Signals mediating nuclear targeting and their regulation: application in drug delivery

Aggressive Impacts Affecting the Biodegradable Ultrathin Fibers Based on Poly(3-Hydroxybutyrate), Polylactide and Their Blends: Water Sorption, Hydrolysis and Ozonolysis

Ultrathin electrospun fibers of pristine biopolyesters, poly(3-hydroxybutyrate) (PHB) and polylactic acid (PLA), as well as their blends, have been obtained and then explored after exposure to hydrolytic (phosphate buffer) and oxidative (ozone) media. All the fibers were obtained from a co-solvent, chloroform, by solution-mode electrospinning. The structure, morphology, and segmental dynamic behavior of the fibers have been determined by optical microscopy, SEM, ESR, and others. The isotherms of water absorption have been obtained and the deviation from linearity (the Henry low) was analyzed by the simplified model. For PHB-PLA fibers, the loss weight increments as the reaction on hydrolysis are symbate to water absorption capacity. It was shown that the ozonolysis of blend fibrils has a two-stage character which is typical for O₃ consumption, namely, the pendant group's oxidation and the autodegradation of polymer molecules with chain rupturing. The first stage of ozonolysis has a quasi-zero-order reaction. A subsequent second reaction stage comprising the back-bone destruction has a reaction order that differs from the zero order. The fibrous blend PLA/PHB ratio affects the rate of hydrolysis and ozonolysis so that the fibers with prevalent content of PLA display poor resistance to degradation in aqueous and gaseous media.

Nanotechnology and immunoengineering: How nanotechnology can boost CAR-T therapy

Chimeric antigen receptor (CAR) therapy has achieved remarkable clinical efficacy against hematological cancers and has been approved by FDA for treatment of B-cell tumors. However, the complex manufacturing process and limited success in solid tumors hamper its widespread applications, thus prompting the development of new strategies for overcoming the abovementioned hurdles. In the last decade, nanotechnology has provided sustainable strategies for improving cancer immunotherapy through vaccine development and delivery of immunomodulatory drugs. Nanotechnology can boost CAR-T therapy and may overcome the existing challenges by emerging as a carrier for CAR-T therapy or in combination with CAR-T, it may inhibit solid tumors more effectively than conventional approaches. The revealing of cellular mechanisms, barriers and potential strategies that could be used to manipulate and/or modify cells would enable unprecedented advances in nanotechnology for biologics delivery. This review outlines the journey and barriers of nanoparticles (NPs) across the cell. Subsequently, the approaches to tackle the barriers and strategies to modulate NPs as a carrier for CAR-T therapy are discussed. Finally, the role of NPs in CAR-T therapy and the potential challenges are summarized. This review aims to provide the readers with a detailed overview of NP-based CAR-T therapy research and distil this information into an accessible form conducive to design desired CAR-T therapy using NP approach. STATEMENT OF SIGNIFICANCE: Chimeric antigen receptor (CAR) T-cell therapy is the most vibrant field in immuno-oncology today, with enormous benefits to patients with B-cell malignancies. However, a rapid and straightforward procedure for CAR-T generation is an exigent need to broaden its therapeutic avenue. Nanotechnology has emerged as a novel alternative approach for CAR-T generation. To the best of our knowledge, this is the first in-depth review that briefly highlights the various aspects of nanotechnology in CAR-T therapy, including the strategies to brand NPs as an effective carrier for CAR cargo, its potential advantages, challenges, and future roadmap. It provides readers with a detailed overview of NP-based CAR-T therapy research, and researchers would be able to distill this information into an accessible form conducive to design the desired CAR therapy using the nanotechnology approach.

Journey to the Center of the Cell: Current Nanocarrier Design Strategies Targeting Biopharmaceuticals to the Cytoplasm and Nucleus

New biopharmaceutical molecules, potentially able to provide more personalized and effective treatments, are being identified through the advent of advanced synthetic biology strategies, sophisticated chemical synthesis approaches, and new analytical methods to assess biological potency. However, translation of many of these structures has been significantly limited due to the need for more efficient strategies to deliver macromolecular therapeutics to desirable intracellular sites of action. Engineered nanocarriers that encapsulate peptides, proteins, or nucleic acids are generally internalized into target cells via one of several endocytic pathways. These nanostructures, entrapped within endosomes, must navigate the intracellular milieu to orchestrate delivery to the intended destination, typically the cytoplasm or nucleus. For therapeutics active in the cytoplasm, endosomal escape continues to represent a limiting step to effective treatment, since a majority of nanocarriers trapped within endosomes are ultimately marked for enzymatic degradation in lysosomes. Therapeutics active in the nucleus have the added challenges of reaching and penetrating the nuclear envelope, and nuclear delivery remains a preeminent challenge preventing clinical translation of gene therapy applications. Herein, we review cutting-edge peptide- and polymer-based design strategies with the potential to enable significant improvements in biopharmaceutical efficacy through improved intracellular targeting. These strategies often mimic the activities of pathogens, which have developed innate and highly effective mechanisms to penetrate plasma membranes and enter the nucleus of host cells. Understanding these mechanisms has enabled advances in synthetic peptide and polymer design that may ultimately improve intracellular trafficking and bioavailability, leading to increased access to new classes of biotherapeutics.

LocSigDB: a database of protein localization signals

LocSigDB (http://genome.unmc.edu/LocSigDB/) is a manually curated database of experimental protein localization signals for eight distinct subcellular locations; primarily in a eukaryotic cell with brief coverage of bacterial proteins. Proteins must be localized at their appropriate subcellular compartment to perform their desired function. Mislocalization of proteins to unintended locations is a causative factor for many human diseases; therefore, collection of known sorting signals will help support many important areas of biomedical research. By performing an extensive literature study, we compiled a collection of 533 experimentally determined localization signals, along with the proteins that harbor such signals. Each signal in the LocSigDB is annotated with its localization, source, PubMed references and is linked to the proteins in UniProt database along with the organism information that contain the same amino acid pattern as the given signal. From LocSigDB webserver, users can download the whole database or browse/search for data using an intuitive query interface. To date, LocSigDB is the most comprehensive compendium of protein localization signals for eight distinct subcellular locations.

