Enhanced Intracellular Mobility and Nuclear Accumulation of DNA Plasmids Associated with a Karyophilic Protein

Nucleic acid therapies for CNS diseases: Pathophysiology, targets, barriers, and delivery strategies

Nucleic acid therapeutics have emerged as one of the very advanced and efficacious treatment approaches for debilitating health conditions, including those diseases affecting the central nervous system (CNS). Precise targeting with an optimal control over gene regulation confers long-lasting benefits through the administration of nucleic acid payloads via viral, non-viral, and engineered vectors. The current review majorly focuses on the development and clinical translational potential of non-viral vectors for treating CNS diseases with a focus on their specific design and targeting approaches. These carriers must be able to surmount the various intracellular and extracellular barriers, to ensure successful neuronal transfection and ultimately attain higher therapeutic efficacies. Additionally, the specific challenges associated with CNS administration also include the presence of blood-brain barrier (BBB), the complex pathophysiological and biochemical changes associated with different disease conditions and the existence of non-dividing cells. The advantages offered by lipid-based or polymeric systems, engineered proteins, particle-based systems coupled with various approaches of neuronal targeting have been discussed in the context of a variety of CNS diseases. The possibilities of rapid yet highly efficient gene modifications rendered by the breakthrough methodologies for gene editing and gene manipulation have also opened vast avenues of research in neuroscience and CNS disease therapy. The current review also underscores the extensive scientific efforts to optimize specialized, efficacious yet non-invasive and safer administration approaches to overcome the therapeutic delivery challenges specifically posed by the CNS transport barriers and the overall obstacles to clinical translation.

A Review of Brain-Targeted Nonviral Gene-Based Therapies for the Treatment of Alzheimer's Disease

Diseases of the central nervous system (CNS) are difficult to treat owing to the complexity of the brain and the presence of a natural blood-brain-barrier (BBB). Alzheimer's disease (AD) is one of the major progressive and currently incurable neurodegenerative disorders of the CNS, which accounts for 60-80% of cases of dementia. The pathophysiology of AD involves the accumulation of amyloid beta (Aβ) plaques and neurofibrillary tangles (NFTs) in the brain. Additionally, synaptic loss and imbalance of neuronal signaling molecules are characterized as important markers of AD. Existing treatments of AD help in the management of its symptoms and aim toward the maintenance of cognitive functions, behavior, and attenuation of gradual memory loss. Over the past decade, nonviral gene therapy has attracted increasing interest due to its various advantages over its viral counterparts. Moreover, advancements in nonviral gene technology have led to their increasing contributions in clinical trials. However, brain-targeted nonviral gene delivery vectors come across various extracellular and intracellular barriers, limiting their ability to transfer the therapeutic gene into the target cells. Chief barriers to nonviral gene therapy have been discussed briefly in this review. We have also highlighted the rapid advancement of several nonviral gene therapies for AD, which are broadly categorized into physical and chemical methods. These methods aim to modulate Aβ, beta-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1), apolipoprotein E, or neurotrophic factors' expression in the CNS. Overall, this review discusses challenges and recent advancements of nonviral gene therapy for AD.

Nano-Particles Carried by Multiple Dynein Motors Self-Regulate Their Number of Actively Participating Motors

Intra-cellular active transport by native cargos is ubiquitous. We investigate the motion of spherical nano-particles (NPs) grafted with flexible polymers that end with a nuclear localization signal peptide. This peptide allows the recruitment of several mammalian dynein motors from cytoplasmic extracts. To determine how motor–motor interactions influenced motility on the single microtubule level, we conducted bead-motility assays incorporating surface adsorbed microtubules and combined them with model simulations that were based on the properties of a single dynein. The experimental and simulation results revealed long time trajectories: when the number of NP-ligated motors N_m increased, run-times and run-lengths were enhanced and mean velocities were somewhat decreased. Moreover, the dependence of the velocity on run-time followed a universal curve, regardless of the system composition. Model simulations also demonstrated left- and right-handed helical motion and revealed self-regulation of the number of microtubule-bound, actively transporting dynein motors. This number was stochastic along trajectories and was distributed mainly between one, two, and three motors, regardless of N_m. We propose that this self-regulation allows our synthetic NPs to achieve persistent motion that is associated with major helicity. Such a helical motion might affect obstacle bypassing, which can influence active transport efficiency when facing the crowded environment of the cell.

Non-Viral Targeted Nucleic Acid Delivery: Apply Sequences for Optimization

In nature, genomes have been optimized by the evolution of their nucleic acid sequences. The design of peptide-like carriers as synthetic sequences provides a strategy for optimizing multifunctional targeted nucleic acid delivery in an iterative process. The optimization of sequence-defined nanocarriers differs for different nucleic acid cargos as well as their specific applications. Supramolecular self-assembly enriched the development of a virus-inspired non-viral nucleic acid delivery system. Incorporation of DNA barcodes presents a complementary approach of applying sequences for nanocarrier optimization. This strategy may greatly help to identify nucleic acid carriers that can overcome pharmacological barriers and facilitate targeted delivery in vivo. Barcode sequences enable simultaneous evaluation of multiple nucleic acid nanocarriers in a single test organism for in vivo biodistribution as well as in vivo bioactivity.

Cytoplasmic transport and nuclear import of plasmid DNA

Productive transfection and gene transfer require not simply the entry of DNA into cells and subsequent transcription from an appropriate promoter, but also a number of intracellular events that allow the DNA to move from the extracellular surface of the cell into and through the cytoplasm, and ultimately across the nuclear envelope and into the nucleus before any transcription can initiate. Immediately upon entry into the cytoplasm, naked DNA, either delivered through physical techniques or after disassembly of DNA-carrier complexes, associates with a large number of cellular proteins that mediate subsequent interactions with the microtubule network for movement toward the microtubule organizing center and the nuclear envelope. Plasmids then enter the nucleus either upon the mitotic disassembly of the nuclear envelope or through nuclear pore complexes in the absence of cell division, using a different set of proteins. This review will discuss our current understanding of these pathways used by naked DNA during the transfection process. While much has been elucidated on these processes, much remains to be discerned, but with the development of a number of model systems and approaches, great progress is being made.

