Differences in intracellular DNA ligation after microinjection and transfection

A comparison of imaging methodologies for 3D tissue engineering

Imaging of cells in two dimensions is routinely performed within cell biology and tissue engineering laboratories. When biology moves into three dimensions imaging becomes more challenging, especially when multiple cell types are used. This review compares imaging techniques used regularly in our laboratory in the culture of cells in both two and three dimensions. The techniques reviewed include phase contrast microscopy, fluorescent microscopy, confocal laser scanning microscopy, electron microscopy, and optical coherence tomography. We compare these techniques to the current "gold standard" for imaging three-dimensional tissue engineered constructs, histology.

Ligation of exogenous linear DNA after gene transfer in vitro and in vivo

BACKGROUND

We have analyzed the physical/topographical state of linear exogenous DNA after gene transfer in vitro and in vivo.

METHODS AND RESULTS

Linear DNA carrying a luciferase expression cassette, either intact or corrupted within the coding region, was tested in gene transfer experiments in vitro and in vivo. To this, a plasmid with a CMV-IE1 promoter-driven luciferase gene was rendered non-functional by the insertion of a 1.2 kb EcoRV-EcoRV fragment. After removal of the insert by digestion with EcoRV, the resulting linear DNA fragments were used to transfect HeLa cells. The recovery of luciferase activity from these cells indicated functional reconstitution of the expression cassette. Recovery of low molecular weight DNA from HeLa cells allowed amplification of an intact luciferase gene, confirming accurate ligation of free DNA ends. In the mouse, rapid intravenous injection of plasmid DNA, linearized within the luciferase gene, resulted in significant luciferase activities in liver and lung. Ligation products could be detected by PCR.

CONCLUSIONS

These data suggest that linear DNA is efficiently circularized after gene transfer in vitro and in vivo. Secondly, equally high luciferase activities were observed in the mouse after rapid intravenous injection of luciferase expression cassettes, either consisting of linear DNA produced by PCR, or carried by linearized plasmid DNA. These findings encourage the use of linear DNA elements for gene transfer applications in vivo.

Molecular characterization of HIV viruses generated after in vivo ligation

During the course of infection, human immunodeficiency virus type 1 (HIV-1) displays wide genotypic and phenotypic differences. Construction of chimeric viruses is useful to determine the genotypic basis that underlies phenotypic variations, but the procedure is time-consuming. Previously, it has been shown that co-transfection of truncated hemi-genomic HIV-1 proviral DNA can lead to generation of full-length infectious virus. In the study of HIV phenotypes, using this technique, it is important to determine whether recombination between the two hemigenomes occurs without mutations. After co-transfection, progeny recombinant viruses replicated at the same rate as the control. We purified progeny viruses from culture supernatants and determined mutations at the recombination site. It appeared that correct in vivo ligation depended on the purity of DNA and the restriction site used. It also appeared that some of the mutations observed affect replication, as progeny viruses bearing one of these mutations disappeared during in vitro cultures, whereas other mutants did not. Although this technique is widely applied to generate chimeric viruses, the results should be evaluated with care, since mutations influencing the phenotype of the progeny viruses may have been introduced.

End-joining of DNA fragments in adenovirus transfection of human cells

Overlapping terminal fragments of adenovirus DNA transfected into human cells either recombine to form standard unit-length genomes, or can join end-to-end to produce internally redundant, viable, genomes. The end-joining reaction in human HeLa and A549 cells is almost as efficient as the recombination reaction, and is relatively insensitive to the nature of the ends, as pairs of fragments terminating in several different single strands or in blunt ends can join. In contrast to the results from transfection with SV40, the ends are usually modified, for example by the loss of 3' single strands or the repair of 5' single strands. The ability to recover viable redundant molecules is not confined to any one area of the adenovirus genome, but can occur in the E1 and L2 regions as well as in the E2b region. The redundant genomes contain extra splice signals and may have the capacity to encode fusion proteins.

Factors influencing transient expression in cytotoxic T cells following DEAE dextran-mediated gene transfer

A number of transfection protocols have been tested for the introduction of exogenous DNA into cytotoxic T cells. These included electroporation, lipofection, calcium phosphate coprecipitation, polybrene-assisted gene transfer, and DEAE dextran-mediated transfer. Only the latter gave significant and reproducible transfection efficiencies coupled with low toxicity. The DEAE dextran protocol was optimized for the transfection of a transcription reporter construct pRSVcat into a cloned cytotoxic cell line. Among the parameters investigated were cell density, amount of input DNA, concentration of DEAE dextran, DNA adsorption time, temperature, use of permeabilization and expression facilitators, and recovery time. The optimized protocol was then used to demonstrate the presence of cis-acting regulatory regions in the 5'-flanking sequences of two cytotoxic cell-specific serine protease genes and, in addition, was shown to be applicable to other cloned T-cell lines.

