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

The intracellular visualization of exogenous DNA in fluorescence microscopy

In the development of non-viral gene delivery vectors, it is essential to reliably localize and quantify transfected DNA inside the cell. To track DNA, fluorescence microscopy methods are commonly applied. These mostly rely on fluorescently labeled DNA, DNA binding proteins fused to a fluorescent protein, or fluorescence in situ hybridization (FISH). In addition, co-stainings are often used to determine the colocalization of the DNA in specific cellular compartments, such as the endolysosomes or the nucleus. We provide an overview of these DNA tracking methods, advice on how they should be combined, and indicate which co-stainings or additional methods are required to draw precise conclusions from a DNA tracking experiment. Some emphasis is given to the localization of exogenous DNA inside the nucleus, which is the last step of DNA delivery. We argue that suitable tools which allow for the nuclear detection of faint signals are still missing, hampering the rational development of more efficient non-viral transfection systems.

Decoding cellular mechanism of recombinant adeno-associated virus (rAAV) and engineering host-cell factories toward intensified viral vector manufacturing

Recombinant adeno-associated virus (rAAV) is one of the prominent gene delivery vehicles that has opened promising opportunities for novel gene therapeutic approaches. However, the current major viral vector production platform, triple transfection in mammalian cells, may not meet the increasing demand. Thus, it is highly required to understand production bottlenecks from the host cell perspective and engineer the cells to be more favorable and tolerant to viral vector production, thereby effectively enhancing rAAV manufacturing. In this review, we provided a comprehensive exploration of the intricate cellular process involved in rAAV production, encompassing various stages such as plasmid entry to the cytoplasm, plasmid trafficking and nuclear delivery, rAAV structural/non-structural protein expression, viral capsid assembly, genome replication, genome packaging, and rAAV release/secretion. The knowledge in the fundamental biology of host cells supporting viral replication as manufacturing factories or exhibiting defending behaviors against viral production is summarized for each stage. The control strategies from the perspectives of host cell and materials (e.g., AAV plasmids) are proposed as our insights based on the characterization of molecular features and our existing knowledge of the AAV viral life cycle, rAAV and other viral vector production in the Human embryonic kidney (HEK) cells.

Circular RNA circRILPL1 promotes nasopharyngeal carcinoma malignant progression by activating the Hippo-YAP signaling pathway

Circular RNAs (circRNAs) play an important regulatory role in the pathogenesis and progression of nasopharyngeal carcinoma (NPC), which have not been thoroughly elucidated. In this study, we revealed for the first time that circRILPL1 was upregulated in NPC, weakened adhesion and decreased stiffness of NPC cells, and promoted NPC proliferation and metastasis in vitro and in vivo. Mechanistically, circRILPL1 inhibited the LATS1-YAP kinase cascade by binding to and activating ROCK1, resulting in decrease of YAP phosphorylation. Binding and cooperating with transport receptor IPO7, circRILPL1 promoted the translocation of YAP from the cytoplasm to the nucleus, where YAP enhanced the transcription of cytoskeleton remodeling genes CAPN2 and PXN. By which, circRILPL1 contributed to the pathogenesis of NPC. Our results demonstrated that circRILPL1 promoted the proliferation and metastasis of NPC through activating the Hippo-YAP signaling pathway by binding to both ROCK1 and IPO7. Highly expressed circRILPL1 in NPC may serve as an important biomarker for tumor diagnosis and may also be a potential therapeutic target.

Polymeric Delivery of Therapeutic Nucleic Acids

The advent of genome editing has transformed the therapeutic landscape for several debilitating diseases, and the clinical outlook for gene therapeutics has never been more promising. The therapeutic potential of nucleic acids has been limited by a reliance on engineered viral vectors for delivery. Chemically defined polymers can remediate technological, regulatory, and clinical challenges associated with viral modes of gene delivery. Because of their scalability, versatility, and exquisite tunability, polymers are ideal biomaterial platforms for delivering nucleic acid payloads efficiently while minimizing immune response and cellular toxicity. While polymeric gene delivery has progressed significantly in the past four decades, clinical translation of polymeric vehicles faces several formidable challenges. The aim of our Account is to illustrate diverse concepts in designing polymeric vectors towards meeting therapeutic goals of in vivo and ex vivo gene therapy. Here, we highlight several classes of polymers employed in gene delivery and summarize the recent work on understanding the contributions of chemical and architectural design parameters. We touch upon characterization methods used to visualize and understand events transpiring at the interfaces between polymer, nucleic acids, and the physiological environment. We conclude that interdisciplinary approaches and methodologies motivated by fundamental questions are key to designing high-performing polymeric vehicles for gene therapy.