Peptides mediating DNA transport on microtubules and their impact on non-viral gene transfer efficiency

Synthetic vectors such as cationic polymers and cationic lipids remain attractive tools for non-viral gene transfer which is a complex process whose effectiveness relies on the ability to deliver a plasmid DNA (pDNA) into the nucleus of non-dividing cells. Once in the cytosol, the transport of pDNAs towards the nuclear envelope is strongly impaired by their very low cytosolic mobility due to their large size. To promote their movement towards the cell nucleus, few strategies have been implemented to exploit dynein, the microtubule’s (MT’s) motor protein, for propagation of cytosolic pDNA along the MTs towards the cell nucleus. In the first part of this review, an overview on MTs, dynein, dynein/virus interaction feature is presented followed by a summary of the results obtained by exploitation of LC8 and TCTEL1 dynein light chain association sequence (DLC-AS) for non-viral transfection. The second part dedicated to the adenoviral protein E3-14.7K, reports the transfection efficiency of polyplexes and lipoplexes containing the E3-14.7K-derived P79-98 peptide linked to pDNA. Here, several lines of evidence are given showing that dynein can be targeted to improve cytosolic pDNA mobility and accumulate pDNA near nuclear envelope in order to facilitate its transport through the nuclear pores. The linkage of various DLC-AS to pDNA carriers led to modest transfection improvements and their direct interaction with MTs was not demonstrated. In contrast, pDNA linked to the P79-98 peptide interacting with TCTEL1 via a cytosolic protein (fourteen seven K-interacting protein-1 (FIP-1)), interaction with MTs is evidenced in cellulo and transfection efficiency is improved.

Asn194Lys mutation in RVG29 peptide increases GFP transgene delivery by endocytosis to neuroblastoma and astrocyte cells

OBJECTIVES

A cell-penetrating peptide-based delivery system could target specific types of cells for therapeutic genes delivery. To increase the gene delivery efficiency into neuronal phenotype cells, we introduced an Asn194Lys mutation to RVG29 peptide derived from rabies virus glycoprotein and added a nuclear localization signal to enhance its nuclear import.

METHODS

Mutant RVG or wild-type RVG peptide, a karyophilic peptide (KP) and a plasmid encoding green fluorescent protein (pGL) were bound by electrostatic charges to form four different kinds of RVG complexes. Immunofluorescence was used to assess the gene transfection efficiency into astrocytes, oligodendrocyte precursor cells (OPCs), SH-SY5Y, HeLa and NIH/3T3 cells. The cellular uptake mechanism of RVG29 complexes was examined using endocytosis inhibitors.

KEY FINDINGS

The mRVG29 peptide has the ability to enhance the nuclear import of plasmids. The Asn194Lys mutation in RVG29 peptide of the pGL-mRVG29 complex and the addition of KP to the pGL-RVG29-KP complex increased the capacity to deliver DNA by endocytosis in astrocytes and SH-SY5Y cells.

CONCLUSIONS

The complexes pGL-mRVG29 and pGL-RVG29-KP have specificity for transfecting astrocytes and SH-SY5Y cells. The karyophilic capacity of this new mRVG peptide render it promising candidate to act as gene delivery vector into the brain cells.

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.

Intracellular Availability of pDNA and mRNA after Transfection: A Comparative Study among Polyplexes, Lipoplexes, and Lipopolyplexes

Intracellular availability of nucleic acids from synthetic vectors is critical and directly influences the transfection efficiency (TE). Herein, we evaluated the TE of polymer- and lipid-based nanoplexes (polyplexes, lipoplexes and lipopolyplexes) of EGFP-encoding mRNA and pDNA. To determine the translation and transcription efficiency of each nucleic acid nanoplex, in vitro expression was measured in HEK293T7 cells that permit gene expression in the cytoplasmic region. Globally, mRNA transfection profile was well corroborative with cytoplasmic transfection of pT7-pDNA as well as with nuclear transfection of pCMV-DNA. Irrespective of the nucleic acid, high TE was observed with histidinylated l-polyethylenimine (His-lPEI) polyplexes and dioleyl succinyl paromomycin/O,O-dioleyl-N-histamine phosphoramidate (DOPS/MM27) lipoplexes. Moreover, His-lPEI polyplexes yielded higher in vitro expression of EGFP for pDNA than for mRNA. Furthermore, a significant enhancement in the TE in the presence of an excess of His-lPEI was observed indicating that this polymer promotes cytosolic delivery. Compared to other nanoplexes, His-lPEI polyplex showed high intracellular availability of DNA and mRNA along with low cytotoxicity, owing to its rapid (complete or partial) unpacking in the cytosol and/or endosomes. This study gives an insight that, whether with mRNA or pDNA, enhancing nanoplex unpacking in the endosomes and cytosol would improve the delivery of nucleic acid in the cytosol and particularly in the case of pDNA where a sufficient available amount of pDNA in the cytoplasm would definitely improve its transport toward the nucleus.

Shaping of interphase chromosomes by the microtubule network

It is well established that microtubule dynamics play a major role in chromosome condensation and localization during mitosis. During interphase, however, it is assumed that the metazoan nuclear envelope presents a physical barrier, which inhibits interaction between the microtubules located in the cytoplasm and the chromatin fibers located in the nucleus. In recent years, it has become apparent that microtubule dynamics alter chromatin structure and function during interphase as well. Microtubule motor proteins transport several transcription factors and exogenous DNA (such as plasmid DNA) from the cytoplasm to the nucleus. Various soluble microtubule components are able to translocate into the nucleus, where they bind various chromatin elements leading to transcriptional alterations. In addition, microtubules may apply force on the nuclear envelope, which is transmitted into the nucleus, leading to changes in chromatin structure. Thus, microtubule dynamics during interphase may affect chromatin spatial organization, as well as transcription, replication and repair.

Effect of the Compaction and the Size of DNA on the Nuclear Transfer Efficiency after Microinjection in Synchronized Cells

The nuclear transfer process is one of the critical rate-limiting processes in transgene expression. In the present study, we report on the effect of compaction and the size of the DNA molecule on nuclear transfer efficiency by microinjection. A DNA/protamine complex- or variously-sized naked DNA molecules were injected into the cytoplasm or nucleus of synchronized HeLa cells. To evaluate the nuclear transfer process, a nuclear transfer score (NT score), calculated based on transgene expression after cytoplasmic microinjection divided by that after nuclear microinjection, was employed. The compaction of DNA with protamine decreased the NT score in comparison with the injection of naked DNA when the N/P ratio was increased to >2.0. Moreover, when naked DNA was microinjected, gene expression increased in parallel with the size of the DNA in the following order: minicircle DNA (MC07.CMV-EGFP; 2257 bp) > middle-sized plasmid DNA (pBS-EGFP; 3992 bp) > conventional plasmid DNA (pcDNA3.1-EGFP; 6172 bp), while the level of gene expression was quite comparable among them when the DNAs were injected into the nucleus. The above findings suggest that the intrinsic size of the DNA molecule is a major determinant for nuclear entry and that minicircle DNA has a great advantage in nuclear transfer.

