protein, with the potential to produce proteoform of choice. Using this platform, the full ubiquitinated form of the Parkinson's disease-associated α-synuclein is generated starting from DNA, in a single device. By bringing together all central dogma activities in a single controllable platform, this approach will open up new possibilities for the synthesis of complex targets, will allow to decipher diverse molecular mechanisms in health and disease and to engineer protein-based materials and pharmaceutical agents.

Quantification of Biomolecular Dynamics Inside Real and Synthetic Nuclear Pore Complexes Using Time-Resolved Atomic Force Microscopy

Over the past decades, atomic force microscopy (AFM) has emerged as an increasingly powerful tool to study the dynamics of biomolecules at nanometer length scales. However, the more stochastic the nature of such biomolecular dynamics, the harder it becomes to distinguish them from AFM measurement noise. Rapid, stochastic dynamics are inherent to biological systems comprising intrinsically disordered proteins. One role of such proteins is in the formation of the transport barrier of the nuclear pore complex (NPC): the selective gateway for macromolecular traffic entering or exiting the nucleus. Here, we use AFM to observe the dynamics of intrinsically disordered proteins from two systems: the transport barrier of native NPCs and the transport barrier of a mimetic NPC made using a DNA origami scaffold. Analyzing data recorded with 50-200 ms temporal resolution, we highlight the importance of drift correction and appropriate baseline measurements in such experiments. In addition, we describe an autocorrelation analysis to quantify time scales of observed dynamics and to assess their veracity-an analysis protocol that lends itself to the quantification of stochastic fluctuations in other biomolecular systems. The results reveal the surprisingly slow rate of stochastic, collective transitions inside mimetic NPCs, highlighting the importance of FG-nup cohesive interactions.

Porcine Reproductive and Respiratory Syndrome Virus Nonstructural Protein 1 Beta Interacts with Nucleoporin 62 To Promote Viral Replication and Immune Evasion

Porcine reproductive and respiratory syndrome virus (PRRSV) blocks host mRNA nuclear export to the cytoplasm, and nonstructural protein 1 beta (nsp1β) of PRRSV has been identified as the protein that disintegrates the nuclear pore complex. In the present study, the molecular basis for the inhibition of host mRNA nuclear export was investigated. Nucleoporin 62 (Nup62) was found to bind to nsp1β, and the region representing the C-terminal residues 328 to 522 of Nup62 was determined to be the binding domain for nsp1β. The nsp1β L126A mutant in the SAP domain did not bind to Nup62, and in L126A-expressing cells, host mRNA nuclear export occurred normally. The vL126A mutant PRRSV generated by reverse genetics replicated at a lower rate, and the titer was lower than for wild-type virus. In nsp1β-overexpressing cells or small interfering RNA (siRNA)-mediated Nup62 knockdown cells, viral protein synthesis increased. Notably, the production of type I interferons (IFN-α/β), IFN-stimulated genes (PKR, OAS, Mx1, and ISG15 genes), IFN-induced proteins with tetratricopeptide repeats (IFITs) 1 and 2, and IFN regulatory factor 3 decreased in these cells. As a consequence, the growth of vL126A mutant PRRSV was rescued to the level of wild-type PRRSV. These findings are attributed to nuclear pore complex (NPC) disintegration by nsp1β, resulting in increased viral protein production and decreased host protein production, including antiviral proteins in the cytoplasm. Our study reveals a new strategy of PRRSV for immune evasion and enhanced replication during infection.IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) causes PRRS and is known to effectively suppress host innate immunity. The PRRSV nsp1β protein blocks host mRNA nuclear export, which has been shown to be one of the viral mechanisms for inhibition of antiviral protein production. nsp1β binds to the cellular protein nucleoporin 62 (Nup62), and as a consequence, the nuclear pore complex (NPC) is disintegrated and the nucleocytoplasmic trafficking of host mRNAs and host proteins is blocked. We show the dual benefits of Nup62 and nsp1β binding for PRRSV replication: the inhibition of host antiviral protein expression and the exclusive use of host translation machinery by the virus. Our study unveils a novel strategy of PRRSV for immune evasion and enhanced replication during infection.