Database URL: http://genome.unmc.edu/LocSigDB/

The small nuclear ribonucleoprotein U1A interacts with NS5 from yellow fever virus

The flavivirus NS5 protein is one of the most important proteins of the replication complex, and cellular proteins can interact with it. This study shows for the first time that the yellow fever virus (YFV) NS5 protein is able to interact with U1A, a protein involved in splicing and polyadenylation. We confirmed this interaction by GST-pulldown assay and by co-immunoprecipitation in YFV-infected cells. A region between amino acids 368 and 448 was identified as the site of interaction of the NS5 protein with U1A. This region was conserved among some flaviviruses of medical importance. The implications of this interaction for flavivirus replication are discussed.

Advancing nonviral gene delivery: lipid- and surfactant-based nanoparticle design strategies

Gene therapy is a technique utilized to treat diseases caused by missing, defective or overexpressing genes. Although viral vectors transfect cells efficiently, risks associated with their use limit their clinical applications. Nonviral delivery systems are safer, easier to manufacture, more versatile and cost effective. However, their transfection efficiency lags behind that of viral vectors. Many groups have dedicated considerable effort to improve the efficiency of nonviral gene delivery systems and are investigating complexes composed of DNA and soft materials such as lipids, polymers, peptides, dendrimers and gemini surfactants. The bottom-up approach in the design of these nanoparticles combines components essential for high levels of transfection, biocompatibility and tissue-targeting ability. This article provides an overview of the strategies employed to improve in vitro and in vivo transfection, focusing on the use of cationic lipids and surfactants as building blocks for nonviral gene delivery systems.

Quantification of nuclear protein transport using induced heterodimerization

Trafficking of proteins between the cytoplasm and nucleus occurs exclusively across the nuclear pore complex of eucaryotic cells. Fundamental aspects of this process affect temporal and spatial parameters, the latter carried out by specific import [nuclear localization sequence (NLS)] and export [nuclear export sequence (NES)] sequences. In this study, we focused on the adaptation of a protein heterodimerization assay to kinetically measure Crm1-mediated nuclear export in living cells using the rapalog AP21967, a heterodimerizing agent and NLS- and NES-containing fusion proteins equipped with distinct AP21967-specific binding motifs. In HeLa cells, we observed rapid nuclear export of the NLS-containing fusion protein in the presence of AP21967, with the extent of this process being a function of the number of AP21967-binding motifs. AP21967-induced nuclear export was specifically inhibited by the Crm1-binding molecule leptomycin B. Half maximal export was achieved after approximately 10 min. We further applied protein heterodimerization in HeLa cells to study induced NLS-mediated nuclear import. Only in the presence of heterodimerizer AP21967 nuclear import of a cytoplasmically localizing fusion protein was observed. Induced protein heterodimerization is thus a valuable tool to quantitatively study nucleocytoplasmic protein trafficking in cultured cells, in a non-invasive, time-saving manner.

Enhanced cellular uptake and cytotoxicity studies of organometallic bioconjugates of the NLS peptide in Hep G2 cells

SPACE INVADERS: Organometallic fragments such as the ferrocenyl group (shown in red in the picture) help to enhance cellular entry of NLS peptides. Eventually, these nontoxic conjugates find their way to the cellular nucleus as shown by fluorescence microscopy studies in this work. Intracellular delivery to biomolecular targets is still a major challenge in molecular and cell biology. We recently found that attaching an organometallic group, namely the cobaltocenium cation, to the SV 40 large T antigen nuclear localisation signal (NLS) greatly enhances cellular uptake of the conjugate (Noor et al., Angew. Chem. Int. Ed. 2005, 45, 2429). In addition, nuclear localisation of the conjugate was observed. In this work, we present a thorough investigation of this novel cellular delivery system with respect to the nature of the metal complex and the peptide sequence. A number of ferrocene ((Fe(II)), neutral metal complex) and cobaltocenium ((Co(III)), cationic metal complex) bioconjugates with both the NLS wild-type sequence PKKKRKV and a scrambled sequence (NLS(scr), KKVKPKR) were prepared by solid-phase peptide synthesis (SPPS). Cellular and nuclear uptake of these bioconjugates was studied by fluorescence microscopy on living Hep G2 cells. In addition, cytotoxicity screening on the conjugates was carried out, as the toxic effects of several simple metallocenes have been noted previously. Rapid cellular uptake as well as nuclear localisation was observed for the metal-NLS conjugates, but not for any dipeptide controls, the metal-NLS(scr) conjugates or any metal-free conjugates. It thus appears that the presence of a metallocene, but not its charge, and the correct NLS sequence is essential for cellular uptake. Fluorescence microscopy co-localisation studies did not reveal a significant endosomal entrapment of the conjugates. The metallocene not only provides a hydrophobic handle for membrane translocation but also facilitates the localisation and distribution of the conjugate in the cytoplasm. The NLS peptide on the other hand is responsible for the nuclear localisation of the bioconjugate. Finally, none of the conjugates were found to be toxic up to the highest concentrations that was tested (1 mM) against the Hep G2 cells that were used in this study. In conclusion, this work supports metallocene-NLS bioconjugates, in particular with the very robust cobaltocenium group, as a simple but potent, nontoxic system for cellular uptake and nuclear delivery. Concurrently, our finding is relevant to the still-unresolved question of cytotoxicity of metallocenes because it excludes binding and/or damage to the DNA as a mechanism of metallocene cytotoxicity. This finding is confirmed by a combined yeast cytotoxicity/genotoxicity assay, which also shows very little toxic effects for all organometal-NLS conjugates that were tested.