Stable integration of an optimized inducible promoter system enables spatiotemporal control of gene expression throughout avian development

Precisely altering gene expression is critical for understanding molecular processes of embryogenesis. Although some tools exist for transgene misexpression in developing chick embryos, we have refined and advanced them by simplifying and optimizing constructs for spatiotemporal control. To maintain expression over the entire course of embryonic development we use an enhanced piggyBac transposon system that efficiently integrates sequences into the host genome. We also incorporate a DNA targeting sequence to direct plasmid translocation into the nucleus and a D4Z4 insulator sequence to prevent epigenetic silencing. We designed these constructs to minimize their size and maximize cellular uptake, and to simplify usage by placing all of the integrating sequences on a single plasmid. Following electroporation of stage HH8.5 embryos, our tetracycline-inducible promoter construct produces robust transgene expression in the presence of doxycycline at any point during embryonic development in ovo or in culture. Moreover, expression levels can be modulated by titrating doxycycline concentrations and spatial control can be achieved using beads or gels. Thus, we have generated a novel, sensitive, tunable, and stable inducible-promoter system for high-resolution gene manipulation in vivo.

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.

Ultrasound‑targeted microbubbles combined with a peptide nucleic acid binding nuclear localization signal mediate transfection of exogenous genes by improving cytoplasmic and nuclear import

The development of an efficient delivery system is critical for the successful treatment of cardiovascular diseases using non-viral gene therapies. Cytoplasmic and nuclear membrane barriers reduce delivery efficiency by impeding the transfection of foreign genes. Thus, a gene delivery system capable of transporting exogenous genes may improve gene therapy. The present study used a novel strategy involving ultrasound-targeted microbubbles and peptide nucleic acid (PNA)-binding nuclear localization signals (NLS). Ultrasound-targeted microbubble destruction (UTMD) and PNA-binding NLS were used to improve the cytoplasmic and nuclear importation of the plasmid, respectively. Experiments were performed using antibody-targeted microbubbles (AT-MCB) that specifically recognize the SV40T antigen receptor expressed on the membranes of 293T cells, resulting in the localization of ultrasound microbubbles to 293T cell membranes. Furthermore, PNA containing NLS was inserted into the enhanced green fluorescent protein (EGFP)-N3 plasmid DNA (NLS-PNA-DNA), which increased nuclear localization. The nuclear import and gene expression efficiency of the AT-MCB with PNA-binding NLS were compared with AT-MCB alone or a PNA-binding NLS. The effect of the AT-MCB containing PNA-binding NLS on transfection was investigated. The ultrasound and AT-MCB delivery significantly enhanced the cytoplasmic intake of exogenous genes and maintained high cell viability. The nuclear import and gene expression of combined microbubble- and PNA-transfected cells were significantly greater compared with cells that were transfected with AT-MCB or DNA with only PNA-binding NLS. The quantity of EGFP-N3 plasmids in the nuclei was increased by >5.0-fold compared with control microbubbles (CMCB) and NLS-free plasmids. The gene expression was ~1.7-fold greater compared with NLS-free plasmids and 1.3-fold greater compared with control microbubbles. In conclusion, UTMD combined with AT-MCB and a PNA-binding NLS plasmid significantly improved transfection efficiency by increasing cytoplasmic and nuclear DNA import. This method is a promising strategy for the noninvasive and effective delivery of target genes or drugs for the treatment of cardiovascular diseases.

