Expression and stabilization of microinjected plasmids containing the herpes simplex virus thymidine kinase gene and polyoma virus DNA in mouse cells

Nuclear targeting of DNA

The nuclear membrane is a tight barrier for cytoplasmic proteins, but nuclear proteins have the intrinsic ability to overcome this barrier by an active signal-mediated process. Specific cytoplasmic carrier proteins have the responsibility to escort these proteins into the nucleus through the nuclear pore. The nuclear membrane is also a tight barrier for exogenous DNA delivered by synthetic vehicles, while many of the karyophilic viruses have a mechanism to actively deliver their genome through the nuclear pore. Virus DNA and RNA cannot move into the nucleus by themselves and require the viral structural proteins for efficient nuclear transport. In this article, we review the recent progress in understanding the mechanism of the nuclear transport of proteins and the virus genome, and discuss the possibility of developing synthetic gene-delivery systems based on these outcomes.

PLGA microspheres containing plasmid DNA: preservation of supercoiled DNA via cryopreparation and carbohydrate stabilization

Biodegradable microspheres containing plasmid DNA have potential uses as mediators of transfection in cells, particularly phagocytic cells such as macrophages. However, the hydrophilic nature and the structural instability of supercoiled DNA preclude its facile encapsulation in polymer matrixes such as poly(d, l-lactic-co-glycolic acid) (PLGA) by traditional methods. We initially studied the microencapsulation of plasmid DNA using the established water-in-oil-in-water double-emulsion solvent-evaporation method and found that (1) the encapsulation efficiency was low (about 20%), (2) the microencapsulation procedure nicked (degraded) the supercoiled DNA, and (3) lyophilization of the microsphere also nicked the DNA. We have therefore designed a new microsphere preparation method (called cryopreparation) to specifically address these concerns. Using the cryopreparation method, the aqueous phase of the primary emulsion containing the plasmid DNA is frozen and then subjected to homogenization. Because there is no shear stress inside a solid, we hypothesized that freezing the aqueous phase of the primary emulsion would help to preserve the supercoiled plasmid DNA during formation of the secondary emulsion. We also hypothesized that the formation of crystals from buffers within the primary emulsion was a causative factor for nicking during freezing or lyophilization, and that disruption of the crystal formation by the addition of saccharides into the primary emulsion would improve the supercoiled-DNA content of the spheres. Our results support the two hypotheses. Not only was the supercoiled-DNA content increased from 39% to over 85%, but the encapsulation efficiency was also elevated from 23% to over 85%.

Gene transfer vectors based on Sendai virus

A gene delivery system is a fundamental technology used in human gene therapy. In order to treat patients suffering from incurable metabolic diseases, we must be able to deliver genes efficiently in situ and induce stable gene expression in non-dividing tissue cells. However, none of the current gene transfer systems (both viral and non-viral) satisfies this goal. In order to develop a novel gene delivery system that is free from the defects of existing gene transfer vectors, we analyzed natural biological phenomena that involve gene transfer and expression, and made artificial components that mimic the functioning of these systems. Our recent results shed light on three major aspects of gene transfer and expression: (1) the direct delivery of DNA into cytoplasm using fusogenic liposomes, (2) the transfer of DNA from cytoplasm to nucleus with a nuclear localization signal, and (3) the stabilization of DNA in the nucleus as an independent replicon. The possible development of a hybrid vector by combining these components is discussed.

Visualization of mitochondrial protein import in cultured mammalian cells with green fluorescent protein and effects of overexpression of the human import receptor Tom20