Caspase-mediated cleavage of importin-alpha increases its affinity for MCM and downregulates DNA synthesis by interrupting the binding of MCM to chromatin

Importin-alpha is essential for classical nucleocytoplasmic transport of nuclear proteins. Here, we report that importin-alpha is cleaved by caspases during apoptosis, generating importin-alpha lacking an IBB domain. This truncated importin-alpha binds tightly to the MCM replication licensing factor and, thus, prevents its binding to chromatin and downregulates DNA synthesis. Together, our data reveal for the first time that a dying cell effectively salvages limited supplies of cellular energy to ensure an orderly process of its own demise by simultaneously downregulating nucleocytoplasmic protein transport and DNA synthesis. Strikingly, cells can achieve this multi-task process by simply cleaving-off a key nuclear import protein.

Signal sequences for targeting of gene therapy products to subcellular compartments: the role of CRM1 in nucleocytoplasmic shuttling of the protein switch

PURPOSE

The purpose of this study was to understand the mechanism of nuclear export of the protein switch, used for controlled intracellular delivery of gene products, by studying the involvement of classical export receptor CRM1.

METHOD

Transient transfections of protein switch constructs, isolated nuclear export and import signals were carried out. Effect of leptomycin B (inhibitor of export receptor) and geldanamycin (inhibitor of Hsp90) on localization of these constructs was studied using fluorescence microscopy. Putative nuclear export signals in the glucocorticoid and progesterone receptor ligand binding domains were identified and studied.

RESULTS

It was observed that treatment with leptomycin B caused nuclear accumulation of the protein switch constructs. However, geldanamycin did not have any pronounced effect on the localization. The isolated nuclear export signal from glucocorticoid receptor localized mostly in the cytoplasm, while its mutated version was present everywhere.

CONCLUSION

The localization controlled protein switch constructs are exported out of the nucleus by the classical CRM1 receptors. The ligand binding domain of these protein switch constructs plays an important role in maintaining these constructs in the cytoplasm in the absence of ligand, as well the re-export back to the cytoplasm from the nucleus after ligand washout.

Nucleocytoplasmic transport of DNA: enhancing non-viral gene transfer

Gene therapy, the correction of dysfunctional or deleted genes by supplying the lacking component, has long been awaited as a means to permanently treat or reverse many genetic disorders. To achieve this, therapeutic DNA must be delivered to the nucleus of cells using a safe and efficient delivery vector. Although viral-based vectors have been utilized extensively due to their innate ability to deliver DNA to intact cells, safety considerations, such as pathogenicity, oncogenicity and the stimulation of an immunological response in the host, remain problematical. There has, however, been much progress in the development of safe non-viral gene-delivery vectors, although they remain less efficient than the viral counterparts. The major limitations of non-viral gene transfer reside in the fact that it must be tailored to overcome the intracellular barriers to DNA delivery that viruses already master, including the cellular and nuclear membranes. In particular, nuclear transport of the therapeutic DNA is known to be the rate-limiting step in the gene-delivery process. Despite this, much progress had been made in recent years in developing novel means to overcome these barriers and efficiently deliver DNA to the nuclei of intact cells. This review focuses on the nucleocytoplasmic delivery of DNA and mechanisms to enhance to non-viral-mediated gene transfer.

Geldanamycin, an inhibitor of Hsp90, blocks cytoplasmic retention of progesterone receptors and glucocorticoid receptors via their respective ligand binding domains

Steroid hormone receptors (SHRs), such as glucocorticoid receptors (GR) and progesterone receptors (PR), are shuttling proteins that undergo continuous nuclear import and export. Various mechanisms have been proposed to explain the localization of SHRs. It has been suggested that the ligand-binding domain (LBD) of SHRs is important in determining the subcellular localization. We have studied the localization of GR-LBD and PR-LBD alone, as well as of full-length GR and PR in the presence of geldanamycin (GA), a benzoquinoid ansamycin that specifically inhibits heat shock protection (Hsp90), using transient transfections and fluorescent microscopy. Our studies have indicated that GR-LBD and PR-LBD are retained in the cytoplasm via interaction with Hsp90. It was observed that in the unliganded state, treatment with GA translocates these LBDs to the nucleus. Similar results were obtained for full-length PR and GR. Additionally, it was found that after ligand induction, GA accelerated reexport of SHRs after ligand washout, implicating Hsp90 in nuclear retention of SHRs in the washout state. We also propose that a recently found "export" signal present in the LBD of SHRs is involved in interactions with Hsp90 and hence cytoplasmic retention of these receptors. After ligand induction, Hsp90 also may play a role in nuclear retention of SHRs following hormone washout.

Central role of the respiratory syncytial virus matrix protein in infection

Respiratory syncytial virus is the major respiratory pathogen of infants and children worldwide, with no effective treatment or vaccine available. Steady progress has been made in understanding the respiratory syncytial virus life cycle and the consequences of infection, but many areas of respiratory syncytial virus biology remain poorly understood, including the role of subcellular localisation of respiratory syncytial virus gene products such as the matrix protein in the infected host cell. The matrix protein plays a central role in viral assembly and, intriguingly, has been observed to traffic into and out of the nucleus at specific times during the respiratory syncytial virus infectious cycle. Further, the matrix protein has been shown to be able to inhibit transcription, which may be a key to respiratory syncytial virus pathogenesis. This review will focus on the role of the matrix protein in respiratory syncytial virus infection and what is known of its nucleocytoplasmic trafficking, the understanding of which may lead to new therapeutic approaches to combat respiratory syncytial virus, and/or vaccine development.