Pathogenic mutations in genes encoding nuclear envelope proteins and defective nucleocytoplasmic connections

Mutations in genes encoding nuclear lamins and associated nuclear envelope proteins have been linked to a broad range of inherited diseases affecting different tissues and organs. These diseases are often referred to as laminopathies. Scientists have yet to elucidate exactly how pathogenic mutations leading to alteration of a nuclear envelope protein cause disease. Our relatively recent research has shown that pathogenic mutations in genes encoding nuclear envelope proteins lead to defective nucleocytoplasmic connections that disrupt proper functioning of the linker of nucleoskeleton and cytoskeleton complex in the establishment of cell polarity. These defects may explain, at least in part, pathogenic mechanisms underlying laminopathies.

Impact statement

Mutations in genes encoding nuclear lamins and associated nuclear envelope proteins have been linked to several diseases affecting different tissues and organs. The pathogenic mechanisms underlying these diseases, often called laminopathies, remain poorly understood. Increased knowledge of the functions of different nuclear envelope proteins and the interactions between them is crucial to elucidate these disease mechanisms. Our research has shown that pathogenic mutations in genes encoding nuclear envelope proteins lead to defective nucleocytoplasmic connections that disrupt proper functioning of the linker of nucleoskeleton and cytoskeleton (LINC) complex in the establishment of cell polarity. These defects may contribute to the pathogenesis of laminopathies and provide novel targets for therapeutics.

Nuclear Transport and Accumulation of Smad Proteins Studied by Single-Molecule Microscopy

Nuclear translocation of stimulated Smad heterocomplexes is a critical step in the signal transduction of transforming growth factor β (TGF-β) from transmembrane receptors into the nucleus. Specifically, normal nuclear accumulation of Smad2/Smad4 heterocomplexes induced by TGF-β1 is involved in carcinogenesis. However, the relationship between nuclear accumulation and the nucleocytoplasmic transport kinetics of Smad proteins in the presence of TGF-β1 remains obscure. By combining a high-speed single-molecule tracking microscopy and Förster resonance energy transfer technique, we tracked the entire TGF-β1-induced process of Smad2/Smad4 heterocomplex formation, as well as their transport through nuclear pore complexes in live cells, with a high single-molecule localization precision of 2 ms and <20 nm. Our single-molecule Förster resonance energy transfer data have revealed that in TGF-β1-treated cells, Smad2/Smad4 heterocomplexes formed in the cytoplasm, imported through the nuclear pore complexes as entireties, and finally dissociated in the nucleus. Moreover, we found that basal-state Smad2 or Smad4 cannot accumulate in the nucleus without the presence of TGF-β1, mainly because both of them have an approximately twofold higher nuclear export efficiency compared to their nuclear import. Remarkably and reversely, heterocomplexes of Smad2/Smad4 induced by TGF-β1 can rapidly concentrate in the nucleus because of their almost fourfold higher nuclear import rate in comparison with their nuclear export rate. Thus, we believe that the determined TGF-β1-dependent transport configurations and efficiencies for the basal-state Smad or stimulated Smad heterocomplexes elucidate the basic molecular mechanism to understand their nuclear transport and accumulation.

Development of a novel DsRed-NLS vector with a monopartite classical nuclear localization signal

The nuclear localization signal (NLS) marks proteins for transport to the nucleus and is used in various applications in many fields. NLSs are used to achieve efficient and stable transport of biomolecules. Previously, commercial vectors used in NLS studies contained three iterations of the NLS sequence, but these sequences can affect experimental results and alter protein function. Here, we investigated a new vector using a single classical NLS sequence with a mutation in pDsRed2-C1-wt to reduce experimental artifacts. In the newly constructed pDsRed2-C1-1NLS vector, the NLS sequence is placed near the multiple cloning sites of pDsRed2-C1-wt, and the multiple cloning site region was designed to facilitate insertion of the desired gene by site-directed mutagenesis. Fluorescent protein expression in the nucleus can be visually confirmed. The results show that the fluorescent protein was bound to the transport protein. The constructed vector had a cell survival rate of 89-95% and a transfection efficiency of 39-56% when introduced into animal cells, which are similar to those of other NLS vectors. Additionally, the constructed NLS vector can be used to demonstrate complementary binding between target proteins, and that the target protein is transported by the NLS transport system. Especially, we show that the vector can be useful for experiments involving the S100A10 gene. In addition, the constructed vector is useful for studies of genes and proteins that show potential for gene therapy or drug delivery applications.