The presequence of the ornithine transcarbamylase precursor (pOTC) was fused to green fluorescent protein (GFP), yielding pOTC-GFP and pOTCN-GFP containing the presequence plus 4 and 58 residues of mature ornithine transcarbamylase, respectively. When GFP cDNA was transfected into COS-7 cells, the cytosol and nucleus were fluorescent. On the other hand, pOTC-GFP cDNA gave strong fluorescence of a unique mitochondrial pattern. After fractionation of cells expressing pOTC-GFP with digitonin, fluorescence was recovered mostly in the particulate fraction. Immunoblot analysis showed that processed GFP was present in the particulate fraction, whereas pOTC-GFP was recovered in both the soluble and particulate fractions. pOTC-GFP and pOTCN-GFP synthesized in vitro were imported efficiently into the isolated mitochondria. Single and triple amino acid mutations in the presequence resulted in impaired mitochondrial import and in a loss of mitochondrial fluorescence. Perinuclear aggregation of fluorescent mitochondria was observed when the human mitochondrial import receptor Tom20 (hTom20) was coexpressed with pOTC-GFP. Overexpression of hTom20 (not DeltahTom20, which lacks the anchor sequence) resulted in stimulated mitochondrial import of pOTC-GFP in COS-7 cells. When pOTC-GFP cDNA was microinjected into nuclei of human fibroblast cells, mitochondrial fluorescence was detected as early as 2-3 h after injection. These results show that GFP fusion protein can be used to visualize mitochondrial structures and to monitor mitochondrial protein import in a single cell in real time.

The XPA protein is a zinc metalloprotein with an ability to recognize various kinds of DNA damage

The XPA (xeroderma pigmentosum group A) gene encodes a protein of 273 amino acids with a zinc finger motif. The human XPA cDNA was placed in an Escherichia coli expression vector for the synthesis of the recombinant XPA protein. The molecular weight of the wild-type protein was about 40 kDa in SDS-PAGE. Microinjection of the wild-type protein specifically restored the defect of UV-induced unscheduled DNA synthesis in XP-A cells. Thus, the bacterially expressed XPA protein retains biochemical properties identical to those of natural sources. The wild-type protein binds preferentially to UV-, cis-diamminedichloroplatinum(II) (cisplatin)- or osmium tetroxide (OsO4)-damaged DNA as assayed by retention on nitrocellulose filters. In addition, the data from atomic absorption and UV-CD spectra revealed that the wild-type protein is a zinc metalloprotein with secondary structure. Furthermore, the mutant protein, of which the cysteine-103 residue in the zinc finger motif was replaced with serine, has a vastly different protein conformation resulting in a loss of XP-A correcting and DNA-binding activities. These findings indicate that the XPA protein is a zinc-binding protein with affinity for various DNA damages, and a cysteine residue in the C4-type zinc finger motif is indispensable for normal protein conformation.

An ERCC5 gene with homology to yeast RAD2 is involved in group G xeroderma pigmentosum

We have isolated a human excision repair gene ERCC5 which complements the defect of the mouse UV-sensitive mutant XL216 (rodent complementation group 5). Here we report cDNA cloning of human and mouse ERCC5 genes using an exon containing an ERCC5 fragment as a probe. The ERCC5 cDNA encodes a predicted 133-kDa nuclear protein that shares some homology with the product of the yeast DNA repair gene RAD2. Transfection with mouse ERCC5 cDNA restored normal levels of UV resistance to XL216 cells. Microinjection of ERCC5 cDNA specifically restored the defect of xeroderma pigmentosum group G cells (XP-G) as measured by unscheduled DNA synthesis, and XP-G cells stably transformed with ERCC5 cDNA showed nearly normal UV resistance.

Escherichia coli RecA protein modified with a nuclear location signal binds to chromosomes in living mammalian cells

We tried to make a well-characterized bacterial protein function in mammalian cell nuclei. For this purpose we chose Escherichia coli RecA protein and fused its carboxy terminus to the nuclear location signal of SV40 large T-antigen by oligonucleotide-dependent modification of the gene. When injected into the cytoplasm, the modified RecA protein (T-RecA for the T-antigen signal) accumulated efficiently in the nuclei, whereas the wild-type RecA protein remained in the cytoplasm. The T-RecA protein retained its original in vivo activity, judging from the finding that uv-sensitive bacteria (recA- E. coli) became uv-resistant on transformation with the T-recA plasmid as well as the recA plasmid. For expression of the T-recA gene in mammalian cells, the 5' region was replaced by the chicken beta-actin promoter and Kozak's initiation signal. A high level of expression was observed when Chinese hamster ovary (CHO-K1) cells were transfected with this plasmid. Indirect immunofluorescence examination revealed that the T-RecA protein in nuclei of mammalian cells bound to chromatin.

