Importins and Beyond: Non-Conventional Nuclear Transport Mechanisms

Gasdermin D licenses MHCII induction to maintain food tolerance in small intestine

The intestinal epithelial cells (IECs) constitute the primary barrier between host cells and numerous foreign antigens; it is unclear how IECs induce the protective immunity against pathogens while maintaining the immune tolerance to food. Here, we found IECs accumulate a less recognized 13-kD N-terminal fragment of GSDMD that is cleaved by caspase-3/7 in response to dietary antigens. Unlike the 30-kD GSDMD cleavage fragment that executes pyroptosis, the IEC-accumulated GSDMD cleavage fragment translocates to the nucleus and induces the transcription of CIITA and MHCII molecules, which in turn induces the Tr1 cells in upper small intestine. Mice treated with a caspase-3/7 inhibitor, mice with GSDMD mutation resistant to caspase-3/7 cleavage, mice with MHCII deficiency in IECs, and mice with Tr1 deficiency all displayed a disrupted food tolerance phenotype. Our study supports that differential cleavage of GSDMD can be understood as a regulatory hub controlling immunity versus tolerance in the small intestine.

Nuclear Delivery of Nanoparticle-Based Drug Delivery Systems by Nuclear Localization Signals

Nanomedicine 2.0 refers to the next generation of nanotechnology-based medical therapies and diagnostic tools. This field focuses on the development of more sophisticated and precise nanoparticles (NPs) for targeted drug delivery, imaging, and sensing. It has been established that the nuclear delivery of NP-loaded drugs can increase their therapeutic efficacy. To effectively direct the NPs to the nucleus, the attachment of nuclear localization signals (NLSs) to NPs has been employed in many applications. In this review, we will provide an overview of the structure of nuclear pore complexes (NPCs) and the classic nuclear import mechanism. Additionally, we will explore various nanoparticles, including their synthesis, functionalization, drug loading and release mechanisms, nuclear targeting strategies, and potential applications. Finally, we will highlight the challenges associated with developing nucleus-targeted nanoparticle-based drug delivery systems (NDDSs) and provide insights into the future of NDDSs.

Inhibition of importin-7 attenuates ventilator-induced lung injury by targeting nuclear translocation of p38

BACKGROUND

The ability of p38 to phosphorylate substrates in the nucleus and the role of nuclear p38 in the regulation of inflammation have focused attention on the subcellular localization of the kinase. Although it is clear that p38 shuttles to the nucleus upon stimulation, the mechanisms that regulate p38 nuclear input in response to mechanical stretch remain to be determined.

METHODS

Cyclic stretch (CS)-induced nuclear translocation of p38 was determined by Western blotting and immunofluorescence. The p38 interacting protein was identified using endogenous pull-down and protein binding assays. The potential role of importin-7 (Imp7) in CS-induced nuclear translocation of p38 and p38-dependent gene expression was confirmed using a series of in vitro and in vivo experiments. Furthermore, we tested the therapeutic potential of intratracheal administration of Imp7 siRNA-loaded nanoparticles in the ventilator-induced lung injury (VILI) mouse model.

RESULTS

We show that CS induced phosphorylation-dependent nuclear translocation of p38, which required the involvement of microtubules and dynein. Endogenous pull-down assay revealed Imp7 to be a potential p38-interacting protein, and the direct interaction between p38 and Imp7 was confirmed by in vitro and in vivo binding assays. Furthermore, silencing Imp7 inhibited CS-induced nuclear translocation of p38 and subsequent cytokine production. Notably, intratracheal administration of Imp7 siRNA nanoparticles attenuated lung inflammation and histological damage in the VILI mouse model.

CONCLUSIONS

Our findings uncover a key role for Imp7 in the process of p38 nuclear import after CS stimulation and highlight the potential of preventing p38 nuclear translocation in treatment of VILI.

Elucidating Biological Functions of 9-cis-Epoxycarotenoid Dioxygenase Genes Involved in Seed Dormancy in Paeonia lactiflora

Abscisic acid (ABA) is a major phytohormone affecting seed dormancy and germination in plants. ABA is synthesized mainly through the C40 carotenoid pathway. In the ABA biosynthesis pathway, 9-cis-epoxycarotenoid dioxygenase (NCED) is a key rate-limiting enzyme that regulates the accumulation and content of ABA. However, the role of the NCED gene in perennial plants with complex seed dormancy remains largely unknown. Here, we cloned two differentially expressed paralogs of herbaceous peony NCED genes, named PlNCED1 and PlNCED2, and further identified their involvement in seed dormancy from perennial herbaceous peony experiencing complex double seed dormancy. The deduced PlNCED amino acid sequences had high sequence homology with NCED sequences from other plants and contained the typical conserved RPE65 domain of the NCED family. Phylogenetic analysis showed that PlNCED1 and PlNCED2 have a close relationship with PoNCED in Paeonia ostii and VvNCED6 in Vitis vinifera, respectively. A subcellular localization assay demonstrated that the PlNCED1 protein resided within the nucleus, while the PlNCED2 protein was located in the cytoplasm, indicating their different roles in the biosynthesis of ABA. Furthermore, the content of endogenous ABA in transgenic calluses showed that PlNCEDs were positively correlated with ABA content. Both PlNCED transgenic Arabidopsis lines and the functional complementation of Arabidopsis NCED mutants found that PlNCEDs promoted seed dormancy and delayed seed germination. These results reveal that PlNCEDs participate in the seed dormancy of herbaceous peony by regulating the accumulation of endogenous ABA.