Development and Challenges of Nanovectors in Gene Therapy

In recent years, hundreds of genes have been linked to a variety of human diseases, and the field of gene therapy has emerged as a way to treat this wide range of diseases. The main goal of gene therapy is to find a gene delivery vehicle that can successfully target diseased cells and deliver therapeutic genes directly to their cellular compartment. The two main types of gene delivery vectors currently being investigated in clinical trials are recombinant viral vectors and synthetic nonviral vectors. Recombinant viral vectors take advantage of the evolutionarily optimized viral mechanisms to deliver genes, but they can be hard to specifically target in vivo and are also associated with serious side effects. Synthetic nonviral vectors are made out of highly biocompatible lipids or polymers, but they are much less efficient at delivering their genetic payload due to the lack of any active delivery mechanism. This mini review will introduce the current state of gene delivery in clinical trials, and discuss the specific challenges associated with each of these vectors. It will also highlight some specific gaps in knowledge that are limiting the advancement of this field and touch on the current areas of research being explored to overcome them.

Intracellular co-delivery of zinc ions and plasmid DNA for enhancing gene transfection activity

Zinc ions, methylated poly(1-vinylimidazole) (PVIm-Me) and plasmid DNA (pDNA) have formed ternary complexes for gene delivery. The resulting Zn-PVIm-Me-pDNA complexes have delivered both Zn(2+) ions and pDNA inside cells, leading to the nuclear translocation of the pDNA. By use of the pDNA containing a nuclear protein, NF-κB, binding sequence, the intracellular co-delivery of Zn(2+) ions and pDNA has enhanced gene expression. These results suggest that the intracellular Zn(2+) ions delivered by Zn-PVIm-Me-pDNA complexes activated the NF-κB, enhancing the nuclear translocation of the pDNA. In conclusion, it has been demonstrated that the Zn-PVIm-Me-pDNA complex is capable of enhancing the gene transfection activity by a synergic effect of the PVIm-Me and the co-delivered intracellular Zn(2+) ions.

A novel nonviral gene delivery system: multifunctional envelope-type nano device

In this review we introduce a new concept for developing a nonviral gene delivery system which we call "Programmed Packaging." Based on this concept, we succeeded in developing a multifunctional envelope-type nano device (MEND), which exerts high transfection activities equivalent to those of an adenovirus in a dividing cell. The use of MEND has been extended to in vivo applications. PEG/peptide/DOPE ternary conjugate (PPD)-MEND, a new in vivo gene delivery system for the targeting of tumor cells that dissociates surface-modified PEG in tumor tissue by matrix metalloproteinase (MMP) and exerts significant transfection activities, was developed. In parallel with the development of MEND, a quantitative gene delivery system, Confocal Image-assisted 3-dimensionally integrated quantification (CIDIQ), also was developed. This method identified the rate-limiting step of the nonviral gene delivery system by comparing it with adenoviral-mediated gene delivery. The results of this analysis provide a new direction for the development of rational nonviral gene delivery systems.

Influence of pDNA availability on transfection efficiency of polyplexes in non-proliferative cells

We succeeded in visualizing plasmid DNA (pDNA) in the nucleus and cytosol of non-proliferative cells after transfection with linear polyethylenemine (lPEI) and histidinylated lPEI (His16-lPEI). This was possible with confocal microscope by using pDNA labelled with quantum dots. Indeed pDNA labelled with Cy3 leads to false positive nuclear localization because the saturation of the fluorescence signal overestimated the volume occupied by Cy3-pDNA. Moreover, Cy3 brightness was too weak to detect low amount of pDNA. About 20 to 40 pDNA copies were detected in the nucleus after the transfection of pDNA labelled with quantum dots. Transfection efficiency and cellular imaging data suggested that the cytosolic availability of pDNA, including endosome escape and/or polyplexes dissociation, is crucial for its nuclear delivery. In vitro transcription assay and transfection of cells allowing cytosolic gene expression concluded to better cytosolic availability of pDNA within His16-lPEI polyplexes. Cryo-TEM analyses revealed that His16-lPEI polyplexes exhibited a spherical shape and an amorphous internal structure which differed from the high degree of order of lPEI polyplexes. Altogether, this comparative study indicated that the high transfection efficiency of non-proliferative cells with His16-lPEI polyplexes was related to the amorphous structure and the facilitated dissociation of the assemblies.

A new role for clathrin adaptor proteins 1 and 3 in lipoplex trafficking

Intracellular protein trafficking through secretory and endocytic pathways depends on the function of adaptor proteins that bind motifs on cargo proteins. The adaptor proteins then recruit coat proteins such as clathrin, enabling the formation of a transport vesicle. While studying the role of the clathrin adaptor proteins, AP-1, AP-2 and AP-3 in viral protein trafficking, we discovered that AP-1 and AP-3 potentially have a role in successful transfection of mammalian cells with DNA-liposome complexes (lipoplexes). We showed that AP-1, -2 and -3 are not required for lipoplexes to enter cells, but that lipoplexes and/or released DNA are unable to reach the nucleus in the absence of AP-1 or AP-3, leading to minimal exogenous gene expression. In contrast, gene expression from liposome-delivered mRNA, which does not require nuclear entry, was not impaired by the absence of AP-1 or AP-3. Despite the use of lipoplexes to mediate gene delivery being so widely used in cell biology and, more recently, gene therapy, the mechanism by which lipoplexes or DNA reach the nucleus is poorly characterised. This work sheds light on the components involved in this process, and demonstrates a novel role for AP-1 and AP-3 in trafficking lipoplexes.