DNA Targeting Sequence Improves Magnetic Nanoparticle-Based Plasmid DNA Transfection Efficiency in Model Neurons

Efficient non-viral plasmid DNA transfection of most stem cells, progenitor cells and primary cell lines currently presents an obstacle for many applications within gene therapy research. From a standpoint of efficiency and cell viability, magnetic nanoparticle-based DNA transfection is a promising gene vectoring technique because it has demonstrated rapid and improved transfection outcomes when compared to alternative non-viral methods. Recently, our research group introduced oscillating magnet arrays that resulted in further improvements to this novel plasmid DNA (pDNA) vectoring technology. Continued improvements to nanomagnetic transfection techniques have focused primarily on magnetic nanoparticle (MNP) functionalization and transfection parameter optimization: cell confluence, growth media, serum starvation, magnet oscillation parameters, etc. Noting that none of these parameters can assist in the nuclear translocation of delivered pDNA following MNP-pDNA complex dissociation in the cell’s cytoplasm, inclusion of a cassette feature for pDNA nuclear translocation is theoretically justified. In this study incorporation of a DNA targeting sequence (DTS) feature in the transfecting plasmid improved transfection efficiency in model neurons, presumably from increased nuclear translocation. This observation became most apparent when comparing the response of the dividing SH-SY5Y precursor cell to the non-dividing and differentiated SH-SY5Y neuroblastoma cells.

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.

Electroporation-mediated gene delivery

Electroporation has been used extensively to transfer DNA to bacteria, yeast, and mammalian cells in culture for the past 30 years. Over this time, numerous advances have been made, from using fields to facilitate cell fusion, delivery of chemotherapeutic drugs to cells and tissues, and most importantly, gene and drug delivery in living tissues from rodents to man. Electroporation uses electrical fields to transiently destabilize the membrane allowing the entry of normally impermeable macromolecules into the cytoplasm. Surprisingly, at the appropriate field strengths, the application of these fields to tissues results in little, if any, damage or trauma. Indeed, electroporation has even been used successfully in human trials for gene delivery for the treatment of tumors and for vaccine development. Electroporation can lead to between 100 and 1000-fold increases in gene delivery and expression and can also increase both the distribution of cells taking up and expressing the DNA as well as the absolute amount of gene product per cell (likely due to increased delivery of plasmids into each cell). Effective electroporation depends on electric field parameters, electrode design, the tissues and cells being targeted, and the plasmids that are being transferred themselves. Most importantly, there is no single combination of these variables that leads to greatest efficacy in every situation; optimization is required in every new setting. Electroporation-mediated in vivo gene delivery has proven highly effective in vaccine production, transgene expression, enzyme replacement, and control of a variety of cancers. Almost any tissue can be targeted with electroporation, including muscle, skin, heart, liver, lung, and vasculature. This chapter will provide an overview of the theory of electroporation for the delivery of DNA both in individual cells and in tissues and its application for in vivo gene delivery in a number of animal models.

Enhancement of plasmid-mediated transgene expression

A large number of studies aimed at the treatment of cancer, autoimmune and metabolic diseases, neurodegenerative disorders, allergic diseases, as well as muscle disorders strengthen the fact that gene therapy could represent an alternative method to treat human diseases where conventional approaches are less effective. To improve transgene expression from plasmid vectors, DNA nuclear targeting sequences (DTSs) can be introduced in a vector backbone to increase in vivo expression up to 20-fold using electroporation (EP) delivery in muscle tissue. The purpose of this chapter is to represent a step-by-step strategy for the construction of a plasmid vector with enhanced efficiency of nuclear plasmid uptake and the methodic for the in vivo efficiency evaluation of the obtained expression vector.

On the possible involvement of bovine serum albumin precursor in lipofection pathway

Protein factors involved in lipofection pathways remain elusive. Using avidin-biotin affinity chromatography and mass finger printing analysis technique, herein we report the identification of a 70 kDa size protein (bovine serum albumin precursor, BSAP) which binds strongly with lipoplexes and may play role in lipofection pathway. Using multiple cultured animal cells and three structurally different cationic transfection lipids, we show that the efficiencies of liposomal transfection vectors get significantly enhanced (by ~2.5- to 5.0-fold) in cells pre-transfected with lipoplexes of reporter plasmid construct encoding BSAP. Findings in the cellular uptake experiments in A549 cells cultured in DMEM supplemented with 10 percent (w/w) BODIPY-labelled BSAP are consistent with the supposition that BSAP enters cell cytoplasm from the cell culture medium (DMEM supplemented with 10 percent FBS) used in lipofection. Cellular uptake studies by confocal microscopy using BODIPY-labelled BSAP and FITC-labelled plasmid DNA revealed co-localization of plasmid DNA and BSAP within the cell cytoplasm and nucleus. In summary, the present findings hint at the possible involvement of BSAP in lipofection pathway.