The Nuclear Transporter Importin 13 Can Regulate Stress-Induced Cell Death through the Clusterin/KU70 Axis

The cellular response to environmental stresses, such as heat and oxidative stress, is dependent on extensive trafficking of stress-signalling molecules between the cytoplasm and nucleus, which potentiates stress-activated signalling pathways, eventually resulting in cell repair or death. Although Ran-dependent nucleocytoplasmic transport mediated by members of the importin (IPO) super family of nuclear transporters is believed to be responsible for nearly all macromolecular transit between nucleus and cytoplasm, it is paradoxically known to be significantly impaired under conditions of stress. Importin 13 (IPO13) is a unique bidirectional transporter that binds to and releases cargo in a Ran-dependent manner, but in some cases, cargo release from IPO13 is affected by loading of another cargo. To investigate IPO13's role in stress-activated pathways, we performed cell-based screens to identify a multitude of binding partners of IPO13 from human brain, lung, and testes. Analysis of the IPO13 interactome intriguingly indicated more than half of the candidate binding partners to be annotated for roles in stress responses; these included the pro-apoptotic protein nuclear clusterin (nCLU), as well as the nCLU-interacting DNA repair protein KU70. Here, we show, for the first time, that unlike other IPOs which are mislocalised and non-functional, IPO13 continues to translocate between the nucleus and cytoplasm under stress, retaining the capacity to import certain cargoes, such as nCLU, but not export others, such as KU70, as shown by analysis using fluorescence recovery after photobleaching. Importantly, depletion of IPO13 reduces stress-induced import of nCLU and protects against stress-induced cell death, with concomitant protection from DNA damage during stress. Overexpression/FACS experiments demonstrate that nCLU is dependent on IPO13 to trigger stress-induced cell death via apoptosis. Taken together, these results implicate IPO13 as a novel functional nuclear transporter in cellular stress, with a key role thereby in cell fate decision.

Organelle-targeted therapies: a comprehensive review on system design for enabling precision oncology

Cancer is a major threat to human health. Among various treatment methods, precision therapy has received significant attention since the inception, due to its ability to efficiently inhibit tumor growth, while curtailing common shortcomings from conventional cancer treatment, leading towards enhanced survival rates. Particularly, organelle-targeted strategies enable precise accumulation of therapeutic agents in organelles, locally triggering organelle-mediated cell death signals which can greatly reduce the therapeutic threshold dosage and minimize side-effects. In this review, we comprehensively discuss history and recent advances in targeted therapies on organelles, specifically including nucleus, mitochondria, lysosomes and endoplasmic reticulum, while focusing on organelle structures, organelle-mediated cell death signal pathways, and design guidelines of organelle-targeted nanomedicines based on intervention mechanisms. Furthermore, a perspective on future research and clinical opportunities and potential challenges in precision oncology is presented. Through demonstrating recent developments in organelle-targeted therapies, we believe this article can further stimulate broader interests in multidisciplinary research and technology development for enabling advanced organelle-targeted nanomedicines and their corresponding clinic translations.

Nuclear Transporter IPO13 Is Central to Efficient Neuronal Differentiation

Molecular transport between the nucleus and cytoplasm of the cell is mediated by the importin superfamily of transport receptors, of which the bidirectional transporter Importin 13 (IPO13) is a unique member, with a critical role in early embryonic development through nuclear transport of key regulators, such as transcription factors Pax6, Pax3, and ARX. Here, we examined the role of IPO13 in neuronal differentiation for the first time, using a mouse embryonic stem cell (ESC) model and a monolayer-based differentiation protocol to compare IPO13^−/− to wild type ESCs. Although IPO13^−/− ESCs differentiated into neural progenitor cells, as indicated by the expression of dorsal forebrain progenitor markers, reduced expression of progenitor markers Pax6 and Nestin compared to IPO13^−/− was evident, concomitant with reduced nuclear localisation/transcriptional function of IPO13 import cargo Pax6. Differentiation of IPO13^−/− cells into neurons appeared to be strongly impaired, as evidenced by altered morphology, reduced expression of key neuronal markers, and altered response to the neurotransmitter glutamate. Our findings establish that IPO13 has a key role in ESC neuronal differentiation, in part through the nuclear transport of Pax6.

Structure, Maintenance, and Regulation of Nuclear Pore Complexes: The Gatekeepers of the Eukaryotic Genome

In eukaryotic cells, the genetic material is segregated inside the nucleus. This compartmentalization of the genome requires a transport system that allows cells to move molecules across the nuclear envelope, the membrane-based barrier that surrounds the chromosomes. Nuclear pore complexes (NPCs) are the central component of the nuclear transport machinery. These large protein channels penetrate the nuclear envelope, creating a passage between the nucleus and the cytoplasm through which nucleocytoplasmic molecule exchange occurs. NPCs are one of the largest protein assemblies of eukaryotic cells and, in addition to their critical function in nuclear transport, these structures also play key roles in many cellular processes in a transport-independent manner. Here we will review the current knowledge of the NPC structure, the cellular mechanisms that regulate their formation and maintenance, and we will provide a brief description of a variety of processes that NPCs regulate.

Nuclear Exportin 1 (XPO1) Binds to the Nuclear Localization/Export Signal of the Turnip Mosaic Virus NIb to Promote Viral Infection

The nuclear localization signal (NLS) and nuclear export signal (NES) are key signatures of proteins for controlling nuclear import and export. The NIb protein of turnip mosaic virus (TuMV) is an RNA-dependent RNA polymerase (RdRP) that is absolutely required for viral genome replication. Previous studies have shown that NIb is a nucleocytoplasmic shuttling protein and contains four putative NES and four putative NLS motifs. Here, we analyzed the function of these NESs and NLSs, and identified two functional NESs and one functional NLS. Mutation of the identified functional NESs or NLS inhibited viral RNA accumulation and systemic infection. Exportin 1 (XPO1) is a nuclear export receptor that binds directly to cargo proteins harboring a leucine-rich NES and translocates them to the cytoplasm. We found that XPO1 contains two NIb-binding domains, which recognize the NLS and NES of NIb, respectively, to mediate the nucleocytoplasmic transport of NIb and promote viral infection. Taken together, these data suggest that the nucleocytoplasmic transport of NIb is modulated by XPO1 through its interactions with the functional NLS and NES of NIb to promote viral infection.

Bimolecular Fluorescence Complementation: Quantitative Analysis of In Cell Interaction of Nuclear Transporter Importin α with Cargo Proteins

Bimolecular fluorescence complementation utilizes the ability of two complementary nonfluorescent fragments to reconstitute and emit fluorescence when brought together through specific interaction of attached protein fragments of interest. It has been used in several different contexts to study protein-protein interaction. Here we apply the method for the first time to study interaction of the nuclear transporter importin α and its cargoes in a cellular context. By using image analysis to quantify the extent of nuclear complexation, it is possible to gain insight into the strength of interaction in cells.