Development of a non-viral gene delivery vector based on the dynein light chain Rp3 and the TAT peptide

Gene therapy and DNA vaccination trials are limited by the lack of gene delivery vectors that combine efficiency and safety. Hence, the development of modular recombinant proteins able to mimic mechanisms used by viruses for intracellular trafficking and nuclear delivery is an important strategy. We designed a modular protein (named T-Rp3) composed of the recombinant human dynein light chain Rp3 fused to an N-terminal DNA-binding domain and a C-terminal membrane active peptide, TAT. The T-Rp3 protein was successfully expressed in Escherichia coli and interacted with the dynein intermediate chain in vitro. It was also proven to efficiently interact and condense plasmid DNA, forming a stable, small (∼100nm) and positively charged (+28.6mV) complex. Transfection of HeLa cells using T-Rp3 revealed that the vector is highly dependent on microtubule polarization, being 400 times more efficient than protamine, and only 13 times less efficient than Lipofectamine 2000™, but with a lower cytotoxicity. Confocal laser scanning microcopy studies revealed perinuclear accumulation of the vector, most likely as a result of transport via microtubules. This study contributes to the development of more efficient and less cytotoxic proteins for non-viral gene delivery.

Strategies on the nuclear-targeted delivery of genes

To improve the nuclear-targeted delivery of non-viral vectors, extensive effort has been carried out on the development of smart vectors which could overcome multiple barriers. The nuclear envelope presents a major barrier to transgene delivery. Viruses are capable of crossing the nuclear envelope to efficiently deliver their genome into the nucleus through the specialized protein components. However, non-viral vectors are preferred over viral ones because of the safety concerns associated with the latter. Non-viral delivery systems have been designed to include various types of components to enable nuclear translocation at the periphery of the nucleus. This review summarizes the progress of research regarding nuclear transport mechanisms. "Smart" non-viral vectors that have been modified by peptides and other small molecules are able to facilitate the nuclear translocation and enhance the efficacy of gene expression. The resulting technology may also enhance delivery of other macromolecules to the nucleus.

Kinesin KIFC1 actively transports bare double-stranded DNA

During the past years, exogenous DNA molecules have been used in gene and molecular therapy. At present, it is not known how these DNA molecules reach the cell nucleus. We used an in cell single-molecule approach to observe the motion of exogenous short DNA molecules in the cytoplasm of eukaryotic cells. Our observations suggest an active transport of the DNA along the cytoskeleton filaments. We used an in vitro motility assay, in which the motion of single-DNA molecules along cytoskeleton filaments in cell extracts is monitored; we demonstrate that microtubule-associated motors are involved in this transport. Precipitation of DNA-bound proteins and mass spectrometry analyses reveal the preferential binding of the kinesin KIFC1 on DNA. Cell extract depletion of kinesin KIFC1 significantly decreases DNA motion, confirming the active implication of this molecular motor in the intracellular DNA transport.

Synergistic effect of a biosurfactant and protamine on gene transfection efficiency

Several barriers need to be overcome to ensure successful gene transfection, including passing of the foreign gene through the plasma membrane, escape of this material from lysosomal degradation, and its translocation into the nucleus. We previously showed that the biosurfactant mannosylerythritol lipid-A (MEL-A) enhanced the efficiency of gene transfection mediated by cationic liposomes by facilitating rapid delivery of foreign genes into target cells through membrane fusion between liposomes and the plasma membrane. Moreover, using MEL-A-containing cationic liposomes, the foreign gene was efficiently delivered into the nucleus because it was released directly into the cytosol and thus escaped lysosomal degradation. Here we investigated the effect of pre-condensation of plasmid DNA by a cationic polymer, protamine, on gene transfection. We found that the efficiency of pre-condensed DNA transfection mediated by MEL-A-containing OH liposomes was >10 times higher than that of non-condensed DNA transfection. In contrast, the efficiency of pre-condensed DNA transfection mediated by OH liposomes was only 1.5 times higher than that of non-condensed DNA transfection. MEL-A did not influence plasmid DNA encapsulation by cationic liposomes, but it greatly accelerated the nuclear delivery of pre-condensed plasmid DNA. Our findings indicate that MEL-A and protamine synergistically accelerate the nuclear delivery of foreign gene and consequently promote gene transfection efficiency.

Importin-7 mediates nuclear trafficking of DNA in mammalian cells

Eukaryotic cells have the ability to uptake and transport endogenous and exogenous DNA in their nuclei, however little is known about the specific pathways involved. Here we show that the nuclear transport receptor importin 7 (imp7) supports nuclear import of supercoiled plasmid DNA and human mitochondrial DNA in a Ran and energy-dependent way. The imp7-dependent pathway was specifically competed by excess DNA but not by excess of maltose-binding protein fused with the classical nuclear localizing signal (NLS) or the M9 peptides. Transport of DNA molecules complexed with poly-l-lysine was impaired in intact cells depleted of imp7, and DNA complexes remained localized in the cytoplasm. Poor DNA nuclear import in cells depleted of imp7 directly correlated with lower gene expression levels in these cells compared to controls. Inefficient nuclear import of transfected DNA induced greater upregulation of the interferon pathway, suggesting that rapid DNA nuclear import may prevent uncontrolled activation of the innate immune response. Our results provide evidence that imp7 is a non-redundant component of an intrinsic pathway in mammalian cells for efficient accumulation of exogenous and endogenous DNA in the nucleus, which may be critical for the exchange of genetic information between mitochondria and nuclear genomes and to control activation of the innate immune response.

Particle tracking analysis for the intracellular trafficking of nanoparticles modified with African swine fever virus protein p54-derived peptide

Previous studies showed that the cytoplasmic transport of nanoparticles to the nucleus is driven by a vesicular sorting system. Artificial approaches for targeting a microtubule-associating motor complex is also a challenge. We describe herein the development of a liposomal nanoparticle, the surface of which is modified with stearylated octa-arginine (STR-R8), and a dynein light chain (LC8)-associated peptide derived from an African swine fever virus protein p54 (p54(149-161)) with polyethyleneglycol (PEG) as a spacer (p54(149-161)-PEG/R8-liposomal nanoparticles (LNPs)). The p54(149-161)-PEG/R8-LNPs preferentially gain access to the nucleus, resulting in a one- to two-order of magnitude higher transfection activity in comparison with p54(149-161)-free nanoparticles (PEG/R8-LNPs). Further studies of particle tracking in HeLa cells stably expressing green fluorescent protein (GFP)-tagged tubulin (GFP/Tub-HeLa) indicate that p54(149-161) stimulated the transport of nanoparticles along fibrous tubulin structures. Moreover, a part of the p54(149-161)-PEG/R8-LNPs appeared to undergo quasi-straight transport without sharing the tracks corresponding to PKH67, the plasma membrane of which had been prestained with a marker just before transfection, while corresponding movement was never observed in the case of PEG/R8-LNPs. These findings suggest that a portion of the p54(149-161)-modified nanoparticles can use microtubule-dependent transport without the need for an assist by a vesicular sorting system.