Factors influencing efficiency and reproducibility of polybrene-assisted gene transfer

A systematic investigation of factors influencing the efficiency of polybrene-assisted gene transfer for both transient and stable foreign gene expression was carried out utilizing NIH 3T3 fibroblasts as prototypic recipients for the plasmid expression vectors pSV2cat and pSV2neo. While transfection cocktail composition and cell density, in addition to polybrene exposure conditions and exogenous DNA concentration, each played an important role, the key determinant to achieving excellent transfection efficiency proved to be the DMSO treatment regimen. Under optimal conditions, the yield of colonies resistant to the neomycin analog, G418, increased linearly at the rate of 10 clones/ng of input (native form I pSV2neo) DNA up to a plasmid concentration of 50 ng, whereupon the dose-response for colony recovery became semilogarithmic. The incidence of stable transformants was doubled by linearization of the vector DNA, whereas the addition of carrier DNA to the transfection cocktail was without effect until present at concentrations above 10-fold molar excess, at which point the efficacy of gene transfer declined rapidly. Combined Southern and dot-blot analyses of transformed cell DNA demonstrated that the polybrene-DMSO procedure led to the stable integration of relatively few copies of the marker gene in each transformant; the actual number varied from 1-3 to 10-15 per host genome, depending on the concentration of pSV2neo DNA added. The potential for the adaptation of this DNA transfection procedure for general use with other mammalian cell types, as well as its technical strengths and weaknesses, is discussed.

Polyoma integrates readily in mouse cellular DNA

Although the natural host of polyoma virus is the mouse, its integration in cellular DNA has been investigated almost exclusively in rat cells. We report here studies on the integration of polyoma in mouse cells. We introduced the polyoma virus genome in two different mouse cell lines as an unselected genetic marker, by cotransfection with the tk gene of herpes simplex virus or the neo gene of E. coli. The number of TK+ or G418R clones obtained was reduced up to 50 fold by the presence of the polyoma genome. The gene coding for the early protein large T of polyoma was necessary and sufficient to produce this reduction. However, this effect appeared to be independent of polyoma replication. Surprisingly, all of the 33 clones analysed that had survived cotransfection with polyoma contained polyoma DNA integrated in their genome. Furthermore, in over 50% of these clones, the entire polyoma genome had been integrated. We conclude that polyoma integrates readily in mouse cellular DNA.

Reversible inhibition of protein import into the nucleus by wheat germ agglutinin injected into cultured cells

The importance of glycoproteins located in the nuclear envelope in nuclear transport was tested by microinjection of karyophilic proteins into the cytoplasm of cultured human cells together with various lectins. Wheat germ agglutinin (WGA) blocked the nuclear transport of nucleoplasmin, a nuclear protein of Xenopus laevis oocytes, and of nonnuclear proteins conjugated with a synthetic peptide containing the nuclear localization signal sequence for simian virus 40 (SV40) large T antigen. Its inhibitory activity persisted for about 1 h after its injection into the cells and then gradually decreased. Export of at least some kinds of RNA from the nucleus seemed not to be affected by WGA even when import of the proteins into the nucleus was completely blocked (within 1 h after WGA injection). Moreover, WGA did not inhibit the passive diffusion of fluorescein isothiocyanate (FITC)-dextran (average Mr 17,900) into the nucleus. Wistaria floribunda agglutinin (WFA), concanavalin A (Con A), and lentil lectin did not block nuclear transport. These results indicate that WGA specifically blocks active protein import, but not passive diffusion of materials into the nucleus.