Nuclear transport genes recurrently duplicate by means of RNA intermediates in Drosophila but not in other insects

Background

The nuclear transport machinery is involved in a well-known male meiotic drive system in Drosophila. Fast gene evolution and gene duplications have been major underlying mechanisms in the evolution of meiotic drive systems, and this might include some nuclear transport genes in Drosophila. So, using a comprehensive, detailed phylogenomic study, we examined 51 insect genomes for the duplication of the same nuclear transport genes.

Results

We find that most of the nuclear transport duplications in Drosophila are of a few classes of nuclear transport genes, RNA mediated and fast evolving. We also retrieve many pseudogenes for the Ran gene. Some of the duplicates are relatively young and likely contributing to the turnover expected for genes under strong but changing selective pressures. These duplications are potentially revealing what features of nuclear transport are under selection. Unlike in flies, we find only a few duplications when we study the Drosophila duplicated nuclear transport genes in dipteran species outside of Drosophila, and none in other insects.

Conclusions

These findings strengthen the hypothesis that nuclear transport gene duplicates in Drosophila evolve either as drivers or suppressors of meiotic drive systems or as other male-specific adaptations circumscribed to flies and involving a handful of nuclear transport functions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08170-4.

PACS-1 contains distinct motifs for nuclear-cytoplasmic transport and interacts with the RNA-binding protein PTBP1 in the nucleus and cytosol

Phosphofurin acidic cluster sorting protein 1 (PACS-1) is canonically a cytosolic trafficking protein, yet recent reports have described nuclear roles for PACS-1. Herein, we sought to define the nuclear transport mechanism of PACS-1. We demonstrate that PACS-1 nucleocytoplasmic trafficking is dependent on its interaction with the nuclear transport receptors importin alpha 5 and exportin 1. PACS-1 nuclear entry and exit are defined by a nuclear localization signal (NLS, residues 311-318) and nuclear export signal (NES3, residues 366-375). Mutation of the PACS-1 NLS and NES3 altered the localization of a complex formed between PACS-1 and an RNA-binding protein, polypyrimidine tract-binding protein 1. Overall, we identify the nuclear localization mechanism of PACS-1 and highlight a potential role for PACS-1 in RNA-binding protein trafficking.

Nuclear import of BCL11B is mediated by a classical nuclear localization signal and not the Krüppel-like zinc fingers

The Krüppel-like transcription factor BCL11B is characterized by wide tissue distribution and crucial functions in key developmental and cellular processes and various pathologies including cancer or HIV infection. Although basics of BCL11B activity and relevant interactions with other proteins were uncovered, how this exclusively nuclear protein localizes to its compartment remained unclear. Here, we demonstrate that unlike other KLFs, BCL11B does not require the C-terminal DNA-binding domain to pass through the nuclear envelope but encodes an independent, previously unidentified nuclear localization signal (NLS) which is located distantly from the zinc finger domains and fulfills the essential criteria of an autonomous NLS. First, it can redirect a heterologous cytoplasmic protein to the nucleus. Second, its mutations cause aberrant localization of the protein of origin. Finally, we provide experimental and in silico evidences of the direct interaction with importin alpha. The relative conservation of this motif allows formulating a consensus sequence (K/R)K-X13-14-KR+K++ which can be found in all BCL11B orthologues among vertebrates and in the closely related protein BCL11A.

Assembly and recognition of keratins: A structural perspective

Keratins are one of the major components of cytoskeletal network and assemble into fibrous structures named intermediate filaments (IFs), which are important for maintaining the mechanical properties of cells and tissues. Over the past decades, evidence has shown that the functions of keratins go beyond providing mechanical support for cells, they interact with multiple cellular components and are widely involved in the pathways of cell proliferation, differentiation, motility and death. However, the structural details of keratins and IFs are largely missing and many questions remain regarding the mechanisms of keratin assembly and recognition. Here we briefly review the current structural models and assembly of keratins as well as the interactions of keratins with the binding partners, which may provide a structural view for understanding the mechanisms of keratins in the biological activities and the related diseases.

Cargo transport through the nuclear pore complex at a glance

Bidirectional transport of macromolecules across the nuclear envelope is a hallmark of eukaryotic cells, in which the genetic material is compartmentalized inside the nucleus. The nuclear pore complex (NPC) is the major gateway to the nucleus and it regulates nucleocytoplasmic transport, which is key to processes including transcriptional regulation and cell cycle control. Accordingly, components of the nuclear transport machinery are often found to be dysregulated or hijacked in diseases. In this Cell Science at a Glance article and accompanying poster, we provide an overview of our current understanding of cargo transport through the NPC, from the basic transport signals and machinery to more emerging aspects, all from a 'cargo perspective'. Among these, we discuss the transport of large cargoes (>15 nm), as well as the roles of different cargo properties to nuclear transport, from size and number of bound nuclear transport receptors (NTRs), to surface and mechanical properties.

Identification of a non-classical three-dimensional nuclear localization signal in the intestinal fatty acid binding protein

The intestinal fatty acid binding protein (FABP) is a small protein expressed along the small intestine that bind long-chain fatty acids and other hydrophobic ligands. Several lines of evidence suggest that, once in the nucleus, it interacts with nuclear receptors, activating them and thus transferring the bound ligand into the nucleus. Previous work by our group suggests that FABP2 would participate in the cytoplasm-nucleus translocation of fatty acids. Because the consensus NLS is absent in the sequence of FABP2, we propose that a 3D signal could be responsible for its nuclear translocation. The results obtained by transfection assays of recombinant wild type and mutated forms of Danio rerio Fabp2 in Caco-2 cell cultures, showed that lysine 17, arginine 29 and lysine 30 residues, which are located in the helix-turn-helix region, would constitute a functional non-classical three-dimensional NLS.