Proteomic and functional analyses of protein-DNA complexes during gene transfer

One of the barriers to successful nonviral gene delivery is the crowded cytoplasm, which plasmids need to actively traverse for gene expression. Relatively little is known about how this process occurs, but our lab and others have shown that the microtubule network and motors are required for plasmid movement to the nucleus. To further investigate how plasmids exploit normal physiological processes to transfect cells, we have taken a proteomics approach to identify the proteins that comprise the plasmid-trafficking complex. We have developed a live cell DNA-protein pull-down assay to isolate complexes at certain time points post-transfection (15 minutes to 4 hours) for analysis by mass spectrometry (MS). Plasmids containing promoter sequences bound hundreds of unique proteins as early as 15 minutes post-electroporation, while a plasmid lacking any eukaryotic sequences failed to bind many of the proteins. Specific proteins included microtubule-based motor proteins (e.g., kinesin and dynein), proteins involved in protein nuclear import (e.g., importin 1, 2, 4, and 7, Crm1, RAN, and several RAN-binding proteins), a number of heterogeneous nuclear ribonucleoprotein (hnRNP)- and mRNA-binding proteins, and transcription factors. The significance of several of the proteins involved in protein nuclear localization and plasmid trafficking was determined by monitoring movement of microinjected fluorescently labeled plasmids via live cell particle tracking in cells following protein knockdown by small-interfering RNA (siRNA) or through the use of specific inhibitors. While importin β1 was required for plasmid trafficking and subsequent nuclear import, importin α1 played no role in microtubule trafficking but was required for optimal plasmid nuclear import. Surprisingly, the nuclear export protein Crm1 also was found to complex with the transfected plasmids and was necessary for plasmid trafficking along microtubules and nuclear import. Our results show that various proteins involved in nuclear import and export influence intracellular trafficking of plasmids and subsequent nuclear accumulation.

Subcellular knockout of importin β1 perturbs axonal retrograde signaling

Subcellular localization of mRNA enables compartmentalized regulation within large cells. Neurons are the longest known cells; however, so far, evidence is lacking for an essential role of endogenous mRNA localization in axons. Localized upregulation of Importin β1 in lesioned axons coordinates a retrograde injury-signaling complex transported to the neuronal cell body. Here we show that a long 3' untranslated region (3' UTR) directs axonal localization of Importin β1. Conditional targeting of this 3' UTR region in mice causes subcellular loss of Importin β1 mRNA and protein in axons, without affecting cell body levels or nuclear functions in sensory neurons. Strikingly, axonal knockout of Importin β1 attenuates cell body transcriptional responses to nerve injury and delays functional recovery in vivo. Thus, localized translation of Importin β1 mRNA enables separation of cytoplasmic and nuclear transport functions of importins and is required for efficient retrograde signaling in injured axons.

Insertion of a nuclear factor kappa B DNA nuclear-targeting sequence potentiates suicide gene therapy efficacy in lung cancer cell lines

Lung cancer currently causes the majority of cancer-related deaths worldwide and new treatments are in high demand. Gene therapy could be a promising treatment but currently lacks sufficient efficiency for clinical use, primarily due to limited cellular and nuclear DNA delivery. In the present study, we investigated whether it was possible to exploit the endogenous nuclear-shuttling activity by the nuclear factor kappa B (NFκB) system, which is highly prominent in many cancers as well as lung cancer. We observed that insertion of a DNA nuclear-targeting sequence (DTS) recognized by NFκB could improve plasmid nuclear delivery and enhance the therapeutic effect of a validated transcriptionally cancer-targeted suicide gene therapy system. A clear correlation between the number of inserted NFκB-binding sites and the therapeutic effect of the suicide system was observed in both small cell lung cancer (SCLC) and non-SCLC cell lines. The effect was observed to be due to elevated nuclear translocation of the suicide gene-encoding plasmids. The results show that a significant improvement of gene therapeutic efficiency can be obtained by increasing the intracellular trafficking of therapeutic DNA. This is to our knowledge the first time a DTS strategy has been implemented for suicide gene therapy.

Highly acetylated tubulin permits enhanced interactions with and trafficking of plasmids along microtubules

Microtubule-based transport is required for plasmid translocation to the nucleus during transfections, and having stable structures could enhance this movement. In previous studies in which the cytoskeleton was disrupted, we found that populations of microtubules remain that are stable and highly acetylated. By increasing the levels of acetylated tubulin through inhibition of the tubulin deacetylase HDAC6, we observe more rapid plasmid nuclear localization of transfected plasmids and greater levels of gene transfer. In this study, we sought to understand plasmid movement in cells with enhanced tubulin acetylation. Using variations of a microtubule spin down assay, we found that plasmids bound to hyper-acetylated microtubules to a greater degree than they did to unmodified microtubules. To determine if microtubule acetylation also affects cytoplasmic trafficking, plasmid movement was evaluated in real time by particle tracking in cells with varying levels of acetylated microtubules. We found that plasmids display greater net rates of movement, spend more time in productive motion and display longer runs of continuous motion in cells with highly acetylated microtubules compared to those with fewer modifications. These results all suggest that plasmid movement is enhanced along highly acetylated microtubules, reducing the time spent in the cytoplasm prior to nuclear import. Taken together, these findings provide a foundation for determining how modulation of microtubule acetylation can be used as a means to increase intracellular trafficking of plasmids and enhance gene therapy.