Cationic liposome-DNA complexes: from liquid crystal science to gene delivery applications

At present, there is an unprecedented level of interest in the properties and structures of complexes consisting of DNA mixed with oppositely charged cationic liposomes (CLs). The interest arises because the complexes mimic natural viruses as chemical carriers of DNA into cells in worldwide human gene therapy clinical trials. However, since our understanding of the mechanisms of action of CL-DNA complexes interacting with cells remains poor, significant additional insights and discoveries will be required before the development of efficient chemical carriers suitable for long-term therapeutic applications. Recent studies describe synchrotron X-ray diffraction, which has revealed the liquid crystalline nature of CL-DNA complexes, and three-dimensional laser-scanning confocal microscopy, which reveals CL-DNA pathways and interactions with cells. The importance of the liquid crystalline structures in biological function is revealed in the application of these modern techniques in combination with functional transfection efficiency measurements, which shows that the mechanism of gene release from complexes in the cell cytoplasm is dependent on their precise liquid crystalline nature and the physical and chemical parameters (for example, the membrane charge density) of the complexes. In [section sign] 5, we describe some recent new results aimed at developing bionanotube vectors for gene delivery.

Rad52 and Rad59 exhibit both overlapping and distinct functions

Homologous recombination is an important pathway for the repair of DNA double-strand breaks (DSBs). In the yeast Saccharomyces cerevisiae, Rad52 is a central recombination protein, whereas its paralogue, Rad59, plays a more subtle role in homologous recombination. Both proteins can mediate annealing of complementary single-stranded DNA in vitro, but only Rad52 interacts with replication protein A and the Rad51 recombinase. We have studied the functional overlap between Rad52 and Rad59 in living cells using chimeras of the two proteins and site-directed mutagenesis. We find that Rad52 and Rad59 have both overlapping as well as separate functions in DSB repair. Importantly, the N-terminus of Rad52 possesses functions not supplied by Rad59, which may account for its central role in homologous recombination.

Controlling protein compartmentalization to overcome disease

Over the past decade, considerable progress has been made to improve our understanding of the intracellular transport of proteins. Mechanisms of nuclear import and export involving classical receptors have been studied. Signal sequences required for directing a protein molecule to a specific cellular compartment have been defined. Knowledge of subcellular trafficking of proteins has also increased our understanding of diseases caused due to mislocalization of proteins. A specific protein on deviating from its native cellular compartment may result in disease due to loss of its normal functioning and aberrant activity in the "wrong" compartment. Mislocalization of proteins results in diseases that range from metabolic disorders to cancer. In this review we discuss some of the diseases caused due to mislocalization. We further focus on application of nucleocytoplasmic transport to drug delivery. Various rationales to treat diseases by exploiting intracellular transport machinery have been proposed. Although the pathways for intracellular movement of proteins have been defined, these have not been adequately utilized for management of diseases involving mislocalized proteins. This review stresses the need for designing drug delivery systems utilizing these mechanisms as this area is least exploited but offers great potential.

The Nuclear Pore Complex: The Gateway to Successful Nonviral Gene Delivery

One of the limiting steps in the efficiency of nonviral gene delivery is transport of genetic material across the nuclear membrane. Trafficking of nuclear proteins from the cytoplasm into the nucleus occurs via the nuclear pore complex and is mediated by nuclear localization signals and their nuclear receptors. Several strategies employing this transport mechanism have been designed and explored to improve nonviral gene delivery. In this article, we review the mechanism of nuclear import through the nuclear pore complex and the strategies used to facilitate nuclear import of exogenous DNA and improve gene expression.

The C-terminal tail of protein kinase D2 and protein kinase D3 regulates their intracellular distribution

We generated a set of GFP-tagged chimeras between protein kinase D2 (PKD2) and protein kinase D3 (PKD3) to examine in live cells the contribution of their C-terminal region to their intracellular localization. We found that the catalytic domain of PKD2 and PKD3 can localize to the nucleus when expressed without other kinase domains. However, when the C-terminal tail of PKD2 was added to its catalytic domain, the nuclear localization of the resulting protein was inhibited. In contrast, the nuclear localization of the CD of PKD3 was not inhibited by its C-terminal tail. Furthermore, the exchange of the C-terminal tail of PKD2 and PKD3 in the full-length proteins was sufficient to exchange their intracellular localization. Collectively, these data demonstrate that the short C-terminal tail of these kinases plays a critical role in determining their cytoplasmic/nuclear localization.

Effect of different nuclear localization sequences on the immune responses induced by a MIDGE vector encoding bovine herpesvirus-1 glycoprotein D

To optimize the efficiency of a Minimalistic Immunogenically Defined Gene Expression (MIDGE) vector, peptides containing proven (SV40 T-antigen and bovine herpesvirus-1 VP8) or putative (herpes simplex virus-1 VP22) nuclear localization signals (NLSs) were linked to a MIDGE vector encoding a truncated, secreted form of BHV-1 glycoprotein D (tgD) (MIDGE-tgD). Conjugation of an NLS to the MIDGE-tgD vector improved the tgD expression in vitro and the humoral and cellular immune responses induced in mice in vivo. The NLS from BHV-1 VP8 was most efficient at enhancing the tgD production and tgD-specific immune responses when conjugated to the MIDGE-tgD vector.

Cationic lipid–DNA complexes for non-viral gene therapy: relating supramolecular structures to cellular pathways

Cationic liposomes (CLs) are used as nonviral vectors in worldwide human clinical trials of gene therapy. Among other advantages, lipid-DNA complexes have the ability to transfer very large genes into cells, but their efficiency is much lower than that of viruses. Recent studies combining structural and biological techniques are beginning to unravel the relationship between the distinctly structured CL-DNA complexes and their transfection efficiency. Most CL-DNA complexes form a multilayered structure with DNA sandwiched between the cationic lipids (lamellar complexes, LalphaC). On rare occasions, an inverted hexagonal structure (HIIC) is observed. An important recent insight is that the membrane charge density (sigmaM) of the CL-vector is a universal parameter governing the transfection efficiency of LalphaC (but not HIIC) complexes. This has led to a new model of the cellular uptake of LalphaC complexes through activated fusion with endosomal membranes. Surface-functionalised complexes with poly(ethylene glycol)-lipids, potentially suitable for transfection invivo, have also been investigated, and the novel aspects of these complexes are discussed.