Dynein-mediated nuclear translocation of yes-associated protein through microtubule acetylation controls fibroblast activation

Myofibroblasts are the major cell type that is responsible for increase in the mechanical stiffness in fibrotic tissues. It has well documented that the TGF-β/Smad axis is required for myofibroblast differentiation under the rigid substrate condition. However, the mechanism driving myofibroblast differentiation in soft substrates remains unknown. In this research, we demonstrated that interaction of yes-associated protein (YAP) and acetylated microtubule via dynein, a microtubule motor protein drives nuclear localization of YAP in the soft matrix, which in turn increased TGF-β1-induced transcriptional activity of Smad for myofibroblast differentiation. Pharmacological and genetical disruption of dynein impaired the nuclear translocation of YAP and decreased the TGF-β1-induced Smad activity even though phosphorylation and nuclear localization of Smad occurred normally in α-tubulin acetyltransferase 1 (α-TAT1) knockout cell. Moreover, microtubule acetylation prominently appeared in the fibroblast-like cells nearby the blood vessel in the fibrotic liver induced by CCl4 administration, which was conversely decreased by TGF-β receptor inhibitor. As a result, quantitative inhibition of microtubule acetylation may be suggested as a new target for overcoming fibrotic diseases.

Analysis of rice nuclear-localized seed-expressed proteins and their database (RSNP-DB)

Nuclear proteins are primarily regulatory factors governing gene expression. Multiple factors determine the localization of a protein in the nucleus. An upright identification of nuclear proteins is way far from accuracy. We have attempted to combine information from subcellular prediction tools, experimental evidence, and nuclear proteome data to identify a reliable list of seed-expressed nuclear proteins in rice. Depending upon the number of prediction tools calling a protein nuclear, we could sort 19,441 seed expressed proteins into five categories. Of which, half of the seed-expressed proteins were called nuclear by at least one out of four prediction tools. Further, gene ontology (GO) enrichment and transcription factor composition analysis showed that 6116 seed-expressed proteins could be called nuclear with a greater assertion. Localization evidence from experimental data was available for 1360 proteins. Their analysis showed that a 92.04% accuracy of a nuclear call is valid for proteins predicted nuclear by at least three tools. Distribution of nuclear localization signals and nuclear export signals showed that the majority of category four members were nuclear resident proteins, whereas other categories have a low fraction of nuclear resident proteins and significantly higher constitution of shuttling proteins. We compiled all the above information for the seed-expressed genes in the form of a searchable database named Rice Seed Nuclear Protein DataBase (RSNP-DB) https://pmb.du.ac.in/rsnpdb. This information will be useful for comprehending the role of seed nuclear proteome in rice.

Systematic Discovery of Short Linear Motifs Decodes Calcineurin Phosphatase Signaling

Short linear motifs (SLiMs) drive dynamic protein-protein interactions essential for signaling, but sequence degeneracy and low binding affinities make them difficult to identify. We harnessed unbiased systematic approaches for SLiM discovery to elucidate the regulatory network of calcineurin (CN)/PP2B, the Ca²⁺-activated phosphatase that recognizes LxVP and PxIxIT motifs. In vitro proteome-wide detection of CN-binding peptides, in vivo SLiM-dependent proximity labeling, and in silico modeling of motif determinants uncovered unanticipated CN interactors, including NOTCH1, which we establish as a CN substrate. Unexpectedly, CN shows SLiM-dependent proximity to centrosomal and nuclear pore complex (NPC) proteins-structures where Ca²⁺ signaling is largely uncharacterized. CN dephosphorylates human and yeast NPC proteins and promotes accumulation of a nuclear transport reporter, suggesting conserved NPC regulation by CN. The CN network assembled here provides a resource to investigate Ca²⁺ and CN signaling and demonstrates synergy between experimental and computational methods, establishing a blueprint for examining SLiM-based networks.

Agonist binding directs dynamic competition among nuclear receptors for heterodimerization with retinoid X receptor

Retinoid X receptor (RXR) plays a pivotal role as a transcriptional regulator and serves as an obligatory heterodimerization partner for at least 20 other nuclear receptors (NRs). Given a potentially limiting/sequestered pool of RXR and simultaneous expression of several RXR partners, we hypothesized that NRs compete for binding to RXR and that this competition is directed by specific agonist treatment. Here, we tested this hypothesis on three NRs: peroxisome proliferator-activated receptor gamma (PPARγ), vitamin D receptor (VDR), and retinoic acid receptor alpha (RARα). The evaluation of competition relied on a nuclear translocation assay applied in a three-color imaging model system by detecting changes in heterodimerization between RXRα and one of its partners (NR1) in the presence of another competing partner (NR2). Our results indicated dynamic competition between the NRs governed by two mechanisms. First, in the absence of agonist treatment, there is a hierarchy of affinities between RXRα and its partners in the following order: RARα > PPARγ > VDR. Second, upon agonist treatment, RXRα favors the liganded partner. We conclude that recruiting RXRα by the liganded NR not only facilitates a stimulus-specific cellular response but also might impede other NR pathways involving RXRα.

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.

Multifunctional Nanoparticles in Precise Cancer Treatment: Considerations in Design and Functionalization of Nanocarriers

Nanotechnology has revolutionized cancer treatment in both diagnosis and therapy. Since the initial application of nanoparticles (NPs) in cancer treatment, the main objective of nanotechnology was developing effective nanosystems with high selectivity and specificity for cancer treatment and diagnosis. To achieve this, different encapsulation and conjugation strategies along with surface functionalization techniques have been developed to synthesize anticancer drugs loaded NPs with effective targeting to specific tumor cells. The unique physicochemical attributes of NPs make them promising candidates for targeted drug delivery, localized therapies, sensing, and targeting at cellular levels. However, a nanosystem for localized and targeted cancer managements should overcome several biological barriers and biomedical challenges such as endothelial barriers, blood brain barrier, reticuloendothelial system, selective targeting, biocompatibility, acute/chronic toxicity, tumor-targeting efficacy. The NPs for in vivo applications encounter barriers at system, organ, and the cellular level. To overcome these barriers, different strategies during the synthesis and functionalization of NPs should be adapted. Pharmacokinetics and cellular uptake of NPs are largely associated with physicochemical attributes of NPs, morphology, hydrodynamic size, charge, and other surface properties. These properties can be adjusted during different phases of synthesis and functionalization of the NPs. This study reviews the advances in targeted cancer treatment and the parameters influencing the efficacies of NPs as therapeutics. Different strategies for overcoming the biological barriers at cellular, organ and system levels and biomedical challenges are discussed. Moreover, the applications of NPs in preclinical and clinical practice are reviewed.

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.