Ultrasound microbubbles combined with the NFκB binding motif increase transfection efficiency by enhancing the cytoplasmic and nuclear import of plasmid DNA

Inefficient gene delivery poses a challenge for non‑viral gene therapy. Cytoplasmic and nuclear membrane barriers are responsible for the inefficiency as they restrict the import of exogenous genes. The present study aimed to improve the transfection efficiency using a novel gene delivery system, which consisted of two components: ultrasound microbubbles and the nuclear factor κB (NFκB) binding motif. Ultrasound-targeted microbubble destruction (UTMD) was used to enhance the cytoplasmic import of plasmids and the NFκB binding motif was added to promote the nuclear intake of the plasmid from the cytoplasm. In the present study, human umbilical vein endothelial cells were transfected using UTMD with two different Cy3-labeled plasmids, phSDF-1α and phSDF‑1α‑NFκB. phSDF-1α-NFκB was constructed by inserting a specific DNA targeting sequence (five optimal repeats of the binding motif for the inducible transcription factor NFκB) into phSDF‑1α. The nuclear import and gene expression efficiency of phSDF-1α-NFκB were compared with those of phSDF-1α to investigate the effect of the NFκB binding motif on transfection. The results showed that UTMD significantly increased the cytoplasmic intake of pDNA and maintained high cell viability. The nuclear import and gene expression of phSDF-1α‑NFκB‑transfected cells were significantly higher than those transfected with phSDF-1α. Compared with the NFκB‑free plasmids, the quantity of NFκB plasmids in the nucleus increased 6.5-fold and the expression of SDF-1α was 4.4-fold greater. These results suggest that UTMD combined with NFκB binding motif significantly improve transfection efficiency by enhancing the cytoplasmic and nuclear import of exogenous plasmid DNA.

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.

Assessment of the nuclear pore dilating agent trans-cyclohexane-1,2-diol in differentiated airway epithelium

BACKGROUND

The nuclear membrane of differentiated airway epithelial cells is a significant barrier for nonviral vectors. Trans-cyclohexane-1,2-diol (TCHD) is an amphipathic alcohol that has been shown to collapse nuclear pore cores and allow the uptake of macromolecules that would otherwise be too large for nuclear entry. Previous studies have shown that TCHD can increase lipid-mediated transfection in vitro.

METHODS

We aimed to reproduce these in vitro studies using the cationic lipid GL67A, which we are currently assessing in cystic fibrosis trials and, more importantly, we assessed the effects of TCHD on transfection efficiency in differentiated airway epithelium ex vivo and in mouse lung in vivo using three different drug delivery protocols (nebulisation and bolus administration of TCHD to the mouse lung, as well as perfusion of TCHD to the nasal epithelium, which prolongs contact time between the airway epithelium and drug).

RESULTS

TCHD (0.5-2%) dose-dependently increased Lipofectamine 2000 and GL67A-mediated transfection of 293T cells by up to 2 logs. Encouragingly, exposure to 8% TCHD (but not 0.5% or 2.0%) increased gene expression in fully differentiated human air liquid interface cultures by approximately 20-fold, although this was accompanied by significant cell damage. However, none of the TCHD treated mice in any of the three protocols had higher gene expression compared to no TCHD controls.

CONCLUSIONS

Although TCHD significantly increases gene transfer in cell lines and differentiated airway epithelium ex vivo, this effect is lost in vivo and further highlights that promising in vitro findings often cannot be translated into in vivo applications.

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.

Nanoparticle-mediated gene transfer specific to retinal pigment epithelial cells

Previously, we demonstrated that CK30PEG10k-compacted DNA nanoparticles (NPs) efficiently target photoreceptor cells and improve visual function in a retinitis pigmentosa model. Here, we test the ability of these NPs in driving transgene expression in the retinal pigment epithelium (RPE), using an RPE-specific reporter vector (VMD2-eGFP). NPs, uncompacted plasmid, or saline were subretinally delivered to adult BALB/c mice. NP-based expression was specific to RPE cells and caused no deleterious effects on retinal structure and function. eGFP expression levels in NP-injected eyes peaked at post-injection day 2 (PI-2), stabilized at levels ~3-fold higher than in naked DNA-injected eyes, and remained elevated at the latest time-point examined (PI-30). Unlike naked DNA, which only transfected cells at the site of injection, NPs were able to transfect cells throughout the RPE. Subretinal injections of rhodamine labeled NPs and naked DNA showed comparable initial uptake into RPE cells. However, at PI-7 and -30 days significantly more fluorescence was detected inside the RPE of NP-injected eyes compared to naked DNA, suggesting NPs are stable inside the cell which could possibly lead to higher and sustained expression. Overall, our results demonstrate that NPs can efficiently deliver genes to the RPE and hold great potential for the treatment of RPE-associated diseases.