The improved efficient method for introducing macromolecules into cells using HVJ (Sendai virus) liposomes with gangliosides

Macromolecules such as DNA and RNA can be entrapped within liposomes associated with gangliosides by reverse-phase evaporation. When these liposomes are incubated with HVJ2 (Sendai virus), they deliver their contents into cultured cells efficiently. More than 95% cells of a Ltk- cell line (thymidine kinase-deficient cells) transiently expressed thymidine kinase activity by thymidine kinase gene transfer using HVJ liposomes with gangliosides. Stable transformants could be obtained efficiently from various cell lines by use of HVJ liposomes containing the neoR gene. The neo+ transformants were obtained at frequencies of about 0.2-1.0, 0.06-0.25, and 0.06-0.1% in monolayers of L, CHO-Kl, and HeLa-S3 cells, respectively. Moreover, in Ehrlich ascites tumor cells which grow in suspension, the frequency was more than 0.01%. On introduction of plasmid pTK4 into Ltk- cells, about 0.5-1.0% TK+ transformants were obtained. Cosmid DNA containing the neoR gene (about 45 kbp) was also introduced into L cells by this method and neo+ transformants were obtained at a frequency of 0.1%. When rat liver mRNA was introduced into L cells by HVJ liposomes with gangliosides, immunoprecipitation studies showed that the L cells secreted rat albumin and some other proteins into the cultured medium. Moreover, using erythrocyte membrane vesicles containing IgM that had been incubated with HVJ empty liposomes with gangliosides, the IgM could be introduced into all the L cells.

The expression of integrated plasmid DNA depends on copy number

The effect of copy number, integration site, and enhancers on the expression of stably integrated exogenous DNA was examined in Chinese hamster cells. Three similar plasmids were constructed with the mouse beta maj-globin promoter fused to the galK gene either with no enhancer or with the SV40 or Harvey sarcoma virus (HaSV) enhancer. Eighteen stable cell lines were obtained and characterized with respect to plasmid copy number and galactokinase activity. At copy numbers of four or less, the enhancers showed detectable activity and a DNase I hypersensitive site was present. Above four copies, gene activity decreased as the copy number increased, the enhancer sequences were apparently inactive, and the DNase I hypersensitive site disappeared. These data suggest that, at least in this model system, when exogenous DNA is integrated as multiple head-to-tail copies, the entire multigene unit expresses poorly and inappropriately. When the same exogenous DNA integrates as a single (or low number) copy, expression appears to be relatively normal as judged by enhancer stimulation and DNase I hypersensitivity.

Gene transfer into mouse embryos

Gene transfer into the murine genome was accomplished nearly a decade ago by use of chimeras and teratocarcinomas; however, the low frequencies of transfer into the germ line and other difficulties stemming from mosaicism and karyotypic abnormalities in chimeric mice have limited the general usefulness of this procedure in achieving transformation in mammalian embryos. The introduction of cloned genes into teratocarcinoma cells, selection for a mutant phenotype, and transfer of those cells into mouse embryos holds some promise as a technique to employ mouse chimeras for gene transfer into mice. Infection with animal viruses and retroviral vectors provides another way to introduce exogenous DNA into mouse embryos. Infection with Mo-MuLV has been utilized to characterize the relationship between sites of integration and gene function in developing and adult mice. Gene transfer by microinjection of cloned recombinant DNA has been used by many laboratories for the transfer of DNAs into mouse embryos. The factors affecting transformation frequencies and sites of integration are unknown at present, although it seems that integration is not strictly mediated by homology-dependent events. Many genes have been introduced into mouse embryos by these procedures and many of these are expressed at high levels in appropriate tissues. No realistic possibility exists at the present time for the utilization of embryo gene transfer in the medical field for the correction of genetic defects for several reasons. First, in order to effectively provide "gene therapy" it would be necessary to determine the genotype of each recipient egg, a technical impossibility. The genetic diseases that would be amenable to germ line intervention are recessive diseases and there would be only a 25% chance of any one embryo derived from heterozygous parents being a homozygous recessive. Moreover, it would be impossible to distinguish the normal from abnormal embryos. Second, the frequencies of transformation are so low as to exclude work on human beings on ethical grounds. Third, the parameters effecting chromosomal integration sites and gene expression have not been fully characterized. Until it becomes experimentally possible to target the newly introduced DNA into expressable chromosomal sites and actively replace or supplement defective genes, the possibility of gene therapy through manipulation of embryos is remote. Yet, efforts to provide gene therapy in somatic tissues have been promising, leading to expression of a modified phenotype (Anderson, 1984). In contrast to embryo gene therapy, gene therapy in somatic tissues would not lead to germ line propagation of the manipulated genotype.(ABSTRACT TRUNCATED AT 400 WORDS)