RIP at the Synapse and the Role of Intracellular Domains in Neurons

Regulated intramembrane proteolysis (RIP) occurs in a cell when transmembrane proteins are cleaved by intramembrane proteases such as secretases to generate soluble protein fragments in the extracellular environment and the cytosol. In the cytosol, these soluble intracellular domains (ICDs) have local functions near the site of cleavage or in many cases, translocate to the nucleus to modulate gene expression. While the mechanism of RIP is relatively well studied, the fate and function of ICDs for most substrate proteins remain poorly characterized. In neurons, RIP occurs in various subcellular compartments including at the synapse. In this review, we summarize current research on RIP in neurons, focusing specifically on synaptic proteins where the presence and function of the ICDs have been reported. We also briefly discuss activity-driven processing of RIP substrates at the synapse and the cellular machinery that support long-distance transport of ICDs from the synapse to the nucleus. Finally, we describe future challenges in this field of research in the context of understanding the contribution of ICDs in neuronal function.

Viral Appropriation: Laying Claim to Host Nuclear Transport Machinery

Protein nuclear transport is an integral process to many cellular pathways and often plays a critical role during viral infection. To overcome the barrier presented by the nuclear membrane and gain access to the nucleus, virally encoded proteins have evolved ways to appropriate components of the nuclear transport machinery. By binding karyopherins, or the nuclear pore complex, viral proteins influence their own transport as well as the transport of key cellular regulatory proteins. This review covers how viral proteins can interact with different components of the nuclear import machinery and how this influences viral replicative cycles. We also highlight the effects that viral perturbation of nuclear transport has on the infected host and how we can exploit viruses as tools to study novel mechanisms of protein nuclear import. Finally, we discuss the possibility that drugs targeting these transport pathways could be repurposed for treating viral infections.

Identification of a putative nuclear localization signal in the tumor suppressor maspin sheds light on its nuclear import regulation

The tumor suppressor activity of maspin (mammary serine protease inhibitor) has been associated with its nuclear localization. In this study we explore the regulation of maspin nuclear translocation. An in vitro nuclear import assay suggested that maspin can passively enter the nucleus. However, in silico analysis identified a putative maspin nuclear localization signal (NLS), which was able to mediate the nuclear translocation of a chimeric protein containing this NLS fused to five green fluorescent protein molecules in tandem (5GFP). Dominant‐negative Ran‐GTPase mutants RanQ69L or RanT24N suppressed this process. Unexpectedly, the full‐length maspin fused to 5GFP failed to enter the nucleus. As maspin's putative NLS is partially hidden in its three‐dimensional structure, we suggest that maspin nuclear transport could be conformationally regulated. Our results suggest that maspin nuclear translocation involves both passive and active mechanisms.

Protein Phosphatase 1α and Cofilin Regulate Nuclear Translocation of NF-κB and Promote Expression of the Anti-Inflammatory Cytokine Interleukin-10 by T Cells

While several protein serine/threonine kinases control cytokine production by T cells, the roles of serine/threonine phosphatases are largely unexplored. Here, we analyzed the involvement of protein phosphatase 1α (PP1α) in cytokine synthesis following costimulation of primary human T cells. Small interfering RNA (siRNA)-mediated knockdown of PP1α (PP1^KD) or expression of a dominant negative PP1α (D95N-PP1) drastically diminished interleukin-10 (IL-10) production. Focusing on a key transcriptional activator of human IL-10, we demonstrate that nuclear translocation of NF-κB was significantly inhibited in PP1^KD or D95N-PP1 cells. Interestingly, knockdown of cofilin, a known substrate of PP1 containing a nuclear localization signal, also prevented nuclear accumulation of NF-κB. Expression of a constitutively active nonphosphorylatable S3A-cofilin in D95N-PP1 cells restored nuclear translocation of NF-κB and IL-10 expression. Subpopulation analysis revealed that defective nuclear translocation of NF-κB was most prominent in CD4⁺ CD45RA^- CXCR3^- T cells that included IL-10-producing T_H2 cells. Together these findings reveal novel functions for PP1α and its substrate cofilin in T cells namely the regulation of the nuclear translocation of NF-κB and promotion of IL-10 production. These data suggest that stimulation of PP1α could limit the overwhelming immune responses seen in chronic inflammatory diseases.

NCAM140 is translocated into the nucleus by an importin-β1-dependent mechanism

The neural cell adhesion molecule (NCAM) is important for neural development and for plasticity in adult brain. Previous studies demonstrated a calmodulin-dependent import of a transmembrane fragment of NCAM into the nucleus that regulates gene expression. In a protein macroarray we identified importin-β1 as a potential interaction partner of NCAM's cytoplasmic tail. The interaction was verified and an importin-β1-dependent import of NCAM into the nucleus could be demonstrated using quantitative immunofluorescence analysis. Generation of NCAM deletion mutants revealed that the last amino acids of the cytoplasmic region of NCAM are dispensable whereas other parts of NCAM's cytoplasmic tail take part in its nuclear translocation. With this study we propose an alternative nuclear route for NCAM via the classical importin-mediated import.

An ankyrin-binding motif regulates nuclear levels of L1-type neuroglian and expression of the oncogene Myc in Drosophila neurons

L1 cell adhesion molecule (L1CAM) is well-known for its importance in nervous system development and cancer progression. In addition to its role as a plasma membrane protein in cytoskeletal organization, recent in vitro studies have revealed that both transmembrane and cytosolic fragments of proteolytically cleaved vertebrate L1CAM translocate to the nucleus. In vitro studies indicate that nuclear L1CAM affects genes with functions in DNA post-replication repair, cell cycle control, and cell migration and differentiation, but its in vivo role and how its nuclear levels are regulated is less well-understood. Here, we report that mutations in the conserved ankyrin-binding domain affect nuclear levels of the sole Drosophila homolog neuroglian (Nrg) and that it also has a noncanonical role in regulating transcript levels of the oncogene Myc in the adult nervous system. We further show that altered nuclear levels of Nrg correlate with altered transcript levels of Myc in neurons, similar to what has been reported for human glioblastoma stem cells. However, whereas previous in vitro studies suggest that increased nuclear levels of L1CAM promote tumor cell survival, we found here that elevated levels of nuclear Nrg in neurons are associated with increased sensitivity to oxidative stress and reduced life span of adult animals. We therefore conclude that these findings are of potential relevance to the management of neurodegenerative diseases associated with oxidative stress and cancer.