Nuclear Targeting and Cell Cycle Regulatory Function of Human BARD1

The BARD1 gene is mutated in a subset of breast and ovarian cancers, implicating BARD1 as a potential tumor suppressor. BARD1 gains a ubiquitin E3 ligase activity when heterodimerized with BRCA1, but the only known BRCA1-independent BARD1 function is a p53-dependent proapoptotic activity stimulated by nuclear export to the cytoplasm. We described previously the nuclear-cytoplasmic shuttling of BARD1, and in this study, we identify the transport sequences that target BARD1 to the nucleus and show that they are essential for BARD1 regulation of the cell cycle. We used deletion mapping and mutagenesis to define two active nuclear localization signals (NLSs) present in human BARD1 that are not conserved in rodent BARD1. Site-directed mutagenesis of the primary bipartite NLS abolished BARD1 nuclear import and caused its cytoplasmic accumulation. Using flow cytometry and 5-bromo-2-deoxyuridine incorporation assays, we discovered that transiently expressed BARD1 can elicit a p53-independent cell cycle arrest in G1 phase, and that this was abrogated by mutation of the BARD1 NLS but not by mutation of the nuclear export signal. Thus, BARD1 regulation of the cell cycle is a nuclear event and may be linked to its induced expression during mitosis and its possible involvement in the DNA damage checkpoint.

Nuclear-cytoplasmic shuttling of the oncogenic mouse UNP/USP4 deubiquitylating enzyme

The oncogenic deubiquitylating enzyme (DUB) Unp/Usp4, which binds to the retinoblastoma family of tumor suppressor proteins, was originally described as a nuclear protein. However, more recent studies have shown it to be cytoplasmic. In addition, analysis of its subcellular localization has been complicated by the existence of the paralog Usp15. In this study, we resolved this controversy by investigating the localization of exogenously expressed Usp4 (using red fluorescent protein-Usp4) and of endogenous Usp4 (using highly specific antibodies that can distinguish Usp4 from Usp15). We found that by inhibiting nuclear export with leptomycin B, both exogenous and endogenous Usp4 accumulate in the nucleus. Further, using a Rev-green fluorescent protein-based export assay, we confirmed the existence of a nuclear export signal ((133)VEVYLLELKL(142)) in Usp4. In addition, a functional nuclear import signal ((766)QPQKKKK(772)) was also identified, which was specifically recognized by importin alpha/beta. Finally, we show that the equilibrium of Usp4 subcellular localization varies between different cell types. Usp4 is thus the first DUB reported to have nucleocytoplasmic shuttling properties. The implications of this shuttling for its function as a DUB are discussed.

Lipoplex Structures and Their Distinct Cellular Pathways

Cationic liposomes (CLs) are used as non-viral vectors in worldwide clinical trials of gene therapy. Among other advantages, CL-DNA complexes have the ability to transfer very large genes into cells. However, since the understanding of their mechanisms of action is still incomplete, their transfection efficiencies remain low compared to those of viruses. We describe recent studies which have started to unravel the relationship between the distinct structures and physicochemical properties of CL-DNA complexes and their transfection efficiency by combining several techniques: synchrotron X-ray diffraction for structure determination, laser-scanning confocal microscopy to probe the interactions of CL-DNA particles with cells, and luciferase reporter-gene expression assays to measure transfection efficiencies in mammalian cells. Most CL-DNA complexes form a multilayered structure with DNA sandwiched between the cationic lipids (lamellar complexes, L(alpha)(C)). Much more rarely, an inverted hexagonal structure (H(II)(C)) with single DNA strands encapsulated in lipid tubules is observed. An important recent insight is that the membrane charge density sigma(M) of the CL-vector, rather than, for example, the charge of the cationic lipid, is a universal parameter governing the transfection efficiency of L(alpha)(C) complexes. This has led to a new model of the intracellular release of L(alpha)(C) complexes, through activated fusion with endosomal membranes. In contrast to L(alpha)(C) complexes, H(II)(C) complexes exhibit no dependence on sigma(M), since their structure leads to a distinctly different mechanism of cell entry. Surface-functionalized complexes with poly(ethyleneglycol)-lipids (PEG-lipids), potentially suitable for transfection in vivo, have also been investigated, and the novel aspects of these complexes are discussed.

Bidirectional on/off switch for controlled targeting of proteins to subcellular compartments

A regulatable fusion protein was constructed for controlling the localization of plasmid products. A ligand-inducible nuclear localization signal, nuclear export signal (NES) and a truncated form of the ligand binding domain of the progesterone receptor were attached to the desired protein. Enhanced green fluorescent protein (EGFP) was used as a model protein and its trafficking between the nucleus and cytoplasm was studied using fluorescence microscopy in response to the ligand, mifepristone. It was found that the protein trafficking into the nucleus was dose dependent with ligand concentration. Increasing the ligand dose from 1 to 100 nM enhanced import and reduced the rate of export of the fusion protein from the nucleus to the cytoplasm. This study demonstrates the feasibility of using an export signal and a ligand-inducible nuclear import signal as a bi-directional on/off switch with potential use for controlled targeting of therapeutic proteins to subcellular compartments.