Entry into S phase is inhibited in human fibroblasts by rat liver poly(A)+RNA

Initiation of DNA synthesis was inhibited in human fibroblasts following microinjection with poly(A)+RNA derived from normal rat liver. After sucrose gradient sedimentation of the RNA, the inhibitory activity was found to be limited to two adjacent fractions. Dilution experiments suggest a minimum abundance level of 0.015% for this mRNA(s). Studies on the kinetics of this inhibition indicate a reversible inhibition with a duration of approx. 10 h.

Large macromolecules can be introduced into cultured mammalian cells using erythrocyte membrane vesicles

Plasmid 6.4 kbp DNA, 14 kbp DNA, lambda phage particles, all of which contained herpes simplex virus type 1 (HSV-1) thymidine kinase (TK) gene, or IgM molecules, were mixed with erythrocyte membranes and treated with neutral detergent. The transparent mixture was diluted with phosphate-buffered saline (PBS), followed by centrifugation to collect membrane vesicles containing the large macromolecules. 10-15% of 6.4 kbp, 3% of 14 kbp, 4-7% of the lambda phage particles and 14.5% of IgM were trapped within erythrocyte membrane vesicles. The membrane vesicles containing these molecules were fused with L cells, or rat F2408#20 cells, both of which are deficient in thymidine kinase activity. In each case, transformants were obtained. 2 X 10(5) - 7 X 10(5) phage PFU or 1.5 X 10(6) - 8 X 10(7) DNA molecules were required to obtain one transformant from L cells, but 2-3 X 10(7) phage PFU or 2 X 10(9) - 1 X 10(10) DNA molecules were required for one transformant from rat cells. Number of colonies which transiently expressed TK genes in L cells was also determined by autoradiography. The ratio of stable transformants to colonies positive for transient expression in cells treated with low doses of DNA or lambda phage was 46-68%. The transformation efficiency of human fibroblast cells by pSV2-gpt DNA trapped in erythrocyte membrane vesicles was less than that of L cells by HSV-TK DNA, but almost the same as that of rat cells by HSV-TK DNA.

Differential expression of two linked selection genes (HSVI-tk and Eco.gpt) in transformed teratocarcinoma and in L cells

Upon transfection of (TK-)F9 teratocarcinoma stem-cells and (TK-)L fibroblasts with a plasmid carrying two selection genes, Eco.gpt and HSVI-tk, selection for gpt gene yielded ten times fewer colonies than selection for tk. Only the transformed clones selected for gpt had measurable xanthine guanine phosphoribosyltransferase (XGPRT) activity (Jami et al., 1983). Eco.gpt coding for XGPRT was under the control of simian virus 40 (SV40) early genes' regulating sequences (SV-gpt). In the present study, it was verified that the low efficiency of gpt selection in mouse cells was not due to the eucaryotic controlling sequences added to the bacterial gene. The transformed clones selected for tk that had no XGPRT activity possessed at least one uninterrupted copy of the composite SV-gpt gene and as many copies of the transforming plasmid as the cells selected for gpt expression. In a further test, the gpt gene was placed under the control of tk-regulating sequences and inserted with the tk gene in the same vector. Under these conditions, expression of XGPRT in the transformed clones selected for tk was improved, even though relative selection for gpt remained low.

Homologous recombination of polyoma virus DNA in mouse cells

We have produced nonviable deletion mutants of polyoma virus in order to study homologous recombination after DNA transfection into mouse cells. The frequency of recombination was determined by the formation of infectious virus. It was dependent on the amount of DNA transfected and the size of the region of homology between the mutations. Recombination frequencies were highest when both mutated genomes were transfected in closed circular form rather than after linearization of one or both of the recombination partners. The system described may be useful for a more detailed analysis of physiological and genetic conditions influencing the frequency of homologous recombination in mouse cells as well as to study enzymes involved and intermediates produced in this process.