Investigating nucleo-cytoplasmic shuttling of the human DEAD-box helicase DDX3

The human DEAD-box helicase DDX3 is a multi-functional protein involved in the regulation of gene expression and additional non-conventional roles as signalling adaptor molecule that are independent of its enzymatic RNA remodeling activity. It is a nucleo-cytoplasmic shuttling protein and it has previously been suggested that dysregulation of its subcellular localization could contribute to tumourigenesis. Indeed, both tumour suppressor and oncogenic functions have been attributed to DDX3. In this study, we investigated the regulation of DDX3's nucleocytoplasmic shuttling. We confirmed that an N-terminal conserved Nuclear Export Signal (NES) is required for export of human DDX3 from the nucleus, and identified three regions within DDX3 that can independently facilitate its nuclear import. We also aimed to identify conditions that alter DDX3's subcellular localisation. Viral infection, cytokine treatment and DNA damage only induced minor changes in DDX3's subcellular distribution as determined by High Content Analysis. However, DDX3's nuclear localization increased in early mitotic cells (during prophase) concomitant with an increase in DDX3 expression levels. Our results are likely to have implications for the proposed use of (nuclear) DDX3 as a prognostic biomarker in cancer.

PKA-site phosphorylation of importin13 regulates its subcellular localization and nuclear transport function

Importin 13 (IPO13) is a key member of the importin β superfamily, which can transport cargoes both into and out of the nucleus to contribute to a variety of important cellular processes. IPO13 is known to undergo phosphorylation, but the impact of this on function has not been investigated. Here, we show for the first time that IPO13 is phosphorylated by cAMP-dependent protein kinase A specifically at serine 193. Results from fluorescence recovery after photobleaching and fluorescence loss in photobleaching approaches establish that negative charge at serine 193 through phosphorylation or point mutation both reduces IPO13 nuclear import and increases its nuclear export. Importantly, phosphorylation also appears to enhance cargo interaction on the part of IPO13, with significant impact on localization, as shown for the Pax6 homeobox-containing transcription partner. This is the first report that IPO13 can be phosphorylated at Ser193 and that this modification regulates IPO13 subcellular localization and nucleocytoplasmic transport function, with important implications for IPO13's role in development and other processes.

GaHV-2 ICP22 protein is expressed from a bicistronic transcript regulated by three GaHV-2 microRNAs

Herpesviruses have a lifecycle consisting of successive lytic, latent and reactivation phases. Only three infected cell proteins (ICPs) have been described for the oncogenic Marek's disease virus (or Gallid herpes virus 2, GaHV-2): ICP4, ICP22 and ICP27. We focus here on ICP22, confirming its cytoplasmic location and showing that ICP22 is expressed during productive phases of the lifecycle, via a bicistronic transcript encompassing the US10 gene. We also identified the unique promoter controlling ICP22 expression, and its core promoter, containing functional responsive elements including E-box, ETS-1 and GATA elements involved in ICP22 transactivation. ICP22 gene expression was weakly regulated by DNA methylation and activated by ICP4 or ICP27 proteins. We also investigated the function of GaHV-2 ICP22. We found that this protein repressed transcription from its own promoter and from those of IE ICP4 and ICP27, and the late gK promoter. Finally, we investigated posttranscriptional ICP22 regulation by GaHV-2 microRNAs. We found that mdv1-miR-M5-3p and -M1-5p downregulated ICP22 mRNA expression during latency, whereas, unexpectedly, mdv1-miR-M4-5p upregulated the expression of the protein ICP22, indicating a tight regulation of ICP22 expression by microRNAs.

Adenomatous Polyposis Coli Defines Treg Differentiation and Anti-inflammatory Function through Microtubule-Mediated NFAT Localization

Adenomatous polyposis coli (APC) is a polarity regulator and tumor suppressor associated with familial adenomatous polyposis and colorectal cancer development. Although extensively studied in epithelial transformation, the effect of APC on T lymphocyte activation remains poorly defined. We found that APC ensures T cell receptor-triggered activation through Nuclear Factor of Activated T cells (NFAT), since APC is necessary for NFAT's nuclear localization in a microtubule-dependent fashion and for NFAT-driven transcription leading to cytokine gene expression. Interestingly, NFAT forms clusters juxtaposed with microtubules. Ultimately, mouse Apc deficiency reduces the presence of NFAT in the nucleus of intestinal regulatory T cells (Tregs) and impairs Treg differentiation and the acquisition of a suppressive phenotype, which is characterized by the production of the anti-inflammatory cytokine IL-10. These findings suggest a dual role for APC mutations in colorectal cancer development, where mutations drive the initiation of epithelial neoplasms and also reduce Treg-mediated suppression of the detrimental inflammation that enhances cancer growth.

Nuclear transport in Entamoeba histolytica: knowledge gap and therapeutic potential

Entamoeba histolytica is the protozoan parasite that causes human amoebiasis. It is one of the leading parasitic disease burdens in tropical regions and developing countries, with spread to developed countries through migrants from and travellers to endemic regions.Understanding E. histolytica's invasion mechanisms requires an understanding of how it interacts with external cell components and how it engulfs and kills cells (phagocytosis). Recent research suggests that optimal phagocytosis requires signalling events from the cell surface to the nucleus via the cytoplasm, and the induction of several factors that are transported to the plasma membrane. Current research in other protozoans suggests the presence of proteins with nuclear localization signals, nuclear export signals and Ran proteins; however, there is limited literature on their functionality and their functional similarity to higher eukaryotes.Based on learnings from the development of antivirals, nuclear transport elements in E. histolytica may present viable, specific, therapeutic targets.In this review, we aim to summarize our limited knowledge of the eukaryotic nuclear transport mechanisms that are conserved and may function in E. histolytica.

Alterations of the nuclear transport system in hepatocellular carcinoma - New basis for therapeutic strategies

Hepatocellular carcinoma (HCC) is among the most prevalent human malignancies worldwide with rising incidence in industrialised countries, few therapeutic options and poor prognosis. To expand and improve therapeutic strategies, identification of drug targets involved in several liver cancer-related pathways is crucial. Virtually all signal transduction cascades cross the nuclear envelope and therefore require components of the nuclear transport system (NTS), including nuclear transport receptors (e.g. importins and exportins) and the nuclear pore complex. Accordingly, members of the NTS represent promising targets for therapeutic intervention. Selective inhibitors of nuclear export have already entered clinical trials for various malignancies. Herein, we review the current knowledge regarding alterations of the NTS and their potential for targeted therapy in HCC.