Gene transfer by chemical vectors, and endocytosis routes of polyplexes, lipoplexes and lipopolyplexes in a myoblast cell line

Chemical vectors are widely developed for providing safe DNA delivery systems. It is well admitted that their endocytosis and intracellular trafficking are critical for the transfection efficiency. Here, we have compared the endocytic pathways of lipoplexes, polyplexes and lipopolyplexes formed with carriers of various chemical compositions. Engineered C2C12 mouse myoblast cells expressing Rab5-EGFP, Rab7-EGFP or Cav1-GFP were used to monitor the location of the plasmid DNA into the endocytic compartments by real time fluorescence confocal microscopy. We observed that (i) DNA complexes made with dioleyl succinyl paromomycin:O,O-dioleyl-N-histamine phosphoramidate (DOSP/MM27) liposomes or histidinylated lPEI (His-lPEI) allowing the highest transfection efficiency displayed a positive ζ potential and were internalized by clathrin-mediated endocytosis, (ii) DOSP/MM27 lipoplexes were 6-times more internalized than His-lPEI polyplexes, (iii) all negatively charged DNA complexes lead to less efficient transfection and entered the cells via caveolae and (iv) lipopolyplexes allowing high transfection efficiency were weakly internalized via caveolae. Our results indicate that the transfection efficiency is better correlated with the nature of the endocytic pathway than with the uptake efficacy. This study shows also that engineered cells expressing specific fluorescent compartments are convenient tools to monitor endocytosis of a fluorescent plasmid DNA by real time fluorescence confocal microscopy.

Intracellular partitioning of cell organelles and extraneous nanoparticles during mitosis

The nucleocytoplasmic partitioning of nanoparticles as a result of cell division is highly relevant to the field of nonviral gene delivery. We reviewed the literature on the intracellular distribution of cell organelles (the endosomal vesicles, Golgi apparatus, endoplasmic reticulum and nucleus), foreign macromolecules (dextrans and plasmid DNA) and inorganic nanoparticles (gold, quantum dot and iron oxide) during mitosis. For nonviral gene delivery particles (lipid- or polymer-based), indirect proof of nuclear entry during mitosis is provided. We also describe how retroviruses and latent DNA viruses take advantage of mitosis to transfer their viral genome and segregate their episomes into the host daughter nuclei. Based on this knowledge, we propose strategies to improve nonviral gene delivery in dividing cells with the ultimate goal of designing nonviral gene delivery systems that are as efficient as their viral counterparts but non-immunogenic, non-oncogenic and easy and inexpensive to prepare.

Transcription factor plasmid binding modulates microtubule interactions and intracellular trafficking during gene transfer

For non-viral gene delivery to be successful, plasmids must move through the cytoplasm to the nucleus in order to be transcribed. While the cytoskeletal meshwork acts as a barrier to plasmid DNA movement in the cytoplasm, the microtubule network is required for directed plasmid trafficking to the nucleus. We have shown previously that plasmid-microtubule interactions require cytoplasmic adapter proteins such as molecular motors, transcription factors (TFs) and importins. However, not all plasmid sequences support these interactions to allow movement to the nucleus. We now demonstrate that microtubule-DNA interactions can show sequence specificity with promoters containing binding sites for cyclic AMP response-element binding protein (CREB), including the cytomegalovirus immediate early promoter (CMV(iep)). Plasmids containing CREB-binding sites showed stringent interactions in an in vitro microtubule-binding assay. Using microinjection and real-time particle tracking, we show that the inclusion of TF binding sites within plasmids permits cytoplasmic trafficking of plasmids during gene transfer. We found that CREB-binding sites are bound by CREB in the cytoplasm during transfection, and allow for enhanced rates of movement and subsequent nuclear accumulation. Moreover, small interfering RNA knockdown of CREB prevented this enhanced trafficking. Therefore, TF binding sites within plasmids are necessary for interactions with microtubules and enhance movement to the nucleus.

How to screen non-viral gene delivery systems in vitro?

Screening of new gene delivery candidates regarding transfection efficiency and toxicity is usually performed by reading out transgene expression levels relative to a reference formulation after in vitro transfection. However, over the years and among different laboratories, this screening has been performed in a variety of cell lines, using a variety of conditions and read-out systems, and by comparison to a variety of reference formulations. This makes a direct comparison of results difficult, if not impossible. Reaching a consensus would enable placing new results into context of previous findings and estimate the overall contribution to the improvement of non-viral gene delivery. In this paper we illustrate the sensitivity of transfection outcomes on testing conditions chosen, and propose a screening protocol with the aim of standardization within the field.

DNA nuclear targeting sequences for non-viral gene delivery

Purpose

To evaluate if introduction of DNA nuclear Targeting Sequences (DTS; i.e. recognition sequences for endogenous DNA-binding proteins) in plasmid DNA (pDNA) leads to increased transfection efficiency of non-viral gene delivery by virtue of enhanced nuclear import of the pDNA.

Methods

A set of DTS was identified and cloned into EGFP-reporter plasmids controlled by the CMV-promoter. These pDNA constructs were delivered into A431 and HeLa cells using standard electroporation, pEI-based polyfection or lipofection methods. The amount of pDNA delivered into the nucleus was determined by qPCR; transfection efficiency was determined by flow cytometry.

Results

Neither of these DTS increased transgene expression. We varied several parameters (mitotic activity, applied dose and delivery strategy), but without effect. Although upregulated transgene expression was observed after stimulation with TNF-α, this effect could be ascribed to non-specific upregulation of transcription rather than enhanced nuclear import. Nuclear copy numbers of plasmids containing or lacking a DTS did not differ significantly after lipofectamine-based transfection in dividing and non-dividing cells.

Conclusion

No beneficial effects of DTS on gene expression or nuclear uptake were observed in this study.

Enhancement of dendritic cells transfection in vivo and of vaccination against B16F10 melanoma with mannosylated histidylated lipopolyplexes loaded with tumor antigen messenger RNA

We report the preparation of mannosylated nanoparticles loaded with messenger RNA (mRNA) that enhance the transfection of dendritic cells (DCs) in vivo and the anti-B16F10 melanoma vaccination in mice. Mannosylated and histidylated lipopolyplexes (Man(11)-LPR100) were obtained by adding mannosylated and histidylated liposomes to mRNA-PEGylated histidylated polylysine polyplexes. Upon intravenous injection, ∼9% of the radioactivity of technetium 99 m-labeled lipopolyplexes measured in the liver, spleen, lungs, and kidneys was found in the spleen. We demonstrate that spleen from mice injected with enhanced green fluorescent protein (EGFP) mRNA-loaded Man(11)-LPR100 contained four times more DCs expressing EGFP than that from mice injected with sugar-free LPR100. This better transfection of DCs is correlated with a better inhibition of B16F10 melanoma growth and an increased survival time when mice were immunized with MART-1 mRNA-loaded Man(11)-LPR100. These results indicate that mannosylated and histidylated LPR is an efficient system for the delivery of tumor antigen mRNA in splenic DCs aiming to induce an anticancer immune response.