Nuclear Transport of Yeast Proteasomes

Proteasomes are key proteases in regulating protein homeostasis. Their holo-enzymes are composed of 40 different subunits which are arranged in a proteolytic core (CP) flanked by one to two regulatory particles (RP). Proteasomal proteolysis is essential for the degradation of proteins which control time-sensitive processes like cell cycle progression and stress response. In dividing yeast and human cells, proteasomes are primarily nuclear suggesting that proteasomal proteolysis is mainly required in the nucleus during cell proliferation. In yeast, which have a closed mitosis, proteasomes are imported into the nucleus as immature precursors via the classical import pathway. During quiescence, the reversible absence of proliferation induced by nutrient depletion or growth factor deprivation, proteasomes move from the nucleus into the cytoplasm. In the cytoplasm of quiescent yeast, proteasomes are dissociated into CP and RP and stored in membrane-less cytoplasmic foci, named proteasome storage granules (PSGs). With the resumption of growth, PSGs clear and mature proteasomes are transported into the nucleus by Blm10, a conserved 240 kDa protein and proteasome-intrinsic import receptor. How proteasomes are exported from the nucleus into the cytoplasm is unknown.

Homozygous deletion of the entire AAAS gene in a triple A syndrome patient

Triple A syndrome, a multisystemic autosomal recessive disease, is characterized by the clinical triad of adrenal insufficiency, alacrima and achalasia in combination with progressive neurological impairments. The disorder is caused by homozygous or compound heterozygous mutations in the AAAS gene. Here we present the clinical and molecular data of a ten year old patient with triple A syndrome. Array CGH analysis confirmed the PCR-based assumption of a homozygous deletion of the entire AAAS gene in the patient and a heterozygous deletion in both parents. We demonstrate that the patient carries a 15 kb deletion and identified the 5' and 3' breakpoints outside the AAAS gene. This is the first report of a triple A syndrome patient with a homozygous deletion of the entire AAAS gene.

Structural and biochemical analyses of the nuclear pore complex component ELYS identify residues responsible for nucleosome binding

The nuclear pore complex embedded within the nuclear envelope is the essential architecture for trafficking macromolecules, such as proteins and RNAs, between the cytoplasm and nucleus. The nuclear pore complex assembly occurs on chromatin in the post-mitotic phase of the cell cycle. ELYS (MEL-28/AHCTF1) binds to the nucleosome, which is the basic chromatin unit, and promotes assembly of the complex around the chromosomes in cells. Here we show that the Arg-Arg-Lys (RRK) stretch of the C-terminal ELYS region plays an essential role in the nucleosome binding. The cryo-EM structure and the crosslinking mass spectrometry reveal that the ELYS C-terminal region directly binds to the acidic patch of the nucleosome. These results provide mechanistic insight into the ELYS-nucleosome interaction, which promotes the post-mitotic nuclear pore complex formation around chromosomes in cells.

Patrolling the nucleus: inner nuclear membrane-associated degradation

Protein quality control and transport are important for the integrity of organelles such as the endoplasmic reticulum, but it is largely unknown how protein homeostasis is regulated at the nuclear envelope (NE) despite the connection between NE protein function and human disease. Elucidating mechanisms that regulate the NE proteome is key to understanding nuclear processes such as gene expression, DNA replication and repair as NE components, particularly proteins at the inner nuclear membrane (INM), are involved in the maintenance of nuclear structure, nuclear positioning and chromosome organization. Nuclear pore complexes control the entry and exit of proteins in and out of the nucleus, restricting movement across the nuclear membrane based on protein size, or the size of the extraluminal-facing domain of a transmembrane protein, providing one level of INM proteome regulation. Research in budding yeast has identified a protein quality control system that targets mislocalized and misfolded proteins at the INM. Here, we review what is known about INM-associated degradation, including recent evidence suggesting that it not only targets mislocalized or misfolded proteins, but also contributes to homeostasis of resident INM proteins.