Structural insights into the nuclear import of the histone acetyltransferase males-absent-on-the-first by importin α1

The histone acetyltransferase males-absent-on-the-first (MOF) acetylates the histone H4, a modification important for many biological processes, including chromatin organization, transcriptional regulation, DNA replication, recombination and repair, as well as autophagy. Depletion of MOF induces serious consequences because of the reduction of histone acetylation, such as nuclear morphological defects and cancer. Despite the critical roles of MOF in the nucleus, the structural or functional mechanisms of the nucleocytoplasmic transport of MOF remain elusive. Here, we identified novel importin α1-specific nuclear localization signals (NLSs) in the N-terminal of human MOF. The crystal structure of MOF NLSs in complex with importin α1 further revealed a unique binding mode of MOF, with two independent NLSs binding to importin α1 major and minor sites, respectively. The second NLS of MOF displays an unexpected α-helical conformation in the C-terminus, with more extensive contacts with importin α1 not limited in the minor site. Mutations of the key residues on MOF and importin α1 lead to the reduction of their interaction as well as the nuclear import of MOF, revealing an essential role of NLS2 of MOF in interacting with importin α1 minor site. Taken together, we provide structural mechanisms underlying the nucleocytoplasmic transport of MOF, which will be of great importance in understanding the functional regulation of MOF in various biological processes.

Keratins Are Going Nuclear

Previously thought to reside exclusively in the cytoplasm, the cytoskeletal protein keratin 17 (K17) has been recently identified inside the nucleus of tumor epithelial cells with a direct impact on cell proliferation and gene expression. We comment on fundamental questions raised by this new finding and the associated significance.

Nuclear Import and Export of the Thyroid Hormone Receptor

The thyroid hormone receptors, TRα1 and TRβ1, are members of the nuclear receptor superfamily that forms one of the most abundant classes of transcription factors in multicellular organisms. Although primarily localized to the nucleus, TRα1 and TRβ1 shuttle rapidly between the nucleus and cytoplasm. The fine balance between nuclear import and export of TRs has emerged as a critical control point for modulating thyroid hormone-responsive gene expression. Mutagenesis studies have defined two nuclear localization signal (NLS) motifs that direct nuclear import of TRα1: NLS-1 in the hinge domain and NLS-2 in the N-terminal A/B domain. Three nuclear export signal (NES) motifs reside in the ligand-binding domain. A combined approach of shRNA-mediated knockdown and coimmunoprecipitation assays revealed that nuclear entry of TRα1 is facilitated by importin 7, likely through interactions with NLS-2, and importin β1 and the adapter importin α1 interacting with both NLS-1 and NLS-2. Interestingly, TRβ1 lacks NLS-2 and nuclear import depends solely on the importin α1/β1 heterodimer. Heterokaryon and fluorescence recovery after photobleaching shuttling assays identified multiple exportins that play a role in nuclear export of TRα1, including CRM1 (exportin 1), and exportins 4, 5, and 7. Even single amino acid changes in TRs dramatically alter their intracellular distribution patterns. We conclude that mutations within NLS and NES motifs affect nuclear shuttling activity, and propose that TR mislocalization contributes to the development of some types of cancer and Resistance to Thyroid Hormone syndrome.

Nuclear Import of Hepatitis B Virus Capsids and Genome

Hepatitis B virus (HBV) is an enveloped pararetrovirus with a DNA genome, which is found in an up to 36 nm-measuring capsid. Replication of the genome occurs via an RNA intermediate, which is synthesized in the nucleus. The virus must have thus ways of transporting its DNA genome into this compartment. This review summarizes the data on hepatitis B virus genome transport and correlates the finding to those from other viruses.

Nuclear Import of JAK1 Is Mediated by a Classical NLS and Is Required for Survival of Diffuse Large B-cell Lymphoma

JAKs are non-receptor tyrosine kinases that are generally found in association with cytokine receptors. In the canonical pathway, roles of JAKs have well been established in activating STATs in response to cytokine stimulation to modulate gene transcription. In contrast, a noncanonical role of JAK2 has recently been discovered, in which JAK2 in the nucleus imparts the epigenetic regulation of gene transcription through phosphorylation of tyrosine 41 on the histone protein H3. Recent work further demonstrated that this noncanonical mechanism is conserved with JAK1, which is activated by the autocrine cytokines IL6 and IL10 in activated B-cell-like diffuse large B-cell lymphoma (ABC DLBCL), a cancer type that is particularly difficult to treat and has poor prognosis. However, how JAK1 gains access to the nucleus to enable epigenetic regulation remains undefined. Here, we investigated this question and revealed that JAK1 has a classical nuclear localization signal toward the N-terminal region, which can be recognized by multiple importin α isoforms. Moreover, the nuclear import of JAK1 is independent of its kinase activity but is required for the optimal expansion of ABC DLBCL cells in vitroImplications: This study demonstrates that the nuclear import of JAK1 is essential for the optimal fitness of ABC DLBCL cells, and targeting JAK1 nuclear localization is a potential therapeutic strategy for ABC DLBCL. Mol Cancer Res; 15(3); 348-57. ©2016 AACR.

Identification of protein changes in human spermatozoa throughout the cryopreservation process

Cryoinjury is a consequence of cryopreservation and may have a negative impact on sperm quality regarding motility, morphology, and viability. This study was designed to identify potential proteomic changes in human sperm cells throughout the cryopreservation process. Comparisons made within this study included the detection of the sperm proteomic changes induced by incubation of the sperm cells with a protein-free cryoprotectant (with and without CryoSperm), and the proteomic changes induced by freezing, thawing, and subsequent after-thawing incubation at two different temperatures (0 °C vs. 23 °C). Tandem Mass Tag (TMT) peptide labeling coupled with LC-MS/MS was used for protein quantification. LC-MS/MS resulted in the identification of 769 quantifiable proteins. The abundance of 105 proteins was altered upon CryoSperm incubation. Freezing and thawing also induced substantial protein changes. However, fewer changes were observed when semen was thawed and then maintained after-thawing at approximately 0 °C than when it was maintained after-thawing at 23 °C, with 60 and 99 differential proteins detected, respectively, as compared to unfrozen semen incubated in CryoSperm. Collectively, these differences indicate that substantial changes occur in the sperm proteome at every stage of the cryopreservation process which may ultimately impair the sperm fertilizing capability. This is the first study to compare protein levels in fresh and cryopreserved semen using the TMT technology coupled to LC-MS/MS.