Characterization of nuclear localization and export signals of the major tegument protein VP8 of bovine herpesvirus-1

Bovine herpesvirus-1 (BHV-1) VP8 is found in the nucleus immediately after infection. Transient expression of VP8 fused to yellow fluorescent protein (YFP) in COS-7 cells confirmed the nuclear localization of VP8 in the absence of other viral proteins. VP8 has four putative nuclear localization signals (NLS). Deletion of pat4 ((51)RRPR(54)) or pat7 ((48)PRVRRPR(54)) NLS2 abrogated nuclear accumulation, whereas deletion of (48)PRV(50) did not, so pat4 NLS2 is critical for nuclear localization of VP8. Furthermore, NLS1 ((11)RRPRR(15)), pat4 NLS2, and pat7 NLS2 were all capable of transporting the majority of YFP to the nucleus. Finally, a 12-amino-acid peptide with the sequence RRPRRPRVRRPR directed all of YFP into the nucleus, suggesting that reiteration of the RRPR motif makes the nuclear localization more efficient. Heterokaryon assays demonstrated that VP8 is also capable of shuttling between the nucleus and cytoplasm of the cell. Deletion mutant analysis revealed that this property is attributed to a leucine-rich nuclear export sequence (NES) consisting of amino acids (485)LSAYLTLFVAL(495). This leucine-rich NES caused transport of YFP to the cytoplasm. These results demonstrate that VP8 shuttles between the nucleus and cytoplasm.

Parathyroid hormone-related protein (PTHrP): a nucleocytoplasmic shuttling protein with distinct paracrine and intracrine roles

Parathyroid hormone-related protein (PTHrP) was first discovered as a circulating factor secreted by certain cancers responsible for the syndrome of humoral hypercalcemia of malignancy. PTHrP possesses distinct paracrine and intracrine signaling roles. The similarity of its N-terminus to that of parathyroid hormone (PTH) enables it to share PTH's paracrine signaling properties, whereas the rest of the molecule possesses other functions, largely relating to an intracrine signaling role in the nucleus/nucleolus in regulating apoptosis and cell proliferation. Recent advances have shown that intracellularly expressed PTHrP is able to shuttle in cell-cycle- and signal-dependent fashion between nucleus and cytoplasm through the action of the distinct intracellular transport receptors importin beta 1 and exportin 1 (Crm1) mediating nuclear import and export of PTHrP, respectively. Together, the import and export pathways constitute an integrated system for PTHrP subcellular localization. Intriguingly, PTHrP nuclear/nucleolar import is dependent on microtubule integrity, transport to the nucleus appearing to occur in vectorial fashion along microtubules, mediated in part by the action of importin beta 1. PTHrP has recently been shown to be able to bind to RNA, meaning that PTHrP's nucleocytoplasmic shuttling ability may relate to a specific role within the nucleus/nucleolus to regulate RNA synthesis and/or transport.

Mitochondrial and nuclear localization of kanadaptin

Kanadaptin has originally been isolated as a kidney Cl-/HCO3- anion exchanger 1 (kAE1)-binding protein. Initial studies suggested, that in the kidney of the rabbit kanadaptin is expressed exclusively in all epithelial cells of the collecting duct. Transcripts of kanadaptin were also found in tissues not expressing kAE1, indicating additional roles for kanadaptin. With respect to this, we could recently demonstrate translocation of kanadaptin into the nucleus of mammalian cells in a nuclear localization sequence- and importin-dependent manner (Hübner et al., Biochem. J. 361, 287-296, 2002). In this study, we provide evidence, that kanadaptin is widely expressed in many tissues and that expression of kanadaptin in the mouse occurs early in embryonic development. In rat kidney we found the most intense immunofluorescence for kanadaptin in cells of the proximal tubule, consistent with the detection by in situ hybridization of high amounts of kanadaptin messenger RNA in proximal tubule cells. Immunostaining revealed localization of kanadaptin in two subcellular locations, nuclei and mitochondria. Whereas nuclear localization was demonstrated in virtually all cells, mitochondrial staining was restricted to certain cell types. Nuclear staining was only seen in cryosections, whereas mitochondrial staining was observed in both cryosections and semithin sections of freeze-dried plastic-embedded tissue. In the kidney mitochondrial staining was particularly prominent in proximal tubular epithelium. Most surprisingly, in the collecting duct epithelium (including acid-secreting intercalated cells) only negligible immunostaining, if at all, could be observed. Immunoelectron microscopy showed immunolabelling of the entire cross-sectional profile of mitochondria (matrix/inner membrane). Mitochondrial localization of kanadaptin was further documented by immunoblotting of mitochondria-enriched cellular fractions. Utilizing an interspecies heterokaryon assay, we could further demonstrate that kanadaptin has nuclear export activity. Thus, kanadaptin can be regarded to be a highly mobile nucleocytoplasmic shuttling and multilocalizing protein, but its role in mammalian cells remains still obscure.

Peptides containing cyclin/Cdk-nuclear localization signal motifs derived from viral initiator proteins bind to DNA when unphosphorylated

A single phosphorylation event at T-antigen residue Thr124 regulates initiation of simian virus 40 DNA replication. To explore this regulatory process, a series of peptides were synthesized, centered on Thr124. These peptides contain a nuclear localization signal (NLS) and a recognition site for cyclin/Cdk kinases. When unphosphorylated, the "CDK/NLS" peptides inhibit T-antigen assembly and bind non-sequence specifically to DNA. However, these activities are greatly reduced upon phosphorylation of Thr124. Similar results were obtained by using peptides derived from the CDK/NLS region of bovine papillomavirus E1. Related studies indicate that residues in the NLS bind to DNA, whereas those in the CDK motif regulate binding. These findings are discussed in terms of the control of T-antigen double hexamer assembly and initiation of viral replication.