Structure and Assembly of the Nuclear Pore Complex

Nuclear pore complexes (NPCs) mediate nucleocytoplasmic exchange. They are exceptionally large protein complexes that fuse the inner and outer nuclear membranes to form channels across the nuclear envelope. About 30 different protein components, termed nucleoporins, assemble in multiple copies into an intricate cylindrical architecture. Here, we review our current knowledge of the structure of nucleoporins and how those come together in situ. We delineate architectural principles on several hierarchical organization levels, including isoforms, posttranslational modifications, nucleoporins, and higher-order oligomerization of nucleoporin subcomplexes. We discuss how cells exploit this modularity to faithfully assemble NPCs.

Polymer translocation through a hairy channel mimicking the inner plug of a nuclear pore complex

A microscopic transport model of a polymer translocating through a nuclear pore complex (NPC) is presented based on self-consistent field theory (SCFT), with the NPC and its nucleoporins mimicked by a hairy channel. Multiple cell environment effects (electrolyte effect, excluded volume effect, NPC drag effect, and hydrophobic effect) are all considered in this hairy channel model. The influence of various parameters (polymer chain length, length of NPC, strength of hydrophobic effect, and excluded volume effect) on translocation time is studied through theoretical analysis and numerical calculation. Numerical simulation results show that an area of low nucleoporin number density exists in the NPC, which facilitates the translocation of the polymer. The results also show that the translocation time curves with increasing NPC length and polymer charge number are concave. In addition, there are critical values for NPC length and polymer charge number for which the translocation time has a minimal value. The translocation time decreases with the increasing strength of the hydrophobic effect and excluded volume effect.

Engineering metabolite-responsive transcriptional factors to sense small molecules in eukaryotes: current state and perspectives

Nature has evolved exquisite sensing mechanisms to detect cellular and environmental signals surrounding living organisms. These biosensors have been widely used to sense small molecules, detect environmental cues and diagnose disease markers. Metabolic engineers and synthetic biologists have been able to exploit metabolites-responsive transcriptional factors (MRTFs) as basic tools to rewire cell metabolism, reprogram cellular activity as well as boost cell’s productivity. This is commonly achieved by integrating sensor-actuator systems with biocatalytic functions and dynamically allocating cellular resources to drive carbon flux toward the target pathway. Up to date, most of identified MRTFs are derived from bacteria. As an endeavor to advance intelligent biomanufacturing in yeast cell factory, we will summarize the opportunities and challenges to transfer the bacteria-derived MRTFs to expand the small-molecule sensing capability in eukaryotic cells. We will discuss the design principles underlying MRTF-based biosensors in eukaryotic cells, including the choice of reliable reporters and the characterization tools to minimize background noise, strategies to tune the sensor dynamic range, sensitivity and specificity, as well as the criteria to engineer activator and repressor-based biosensors. Due to the physical separation of transcription and protein expression in eukaryotes, we argue that nuclear import/export mechanism of MRTFs across the nuclear membrane plays a critical role in regulating the MRTF sensor dynamics. Precisely-controlled MRTF response will allow us to repurpose the vast majority of transcriptional factors as molecular switches to achieve temporal or spatial gene expression in eukaryotes. Uncovering this knowledge will inform us fundamental design principles to deliver robust cell factories and enable the design of reprogrammable and predictable biological systems for intelligent biomanufacturing, smart therapeutics or precision medicine in the foreseeable future.

Nucleoporin Seh1 Interacts with Olig2/Brd7 to Promote Oligodendrocyte Differentiation and Myelination

Nucleoporins (Nups) are involved in neural development, and alterations in Nup genes are linked to human neurological diseases. However, physiological functions of specific Nups and the underlying mechanisms involved in these processes remain elusive. Here, we show that tissue-specific depletion of the nucleoporin Seh1 causes dramatic myelination defects in the CNS. Although proliferation is not altered in Seh1-deficient oligodendrocyte progenitor cells (OPCs), they fail to differentiate into mature oligodendrocytes, which impairs myelin production and remyelination after demyelinating injury. Genome-wide analyses show that Seh1 regulates a core myelinogenic regulatory network and establishes an accessible chromatin landscape. Mechanistically, Seh1 regulates OPCs differentiation by assembling Olig2 and Brd7 into a transcription complex at nuclear periphery. Together, our results reveal that Seh1 is required for oligodendrocyte differentiation and myelination by promoting assembly of an Olig2-dependent transcription complex and define a nucleoporin as a key player in the CNS.