FROM THE CLINICAL EDITOR

This paper discusses the preparation of mannosylated nanoparticles loaded with messenger RNA that enhance the transfection of dendritic cells (DCs) in vivo and the anti-B16F10 melanoma vaccination in mice. The authors describe an efficient system for the delivery of tumor antigen mRNA in splenic DCs aiming to induce an anticancer immune response.

Synergistic effects between natural histone mixtures and polyethylenimine in non-viral gene delivery in vitro

Nanoparticles made of plasmid DNA (pDNA) and cationic polymers are promising strategies for non-viral gene delivery. However, many cationic polymers are toxic to cells when used in higher concentrations. Positively charged proteins, such as histones, are biodegradable and a good alternative, especially for potential in vivo applications. It has previously been shown that histones are able to complex DNA and mediate transfection of cells. To investigate possible synergistic effects between the different histone types and to avoid the use of recombinant proteins, we analysed whether natural histone mixtures would be functional as gene carriers. Core and linker histones from calf thymus and from chicken erythrocytes were used to transfect different cell lines. The protein mixtures efficiently complexed the pDNA, and the resulting particles entered the cells. However, only marginal expression of the gene encoded by the pDNA was observed. Transfection rates increased drastically when minimal amounts of the basic polymer polyethylenimine (PEI) were added to the particles. Neither PEI nor histones alone mediated any transfection under the conditions where a combination of both worked efficiently, and the combined particles were well tolerated by the cells. These results demonstrate that histone mixtures from natural sources in combination with minimal amounts of PEI can be used as gene carriers. This might have consequences for the development of novel gene delivery strategies, such as DNA vaccines, with minimal side-effects.

Chemical vectors for gene delivery: uptake and intracellular trafficking

Chemical vectors for non-viral gene delivery are based on engineered DNA nanoparticles produced with various range of macromolecules suitable to mimic some viral functions required for gene transfer. Many efforts have been undertaken these past years to identify cellular barriers that have to be passed for this issue. Here, we summarize the current status of knowledge on the uptake mechanism of DNA nanoparticles made with polymers and liposomes, their endosomal escape, cytosolic diffusion, and nuclear import of pDNA. Studies reported these past years regarding pDNA nanoparticles endocytosis indicated that there is no clear evident relationship between the ways of entry and the transfection efficiency. By contrast, the sequestration of pDNA in intracellular vesicles and the low number of pDNA close to the nuclear envelop are identified as the major intracellular barriers. So, intensive investigations to increase the cytosolic delivery of pDNA and its migration toward nuclear pores make sense to bring the transfection efficiency closer to that of viruses.

Reducible poly(amido ethylenimine)-based gene delivery system for improved nucleus trafficking of plasmid DNA

In a nonviral gene delivery system, localization of a plasmid DNA in the nucleus is a prerequisite for expression of a desired therapeutic protein encoded in the plasmid DNA. In this study, a reducible polymer-based gene delivery system for improved intracellular trafficking and nuclear translocation of plasmid DNA is introduced. The system is consisted of two components, a plasmid DNA having repeated binding sequence for a karyophilic protein, NFkappaB, and a reducible polymer. A reducible poly(amido ethylenimine), poly(TETA-CBA), was synthesized by a Michael-type addition polymerization between cystamine bisacrylamide and triethyl tetramine. The polymer forming tight complexes with plasmid DNA could be degraded in the reductive cytosol to release the plasmid DNA. The triggered release mechanism in the cytosol could facilitate the interaction between cytosolic NFkappaB and the plasmid DNA having repeated NFkappaB biding motif. Upon activation of NFkappaB by interleukin-1beta (IL-1beta), most of the plasmid distributed in the cytoplasm was localized within the nucleus, resulting in significantly higher gene transfection efficiency than controls with nondegradable PEI. The current study suggests an alternative way of improving transfection efficiency by taking advantage of endogenous transport machinery for intracellular trafficking and nuclear translocation of a plasmid DNA.

Delivery of DNA-based cancer vaccine with polyethylenimine

DNA-based vaccine directed to tumor-specific antigens is an attractive strategy in cancer prevention and therapy. In view of the poor immunogenicity of most tumor-associated antigens, studies were carried out here to examine the adjuvant effect of polyethylenimine (PEI), a cationic polymer widely used in non-viral gene delivery, on the efficacy of cancer vaccine strategy. Treatment of animals with the DNA/PEI complexes resulted in antigen-specific cell lysis and activation of B3Z cells, an ovalbumin/K(b)-specific cytotoxic clone that recognizes the target cells through the class I major histocompatibility complex (MHC) molecules. Immunohistochemical examination showed that PEI-mediated DNA vaccination induced cell death and significant lymphocyte infiltration at the injection sites. Immunization of C57BL/6J mice with the DNA/PEI complexes either preceded or after tumor cell inoculation suppressed tumor growth and prolonged the survival rate of the animals. Results obtained in this study illustrated the potential use of PEI as an adjuvant in DNA-based cancer vaccination for induction of protective and therapeutic immunity.

Transportin mediates nuclear entry of DNA in vertebrate systems

Delivery of DNA to the cell nucleus is an essential step in many types of viral infection, transfection, gene transfer by the plant pathogen Agrobacterium tumefaciens and in strategies for gene therapy. Thus, the mechanism by which DNA crosses the nuclear pore complex (NPC) is of great interest. Using nuclei reconstituted in vitro in Xenopus egg extracts, we previously studied DNA passage through the nuclear pores using a single-molecule approach based on optical tweezers. Fluorescently labeled DNA molecules were also seen to accumulate within nuclei. Here we find that this import of DNA relies on a soluble protein receptor of the importin family. To identify this receptor, we used different pathway-specific cargoes in competition studies as well as pathway-specific dominant negative inhibitors derived from the nucleoporin Nup153. We found that inhibition of the receptor transportin suppresses DNA import. In contrast, inhibition of importin beta has little effect on the nuclear accumulation of DNA. The dependence on transportin was fully confirmed in assays using permeabilized HeLa cells and a mammalian cell extract. We conclude that the nuclear import of DNA observed in these different vertebrate systems is largely mediated by the receptor transportin. We further report that histones, a known cargo of transportin, can act as an adaptor for the binding of transportin to DNA.