Linker mutation scanning of the genes encoding the adenovirus type 5 terminal protein precursor and DNA polymerase

The replication of adenovirus DNA requires, in addition to several host factors, three virus-encoded proteins: a DNA binding protein, the precursor of the terminal protein (pTP), and a DNA polymerase (Ad pol). Ad pol and pTP form a tight complex that is necessary for the initiation step in DNA replication. To perform mutation scanning of the adenovirus type 5 pTP and Ad pol a series of in-frame linker insertions of a 12-mer oligonucleotide d(CCCATCGATGGG) were introduced into cloned viral DNA fragments containing coding sequences of these proteins. The insertions are located at recognition sites for several blunt end-cutting restriction endonucleases. Forty different sites were mutagenized and the mutated genes were transferred to a plasmid that contains the left 42% of the adenovirus genome. They were rebuilt into the viral genome by means of in vivo recombination between plasmid DNA and digested adenovirus DNA-TP complex. The resulting viral genomes were tested for viability and rescued virus was analyzed for the presence of the inserted linker oligonucleotide. This procedure resulted in recovery of a number of viable virus mutants with insertions in the pTP or Ad pol genes, all of which are phenotypically silent. The other mutations did not allow virus production. The positions of these apparent lethal codon insertion mutations were useful to identify regions of functional importance in both proteins. It can be concluded that the precursor-specific region of pTP plays an important role in virus multiplication.

Triiodothyronine inhibits transcription from the human growth hormone promoter

Three DNA constructs, the natural human growth hormone gene (hGH-hGH) its 500 bp promoter linked to the chloramphenicol acetyl transferase reporter gene (hGH-CAT), and its structural part linked to the herpes virus thymidine kinase promoter (TK-hGH) were introduced into rat pituitary GC cells by DEAE-dextran transfection. Transient expression was followed as a function of triiodothyronine (T3) concentration. The hGH-CAT expression was specifically inhibited by T3 following a typical dose-response curve while hGH-GH gene expression was not significantly modified. The transient expression of TK-hGH increased as a function of T3 concentration. These results indicate that T3 exerts two opposite effects on hGH gene expression. First, it down-regulates expression by acting on the promoter; second, it up-regulates expression by acting on the structural part of the gene. These action could be due to regulation of transcription and mRNA stabilization, respectively.

Overview of biological effects of addition of DNA molecules to cells

Injected DNA proceeds with certain probabilities through the following steps: degradation by serum nucleases, adsorption to cells, uptake into cells, ligation to other DNA, mutation, expression of unintegrated DNA, integration, expression of integrated DNA, and activation of or inactivation of cellular genes. The maximal probability per DNA molecule of each of these steps is estimated based on experimental results in cell culture with transfection of DNA and with infection by retroviruses. A maximum cumulative probability of having a harmful effect is calculated to be less than 10(-16) to 10(-19) per DNA molecule from a cell without activated proto-oncogenes or active viral oncogenes. The most frequent harmful effects considered are inactivation of a tumor suppressor gene and activation of a proto-oncogene. Such inactivation and activation in a cell that could give rise to cancer would increase the age-standardized incidence of cancer by a small amount. The amount of increase would differ among individuals depending upon their genotypes and their environments. Thus, the magnitude of the increase will depend upon the frequency of more sensitive individuals. The probability of an increased incidence of cancer as a possible effect of the vaccination should be compared to the number of DNA molecules to be injected per person and to the protective effects of a successful HBV vaccine.

Development of new cell lines for animal cell biotechnology

Mammalian cell culture has been an important technique in laboratory-scale experimentation for many decades. Developments in large-scale culture have been due to the need to grow large numbers of cells to support the growth of viruses for vaccine production, and more recently, for growing hybridoma cells as a source of monoclonal antibody. Increasingly, however, pharmaceutical products such as hormones, enzymes, growth factors, and clotting factors are being produced from cell lines which have been manipulated by recombinant DNA techniques. It is clear, therefore, that the high cost of growing mammalian cells on a large scale does not necessarily prohibit their use for biotechnology, and indeed there is considerable evidence to suggest that animal cell biotechnology will continue to be a major growth area in the future.