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.

Improved antibiotic-free plasmid vector design by incorporation of transient expression enhancers

Methods to improve plasmid-mediated transgene expression are needed for gene medicine and gene vaccination applications. To maintain a low risk of insertional mutagenesis-mediated gene activation, expression-augmenting sequences would ideally function to improve transgene expression from transiently transfected intact plasmid, but not from spurious genomically integrated vectors. We report herein the development of potent minimal, antibiotic-free, high-manufacturing-yield mammalian expression vectors incorporating rationally designed additive combinations of expression enhancers. The SV40 72 bp enhancer incorporated upstream of the cytomegalovirus (CMV) enhancer selectively improved extrachromosomal transgene expression. The human T-lymphotropic virus type I (HTLV-I) R region, incorporated downstream of the CMV promoter, dramatically increased mRNA translation efficiency, but not overall mRNA levels, after transient transfection. A similar mRNA translation efficiency increase was observed with plasmid vectors incorporating and expressing the protein kinase R-inhibiting adenoviral viral associated (VA)1 RNA. Strikingly, HTLV-I R and VA1 did not increase transgene expression or mRNA translation efficiency from plasmid DNA after genomic integration. The vector platform, when combined with electroporation delivery, further increased transgene expression and improved HIV-1 gp120 DNA vaccine-induced neutralizing antibody titers in rabbits. These antibiotic-free vectors incorporating transient expression enhancers are safer, more potent alternatives to improve transgene expression for DNA therapy or vaccination.

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.

Progress and prospects: nuclear import of nonviral vectors

The nuclear envelope represents a key barrier to successful nonviral transfection and gene therapy both in vitro and in vivo. Although the main purpose of the nuclear envelope is to partition the cell to maintain cytoplasmic components in the cytoplasm and nuclear components, most notably genomic DNA, in the nucleus, this function poses a problem for transfections in which exogenous DNA is delivered into the cytoplasm. After delivery to the cytoplasm, nucleic acids rapidly become complexed with cellular proteins that mediate interactions with the cellular machinery for trafficking. Thus, it is these proteins that, in essence, control the nuclear import of DNA, and we must also understand their activities in cells. In this review, we will discuss the principles of nuclear import of proteins and DNA-protein complexes, as well as the various approaches that investigators have used to improve nuclear targeting of plasmids. These approaches include complexation of plasmids with peptides, native and engineered proteins, ligands and polymers, as well as the inclusion of transcription factor-binding sites for general and cell-specific delivery.

KEYWORDS

nonviral gene transfermid R:plasmidmid R:nuclear pore complexmid R:importinmid R:nuclear localization signalmid R:karyopherin.

The SP-C promoter facilitates alveolar type II epithelial cell-specific plasmid nuclear import and gene expression

Although nonviral gene therapy has great potential for use in the lung, the relative lack of cell-specific targeting has limited its applications. We have developed a new approach for cell-specific targeting based on selective nuclear import of plasmids in non-dividing cells. Using a microinjection and in situ hybridization approach, we tested several potential DNA sequences for the ability to mediate plasmid nuclear import in alveolar type II epithelial (ATII) cells. Of these, only a sequence within the human surfactant protein C (SP-C) promoter was able to mediate nuclear localization of plasmid DNA specifically in ATII cells but not in other cell types. We have mapped the minimal import sequence to the proximal 318 nucleotides of the promoter, and demonstrate that binding sites for NFI, TTF-1, and GATA-6 and the proteins themselves are required for import activity. Using intratracheal delivery of DNA followed by electroporation, we demonstrate that the SP-C promoter sequence will enhance gene expression specifically in ATII cells in mouse lung. This represents a novel activity for the SP-C promoter and thus ATII cell-specific nuclear import of DNA may prove to be a safe and effective method for targeted and enhanced gene expression in ATII cells.