The interdomain region of dengue NS5 protein that binds to the viral helicase NS3 contains independently functional importin beta 1 and importin alpha/beta-recognized nuclear localization signals

Dengue virus NS5 protein is a multifunctional RNA-dependent RNA polymerase that is essential for virus replication. We have shown previously that the 37- amino acid interdomain spacer sequence (residues (369)X(2)KKX(14)KKKX(11)RKX(3)405) of Dengue2 NS5 contains a functional nuclear localization signal (NLS). In this study, beta-galactosidase fusion proteins carrying point mutations of the positively charged residues or truncations of the interdomain linker region (residues 369-389 or residues 386-405) were analyzed for nuclear import and importin binding activities to show that the N-terminal part of the linker region (residues 369-389, a/bNLS) is critical for nuclear localization and is recognized with high affinity by the conventional NLS-binding importin alpha/beta heterodimeric nuclear import receptor. We also show that the importin beta-binding site (residues 320-368, bNLS) adjacent to the a/bNLS, previously identified by yeast two-hybrid analysis, is functional as an NLS, recognized with high affinity by importin beta, and able to target beta-galactosidase to the nucleus. Intriguingly, the bNLS is highly conserved among Dengue and related flaviviruses, implying a general role for the region and importin beta in the infectious cycle.

Utilization of synthetic peptides containing nuclear localization signals for nonviral gene transfer systems

The ability of nonviral gene delivery systems to overcome extracellular and intracellular barriers is a critical issue for future clinical applications. In recent years, several efforts were focused on the elucidation of the gene transfer mechanisms and on the development of multicomponent systems in order to improve both targeted gene delivery and transfection efficiency. The transport of the therapeutic DNA from the cytoplasm into the nucleus is an inefficient process and is considered as the major limiting step in nondividing cells. One of the strategies to improve nuclear uptake of DNA is taking advantage of the cellular nuclear import machinery. Synthetic peptides containing a nuclear localization signal (NLS) are bound to the DNA so that the resulting DNA-NLS complex can be recognized as a nuclear import substrate by specific intracellular receptor proteins. In this review, we critically summarize recent studies applying this approach with a particular focus on NLS-sequence specificity. Implications of the observed results are also discussed in regards to future developments of this technology.

Roles of cytoskeletal and junctional plaque proteins in nuclear signaling

Cytoplasmic junctional plaque proteins play an important role at intercellular junctions. They link transmembrane cell adhesion molecules to components of the cytoskeleton, thereby playing an important role in the control of many cellular processes. Recent studies on the subcellular distribution of some plaque proteins have revealed that a number of these proteins are able to localize in the nucleus. This dual location indicates that in addition to promoting adhesive interactions, plaque proteins may also play a direct role in nuclear processes, and in particular in the transfer of signals from the membrane to the nucleus. Therefore, translocation of plaque proteins into the nucleus in response to extracellular signals could represent a novel and direct mechanism by which signals can be transmitted from the plasma membrane to the nucleus. This could allow cells to respond to changing environmental conditions in a rapid and efficient way. In addition, conditional sequestration of karyophilic proteins at the sites of cell-cell and cell-substratum adhesion may represent a general mechanism for the regulation of nucleocytoplasmic transport.

Characterisation of inter- and intra-molecular interactions of the dengue virus RNA dependent RNA polymerase as potential drug targets

Our research is directed towards enhancing the understanding of the molecular biology of dengue virus replication with the ultimate goal being to develop novel antiviral strategies based on preventing critical inter- or intra-molecular interactions required for the normal virus life cycle. The viral RNA-dependent RNA polymerase (NS5) and the viral helicase (NS3) interaction offers a possible target for inhibitors to bind and prevent replication. In this study the yeast-two hybrid system was used to show that a small region of NS5 interacts with NS3, and also with the cellular nuclear transport receptor importin-beta. Furthermore, intramolecular interaction between the two putative domains of NS5 can also be detected by the yeast two-hybrid assay. We have also modified the colony lift assay for the beta-galactosidase reporter activity in intact yeast cells which reflects the strength of interaction between two proteins to a microtiter plate format. This assay offers a unique opportunity to screen for small molecule compounds that block physiologically important interactions.

A small region of the dengue virus-encoded RNA-dependent RNA polymerase, NS5, confers interaction with both the nuclear transport receptor importin-beta and the viral helicase, NS3

The dengue virus RNA-dependent RNA polymerase, NS5, and the protease/helicase, NS3, are multidomain proteins that have been shown to interact both in vivo and in vitro. A hyperphosphorylated form of NS5 that does not interact with NS3 has been detected in the nuclei of virus-infected cells, presumably as the result of the action of a functional nuclear localization sequence within the interdomain region of NS5 (residues 369-405). In this study, it is shown by using the yeast two-hybrid system that the C-terminal region of NS3 (residues 303-618) interacts with the N-terminal region of NS5 (residues 320-368). Further, it is shown that this same region of NS5 is also recognized by the cellular nuclear import receptor importin-beta. The interaction between NS5 and importin-beta and competition by NS3 with the latter for the same binding site on NS5 were confirmed by pull-down assays. The direct interaction of importin-beta with NS5 has implications for the mechanism by which this normally cytoplasmic protein may be targetted to the nucleus.

Targeted intracellular delivery of photosensitizers to enhance photodynamic efficiency

Photodynamic therapy (PDT) is a novel treatment, used mainly for anticancer therapy, that depends on the retention of photosensitizers (PS) in tumour cells and irradiation of the tumour with appropriate wavelength light. Photosensitizers are molecules such as porphyrins and chlorins that, on photoactivation, effect strongly localized oxidative damage within target cells. The PS used for PDT localize in various cytoplasmic membranous structures, but are not found in the most vulnerable intracellular sites for reactive oxygen species, such as the cell nucleus. The experimental approaches discussed in the present paper indicate that it is possible to design highly efficient molecular constructs, PS carriers, with specific modules conferring cell-specific targeting, internalization, escape from intracellular vesicles and targeting to the most vulnerable intracellular compartments, such as the nucleus. Nuclear targeting of these PS-carrying constructs results in enhanced photodynamic activity, maximally about 2500-fold that of free PS. Future work is intended to optimize this approach to the point at which tumour cells can be killed rapidly and efficiently, while minimizing normal cell and tissue damage.