Deciphering Nuclear Mechanobiology in Laminopathy

Extracellular mechanical stimuli are translated into biochemical signals inside the cell via mechanotransduction. The nucleus plays a critical role in mechanoregulation, which encompasses mechanosensing and mechanotransduction. The nuclear lamina underlying the inner nuclear membrane not only maintains the structural integrity, but also connects the cytoskeleton to the nuclear envelope. Lamin mutations, therefore, dysregulate the nuclear response, resulting in abnormal mechanoregulations, and ultimately, disease progression. Impaired mechanoregulations even induce malfunction in nuclear positioning, cell migration, mechanosensation, as well as differentiation. To know how to overcome laminopathies, we need to understand the mechanisms of laminopathies in a mechanobiological way. Recently, emerging studies have demonstrated the varying defects from lamin mutation in cellular homeostasis within mechanical surroundings. Therefore, this review summarizes recent findings highlighting the role of lamins, the architecture of nuclear lamina, and their disease relevance in the context of nuclear mechanobiology. We will also provide an overview of the differentiation of cellular mechanics in laminopathy.

The Role of Cohesiveness in the Permeability of the Spatial Assemblies of FG Nucleoporins

Nuclear pore complexes (NPCs) conduct selective, bidirectional transport across the nuclear envelope. The NPC passageway is lined by intrinsically disordered proteins that contain hydrophobic phenylalanine-glycine (FG) motifs, known as FG nucleoporins (FG nups), that play the key role in the NPC transport mechanism. Cohesive interactions among the FG nups, which arise from the combination of hydrophobic, electrostatic, and other forces, have been hypothesized to control the morphology of the assemblies of FG nups in the NPC, as well as their permeability with respect to the transport proteins. However, the role of FG nup cohesiveness is still vigorously debated. Using coarse-grained polymer theory and numerical simulations, we study the effects of cohesiveness on the selective permeability of in vitro FG nup assemblies in different geometries that have served as proxies for the morphological and transport properties of the NPC. We show that in high-density FG nup assemblies, increase in cohesiveness leads to the decrease in their permeability, in accordance with the accepted view. On the other hand, the permeability of low-density assemblies is a nonmonotonic function of the cohesiveness, and a moderate increase in cohesiveness can enhance permeability. The density- and cohesiveness-dependent effects on permeability are explained by considering the free-energy cost associated with penetrating the FG nup assemblies. We discuss the implications of these findings for the organization and function of the NPC.

Global profiling of megalocytivirus-induced proteins in tongue sole (Cynoglossus semilaevis) spleen identifies cellular processes essential to viral infection

Megalocytivirus is a DNA virus with a broad host range among farmed fish including tongue sole (Cynoglossus semilaevis). In this study, label-free proteomics was performed to examine protein expression in tongue sole spleen induced by megalocytivirus at 8 and 12 days post infection (dpi). Compared to uninfected control fish, virus-infected fish displayed 315 up-regulated proteins and 111 down-regulated proteins at 8 dpi, and 48 up-regulated proteins and 43 down-regulated proteins at 12 dpi. The expressions of five differentially expressed proteins were confirmed by Western blot. The differentially expressed proteins were enriched in the pathways and processes associated with innate immune response and viral infection. Interference with the expression of two up-regulated proteins of the ubiquitin proteasome system (UPS), i.e. proteasome assembly chaperone 2 and proteasome maturation protein, significantly reduced viral propagation in fish, whereas overexpression of these two proteins significantly enhanced viral propagation. Consistently, inhibition of the functioning of proteasome significantly impaired viral replication in vivo. This study provided the first global protein profile responsive to megalocytivirus in tongue sole, and revealed an essential role of UPS in viral infection.