Hsp90-binding immunophilin FKBP52 modulates telomerase activity by promoting the cytoplasmic retrotransport of hTERT

Telomerase is a unique ribonucleoprotein enzyme that is required for continued cell proliferation. To generate catalytically active telomerase, human telomerase reverse transcriptase (hTERT) must translocate to the nucleus and assemble with the RNA component of telomerase. The molecular chaperones heat shock protein 90 (Hsp90) and p23 maintain hTERT in a conformation that enables nuclear translocation. However, the regulatory role of chaperones in nuclear transport of hTERT remains unclear. In this work, we demonstrate that immunophilin FK506-binding protein (FKBP)52 linked the hTERT–Hsp90 complex to the dynein–dynactin motor, thereby promoting the transport of hTERT to the nucleus along microtubules. FKBP52 interacted with the hTERT–Hsp90 complex through binding of the tetratricopeptide repeat domain to Hsp90 and binding of the dynamitin (Dyt) component of the dynein-associated dynactin complex to the peptidyl prolyl isomerase domain. The depletion of FKBP52 inhibited nuclear transport of hTERT, resulting in cytoplasmic accumulation. Cytoplasmic hTERT was rapidly degraded through ubiquitin (Ub)-dependent proteolysis, thereby abrogating telomerase activity. In addition, overexpression of dynamitin, which is known to dissociate the dynein–dynactin motor from its cargoes, reduced telomerase activity. Collectively, these results provide a molecular mechanism by which FKBP52 modulates telomerase activity by promoting dynein–dynactin-dependent nuclear import of hTERT.

Androgen receptor variation affects prostate cancer progression and drug resistance

Significant therapeutic progress has been made in treating prostate cancer in recent years. Drugs such as enzalutamide, abiraterone, and cabazitaxel have expanded the treatment armamentarium, although it is not completely clear which of these drugs are the most-effective option for individual patients. Moreover, such advances have been tempered by the development of therapeutic resistance. The purpose of this review is to summarize the current literature pertaining to the biochemical effects of AR variants and their consequences on prostate cancer therapies at both the molecular level and in clinical treatment. We address how these AR splice variants and mutations affect tumor progression and therapeutic resistance and discuss potential novel therapeutic strategies under development. It is hoped that these therapies can be administered with increasing precision as tumor genotyping methods become more sophisticated, thereby lending clinicians a better understanding of the underlying biology of prostate tumors in individual patients.

Enhancing nuclear translocation: perspectives in inhaled corticosteroid therapy

Corticosteroids are widely used in the treatment of asthma and chronic obstructive pulmonary disease (COPD). In contrast to their use in mild-to-moderate asthma, they are less efficacious in improving lung function and controlling the underlying inflammation in COPD. In most clinical trials, corticosteroids have shown little benefit in COPD, but have shown a greater clinical effect in combination with long-acting bronchodilators. Impaired corticosteroid activation of the glucocorticoid receptor (GR) has been reported in corticosteroid-insensitive individuals. Reversal of corticosteroid-insensitivity by enhancing GR nuclear translocation is a potential therapeutic target. Preclinical studies suggest members of the nuclear receptor superfamily may facilitate glucocorticoid receptor nuclear translocation. Unravelling the mechanisms that govern GR nuclear translocation may identify novel therapeutic targets for reversing corticosteroid-insensitivity.

A new mechanism for nuclear import by actin-based propulsion used by a baculovirus nucleocapsid

The transport of macromolecules into the nucleus is mediated by soluble cellular receptors of the importin β superfamily and requires the Ran-GTPase cycle. Several studies have provided evidence that there are exceptions to this canonical nuclear import pathway. Here, we report a new unconventional nuclear import mechanism exploited by the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV). We found that AcMNPV nucleocapsids entered the nucleus of digitonin-permeabilized cells in the absence of exogenous cytosol or under conditions that blocked the Ran-GTPase cycle. AcMNPV contains a protein that activates the Arp2/3 complex and induces actin polymerization at one end of the rod-shaped nucleocapsid. We show that inhibitors of Arp2/3 blocked nuclear import of nucleocapsids in semi-permeabilized cells. Nuclear import of nucleocapsids was also reconstituted in purified nuclei supplemented with G-actin and Arp2/3 under actin polymerization conditions. Thus, we propose that actin polymerization drives not only migration of baculovirus through the cytoplasm but also pushes the nucleocapsid through the nuclear pore complex to enter the cell nucleus. Our findings point to a very distinct role of actin-based motility during the baculovirus infection cycle.

Influenza A viruses escape from MxA restriction at the expense of efficient nuclear vRNP import

To establish a new lineage in the human population, avian influenza A viruses (AIV) must overcome the intracellular restriction factor MxA. Partial escape from MxA restriction can be achieved when the viral nucleoprotein (NP) acquires the critical human-adaptive amino acid residues 100I/V, 283P, and 313Y. Here, we show that introduction of these three residues into the NP of an avian H5N1 virus renders it genetically unstable, resulting in viruses harboring additional single mutations, including G16D. These substitutions restored genetic stability yet again yielded viruses with varying degrees of attenuation in mammalian and avian cells. Additionally, most of the mutant viruses lost the capacity to escape MxA restriction, with the exception of the G16D virus. We show that MxA escape is linked to attenuation by demonstrating that the three substitutions promoting MxA escape disturbed intracellular trafficking of incoming viral ribonucleoprotein complexes (vRNPs), thereby resulting in impaired nuclear import, and that the additional acquired mutations only partially compensate for this import block. We conclude that for adaptation to the human host, AIV must not only overcome MxA restriction but also an associated block in nuclear vRNP import. This inherent difficulty may partially explain the frequent failure of AIV to become pandemic.