Nuclear and nucleolar localization of parathyroid hormone-related protein

Parathyroid hormone-related protein (PTHrP) was first discovered as the factor causing hypercalcaemia produced by solid tumours frequently associated with the head and neck, breast, lung and kidney. The homology of its amino-terminus to parathyroid hormone (PTH; eight of the first 13 residues are identical), enables it to share the same receptor and perform similar biological functions to PTH. The sequences of PTHrP C-terminal to its PTH-like region confer functions such as transplacental calcium transport, renal bicarbonate excretion and in vitro osteoclast inhibition. Recent findings have shown that PTHrP is a nuclear/nucleolar protein in certain tissues and that this localization is cell cycle-regulated, mediated by the middle portion of the molecule, and involves the nuclear import receptor importin beta1. The present review discusses what is known about the pathway by which PTHrP localizes to the nucleus/nucleolus and the putative roles it may have there.

Nuclear targeting signal recognition: a key control point in nuclear transport?

Recent progress indicates that there are multiple pathways of nucleocytoplasmic transport which involve specific targeting sequences, such as nuclear localization sequences (NLSs), and cytosolic receptor molecules of the importin/karyopherin superfamily which recognise and dock the NLS-containing proteins at the nuclear pore. This first step of nuclear import/export is of central importance, with the affinity of the importin-targeting sequence interaction a critical parameter in determining transport efficiency. Different importins possess distinct NLS-binding specificities, which allows the system to be modulated through differential expression of the importins themselves, as well as through competition between different importins for the same protein, and between different proteins for the same importin. The targeting sequence-importin interaction can also be influenced directly by phosphorylation increasing the affinity of the interaction with importins or by targeting sequence masking through phosphorylation or specific protein binding. Targeting sequence recognition thus appears to represent a key control point in the regulation of nuclear transport. BioEssays 22:532-544, 2000.

Regulation of nuclear localization: a key to a door

Information can be transferred between the nucleus and the cytoplasm by translocating macromolecules across the nuclear envelope. Communication of extracellular or intracellular changes to the nucleus frequently leads to a transcriptional response that allows cells to survive in a continuously changing environment. Eukaryotic cells have evolved ways to regulate this movement of macromolecules between the cytoplasm and the nucleus such that the transfer of information occurs only under conditions in which a transcriptional response is required. This review focuses on the ways in which cells regulate movement of proteins across the nuclear envelope and the significance of this regulation for controlling diverse biological processes.

Importin beta recognizes parathyroid hormone-related protein with high affinity and mediates its nuclear import in the absence of importin alpha

Parathyroid hormone-related protein (PTHrP), expressed in a range of tumors, has endocrine, autocrine/paracrine, and intracrine actions, some of which relate to its ability to localize in the nucleus. Here we show for the first time that extracellularly added human PTHrP (amino acids 1-108) can be taken up specifically by receptor-expressing UMR106.01 osteogenic sarcoma cells and accumulate to quite high levels in the nucleus and nucleolus within 40 min. Quantitation of recognition by the nuclear localization sequence (NLS)-binding importin subunits indicated that in contrast to proteins containing conventional NLSs, PTHrP is recognized exclusively by importin beta and not by importin alpha. The sequence of PTHrP responsible for binding was mapped to amino acids 66-94, which includes an SV40 large tumor-antigen NLS-like sequence, although sequence determinants amino-terminal to this region were also necessary for high affinity binding (apparent dissociation constant of approximately 2 nM for importin beta). Nuclear import of PTHrP was assessed in vitro using purified components, demonstrating that importin beta, together with the GTP-binding protein Ran, was able to mediate efficient nuclear accumulation in the absence of importin alpha, whereas the addition of nuclear transport factor NTF2 reduced transport. The polypeptide ligand PTHrP thus appears to be accumulated in the nucleus/nucleolus through a novel, NLS-dependent nuclear import pathway independent of importin alpha and perhaps also of NTF2.

Negative charge at the protein kinase CK2 site enhances recognition of the SV40 large T-antigen NLS by importin: effect of conformation

SV40 large tumor-antigen (T-ag) nuclear import is enhanced by the protein kinase CK2 (CK2) site (Ser111Ser112) flanking the nuclear localization sequence (NLS). Here we use site-directed mutagenesis to examine the influence of negative charge and conformation at the site on T-ag nuclear import and recognition by the NLS-binding importin subunits. Negative charge through aspartic acid in place of Ser111 simulated CK2 phosphorylation in enhancing nuclear accumulation to levels well above those of proteins lacking a functional CK2 site. This was shown to be through enhancement of T-ag NLS recognition by importin using an ELISA-based assay. Asp112-substituted mutants containing proline at positions 109, 110 (wild-type position) or 111 were compared to assess the role of conformation at the CK2 site. Maximal nuclear import of the protein with Pro109 was lower than that of the Pro110 derivative, with the Pro111 variant even lower, these differences also being attributable to effects on importin binding. All results indicate a correlation of the initial nuclear import rate with the importin binding affinity, demonstrating that NLS recognition by importin is a key rate-determining step in nuclear import.

An engineered site for protein kinase C flanking the SV40 large T-antigen NLS confers phorbol ester-inducible nuclear import

Nuclear import of simian virus SV40 large tumour antigen (T-ag) is enhanced by the protein kinase CK2 (CK2) site flanking the nuclear localisation sequence (NLS). We report here that replacement of this site with a consensus site for protein kinase C (PK-C) can alter the regulation of T-ag nuclear import and render it inducible by phorbol ester. Measurement of nuclear import kinetics using fluorescently labelled proteins and confocal laser scanning microscopy show that the introduced PK-C site is functional in enhancing T-ag nuclear import compared to a protein lacking the CK2 site. Treatment with the PK-C activator phorbol 12-myristate 13-acetate (PMA) further increases the level of maximal nuclear accumulation and the initial nuclear import rate. This engineered PMA-responsive NLS may have application in targeting of molecules of interest to the nucleus in response to agents stimulating PK-C.