Chemical vectors for gene delivery: a current review on polymers, peptides and lipids containing histidine or imidazole as nucleic acids carriers

DNA/cationic lipid (lipoplexes), DNA/cationic polymer (polyplexes) and DNA/cationic polymer/cationic lipid (lipopolyplexes) electrostatic complexes are proposed as non-viral nucleic acids delivery systems. These DNA-nanoparticles are taken up by the cells through endocytosis processes, but the low capacity of DNA to escape from endosomes is regarded as the major limitations of their transfection efficiency. Here, we present a current report on a particular class of carriers including the polymers, peptides and lipids, which is based on the exploitation of the imidazole ring as an endosome destabilization device to favour the nucleic acids delivery in the cytosol. The imidazole ring of histidine is a weak base that has the ability to acquire a cationic charge when the pH of the environment drops bellow 6. As it has been demonstrated for poly(histidine), this phenomena can induce membrane fusion and/or membrane permeation in an acidic medium. Moreover, the accumulation of histidine residues inside acidic vesicles can induce a proton sponge effect, which increases their osmolarity and their swelling. The proof of concept has been shown with polylysine partially substituted with histidine residues that has caused a dramatic increase by 3-4.5 orders of magnitude of the transfection efficiency of DNA/polylysine polyplexes. Then, several histidine-rich polymers and peptides as well as lipids with imidazole, imidazolinium or imidazolium polar head have been reported to be efficient carriers to deliver nucleic acids including genes, mRNA or SiRNA in vitro and in vivo. More remarkable, histidylated carriers are often weakly cytotoxic, making them promising chemical vectors for nucleic acids delivery.

Tissue-specific and transcription factor-mediated nuclear entry of DNA

Low levels of gene transfer and a lack of tissue-specific targeting of vectors have limited the therapeutic potential of non-viral gene therapy. This is due to the numerous cellular barriers that hinder nuclear delivery of vectors and the paucity of methods that restrict expression to specific cells types. In non-dividing cells, the nuclear envelope is an especially problematic hurdle to gene transfer. Given that the majority of target tissues are non-dividing in vivo, the nuclear membrane is a major obstacle to therapeutic gene transfer. In this review, the various barriers to gene transfer are discussed. In particular, the role of the nuclear pore complex (NPC) in regulating passage of plasmid vectors during interphase is reviewed. Several methods of modifying plasmid (pDNA) vectors to enhance nuclear import through the NPC are also discussed, including the use of tissue-specific transcription factors to mediate nuclear entry of pDNA in a cell-specific manner.

Identification of protein cofactors necessary for sequence-specific plasmid DNA nuclear import

Although transfections are routinely used in the laboratory, the mechanism(s) by which exogenous DNA is transported into the nucleus is poorly understood. By improving our understanding of how vectors circumvent the numerous cellular barriers to gene transfer, more efficient gene delivery methods can be devised. We have begun to design plasmid constructs that enter the nucleus of specific cell types in the absence of cell division, thereby enhancing levels of expression. We have shown that inclusion of specific DNA sequences in plasmid constructs mediates nuclear import both in vitro and in vivo. Here, we use plasmid affinity chromatography, mass spectrometry (MS), and live-cell pulldowns of transfected plasmid constructs to identify protein cofactors that interact in a sequence-specific manner with these DNA nuclear targeting sequences (DTSs). Importin beta(1), importin 7, and the small guanosine triphosphatase Ran all demonstrate DTS-specific interaction in both MS and pull-down assays, consistent with our model of plasmid nuclear import. In addition, knockdown of importin beta(1) with small interfering RNA (siRNA) abrogates plasmid nuclear import, indicating that it is a necessary cofactor. Our discovery that specific karyopherins mediate plasmid nuclear import can be used to design more effective vectors for gene delivery.

Nanotechnology approaches for gene transfer

In both basic research as well as experimental gene therapy the need to transfer genetic material into a cell is of vital importance. The cellular compartment, which is the target for the genetic material, depends upon application. An siRNA that mediates silencing is preferably delivered to the cytosol while a transgene would need to end up in the nucleus for successful transcription to occur. Furthermore the ability to regulate gene expression has grown substantially since the discovery of RNA interference. In such diverse fields as medical research and agricultural pest control, the capability to alter the genetic output has been a useful tool for pushing the scientific frontiers. This review is focused on nanotechnological approaches to assemble optimised structures of nucleic acid derivatives to facilitate gene delivery as well as promoting down regulation of endogenous genes.

Nuclear transport factors in neuronal function

Active nucleocytoplasmic transport of macromolecules requires soluble transport carriers of the importin/karyopherin superfamily. Although the nuclear transport machinery is essential in all eukaryotic cells, neurons must also mobilise importins and associated proteins to overcome unique spatiotemporal challenges. These include switches in importin alpha subtype expression during neuronal differentiation, localized axonal synthesis of importin beta1 to coordinate a retrograde injury signaling complex on axonal dynein, and trafficking of regulatory and signaling molecules from synaptic terminals to cell bodies. Targeting of RNAs encoding critical components of the importins complex and the Ran system to axons allows sophisticated local regulation of the system for mobilization upon need. Finally, a number of importin family members have been associated with mental or neurodegenerative diseases. The extended roles recently discovered for importins in the nervous system might also be relevant in non-neuronal cells, and the localized modes of importin regulation in neurons offer new avenues to interrogate their cytoplasmic functions.

Ran on tracks – cytoplasmic roles for a nuclear regulator

The GTPase Ran is best known for its crucial roles in the regulation of nucleocytoplasmic transport in interphase cells and in the organization of the spindle apparatus during mitosis. A flurry of recent reports has now implicated Ran in diverse cytoplasmic events, including trafficking of an ephrin receptor homolog in nematode oocytes, control of neurite outgrowth in Drosophila and mammalian neurons, and retrograde signaling in nerve axons after injury. Striking findings suggest that the guanine-nucleotide state of Ran can be regulated by local translation of the Ran-binding protein RanBP1 in axons, and that an additional Ran-binding protein, RanBP10, can act as a microtubule-binding cytoplasmic guanine-nucleotide exchange factor for Ran (RanGEF) in megakaryocytes. Thus, the Ran GTPase system can act as a spatial regulator of importin-dependent transport and signaling in distal cytoplasm, and as a regulator of cytoskeletal dynamics at sites that are distant from the nucleus.