Modulating CRISPR gene drive activity through nucleocytoplasmic localization of Cas9 in S. cerevisiae

Background

The bacterial CRISPR/Cas genome editing system has provided a major breakthrough in molecular biology. One use of this technology is within a nuclease-based gene drive. This type of system can install a genetic element within a population at unnatural rates. Combatting of vector-borne diseases carried by metazoans could benefit from a delivery system that bypasses traditional Mendelian laws of segregation. Recently, laboratory studies in fungi, insects, and even mice, have demonstrated successful propagation of CRISPR gene drives and the potential utility of this type of mechanism. However, current gene drives still face challenges including evolved resistance, containment, and the consequences of application in wild populations. Additional research into molecular mechanisms that would allow for control, titration, and inhibition of drive systems is needed.

Results

In this study, we use artificial gene drives in budding yeast to explore mechanisms to modulate nuclease activity of Cas9 through its nucleocytoplasmic localization. We examine non-native nuclear localization sequences (both NLS and NES) on Cas9 fusion proteins in vivo through fluorescence microscopy and genomic editing. Our results demonstrate that mutational substitutions to nuclear signals and combinatorial fusions can both modulate the level of gene drive activity within a population of cells.

Conclusions

These findings have implications for control of traditional nuclease-dependent editing and use of gene drive systems within other organisms. For instance, initiation of a nuclear export mechanism to Cas9 could serve as a molecular safeguard within an active gene drive to reduce or eliminate editing.

Nuclear export of mRNA molecules studied by SPEED microscopy

The nuclear exit of messenger RNA (mRNA) molecules through the nuclear pore complex (NPC) is an essential step in the translation process of all proteins. The current limitations of conventional fluorescence and electron microscopy have prevented elucidation of how mRNA exports through the NPCs of live cells. In the recent years, various single-molecule fluorescence (SMF) microscopy techniques have been developed to improve the temporal and spatial resolutions of live-cell imaging allowing a more comprehensive understanding of the dynamics of mRNA export through native NPCs. In this review, we firstly evaluate the necessity of single-molecule live-cell microscopy in the study of mRNA nuclear export. Then, we highlight the application of single-point edge-excitation sub-diffraction (SPEED) microscopy that combines high-speed SMF microscopy and a 2D-to-3D transformation algorithm in the studies of nuclear transport kinetics and route for mRNAs. Finally, we summarize the new features of mRNA nuclear export found with SPEED microscopy as well as the reliability and accuracy of SPEED microscopy in mapping the 3D spatial locations of transport routes adopted by proteins and mRNAs through the NPCs.

TorsinA dysfunction causes persistent neuronal nuclear pore defects

A critical challenge to deciphering the pathophysiology of neurodevelopmental disease is identifying which of the myriad abnormalities that emerge during CNS maturation persist to contribute to long-term brain dysfunction. Childhood-onset dystonia caused by a loss-of-function mutation in the AAA+ protein torsinA exemplifies this challenge. Neurons lacking torsinA develop transient nuclear envelope (NE) malformations during CNS maturation, but no NE defects are described in mature torsinA null neurons. We find that during postnatal CNS maturation torsinA null neurons develop mislocalized and dysfunctional nuclear pore complexes (NPC) that lack NUP358, normally added late in NPC biogenesis. SUN1, a torsinA-related molecule implicated in interphase NPC biogenesis, also exhibits localization abnormalities. Whereas SUN1 and associated nuclear membrane abnormalities resolve in juvenile mice, NPC defects persist into adulthood. These findings support a role for torsinA function in NPC biogenesis during neuronal maturation and implicate altered NPC function in dystonia pathophysiology.

Detection of mRNAs Anchored to the Nuclear Envelope During Export Inhibition in Living Cells

Export of mRNA transcripts from the cell nucleus is a complex and multistep process, regulated by various proteins and control mechanisms. Recent studies have demonstrated the rapid passage of mRNA-protein complexes (mRNPs) through the nuclear pore complex (NPC) as well as the ability to detect mRNPs stalled at the NPC during inhibition of the mRNA export process. In this chapter, we describe ways to block mRNA export and present an image analysis method to identify mRNPs stuck at the NPC during such blocks. Using the MS2 mRNA-tagging system to track single mRNPs in living cells we are able to examine their intracellular distribution and dynamics both in the nucleoplasm and at the nuclear periphery. We use this method to identify and count the number of static mRNPs anchored to the nuclear envelope under different conditions of mRNA export inhibition.