Primary cells and established cell lines join DNA ends with the same efficiency relative to homologous recombination

The joining of DNA ends was compared in the established CV1 cell line and in African green monkey kidney primary cells, from which the CV1 cell line was established. Using a linear SV40 genome that carries a terminal repeat of 526 nucleotide pairs, we measured the efficiency of circularization by end joining relative to circularization by homologous recombination between the terminal repeats. The ratios of end joining to homologous recombination were identical in the two types of cells regardless of whether the DNA ends were sticky, blunt, or mismatched. These studies demonstrate that the efficient end joining observed in established cell lines is not a peculiar adaptation to life in culture, but rather reflects a normal aspect of DNA metabolism in mammalian cells. Furthermore, the similar results in primary and established cells support the general validity of using data from studies of cultured cells to formulate models of recombinational processes in primary cells and potentially in multicellular organisms as well.

Optimization of electroporation for transfection of mammalian cell lines

Electroporation can be a highly efficient method for introducing DNA molecules into cultured cells for transient expression of genes or for permanent genetic modification. However, effective transformation by electroporation requires careful optimization of electric field strength and pulse characteristics. We have used the transient expression of the firefly luciferase gene as a rapid and sensitive indicator of gene expression to describe the effects on transfection efficiency of altering electroporation field strength and shape. Using the luciferase assay, we investigated the correlation of cell viability with optimal transfection efficiency and determined the optimal parameters for a number of phenotypically distinct mammalian cell lines derived from the nervous and immune systems. The efficiency of electroporation under optimal conditions was compared with that obtained using DEAE-dextran or calcium phosphate-mediated transformation. Transfection by electroporation using square wave pulses, as opposed to exponentially decaying pulses, was found to be significantly increased by repetitive pulses. These methods improve the ability to obtain high efficiency gene transfer into many mammalian cell types.

The scid defect affects the final step of the immunoglobulin VDJ recombinase mechanism

Abelson murine leukemia virus-transformed precursor B lymphocytes from scid (severe combined immunodeficient) mice, like A-MuLV transformants from normal mice, actively rearrange segments of their Ig heavy chain variable region gene locus during growth in culture. Targeting of recombination to appropriate segments appears normal in these lines as evidenced by initial rearrangement of sequences from within the D and JH locus to form aberrant "DJH" rearrangements and secondary rearrangement of sequences from within the VH locus to the aberrant "DJH" intermediates. A detailed analysis of the joints in these rearrangements indicates that the VDJ recombinase in scid pre-B cells can correctly recognize heptamernonamer signal sequences and perform precise endonucleolytic scissions at these sequences. We propose that the scid defect involves the inability of scid precursor lymphocytes to join correctly the cleaved ends of the coding strands of variable region gene segments.

Growth hormone gene expression in myoepithelial cells directed by various eucaryotic transcriptional regulatory sequences

Mammary gland myoepithelial cells were isolated from cattle and cell lines were established. Cells were plated onto tissue culture dishes with or without collagen. Cells were transfected with bovine growth hormone rDNA containing one of the following eucaryotic transcriptional regulatory sequences: human cytomegalovirus immediate early promoter, simian virus 40 early promoter, mouse metallothionein I promoter and the Rous sarcoma virus long terminal repeat. These sequences were evaluated for their ability to direct recombinant bovine growth hormone DNA expression in myoepithelial cells. The most effective transcriptional regulatory sequences were the cytomegalovirus immediate early and simian virus 40 early promoters.

Intranuclear uptake and persistence of biologically active DNA after electroporation of mammalian cells

Radiolabeled or biologically functional DNA molecules were introduced into cells by electroporation in a variety of forms: double stranded circles, linearized double stranded fragments and single stranded circular molecules. Molecules rapidly entered cells after exposure to a high field-strength electric pulse and then redistributed between the cytoplasm and the nucleus. Maximal intranuclear levels approximated 10(4) molecules per cell. Introduced DNA persisted in a biologically active form with a half-life of 15-24 h. There was no evidence for biologically significant alteration of two double stranded gene sequences. Single stranded DNA molecules also retained biological activity.

A comparison of bovine growth hormone expression directed by bGH genomic or intronless DNA in transiently transfected eukaryotic cells

Two recombinant DNA plasmids were constructed with identical transcriptional and translational regulatory elements controlling expression of the bovine growth hormone (bGH) gene or the bGH gene lacking introns. Transient expression of these plasmids in cultured eukaryotic cells, monitored by assaying secretion of bGH into the culture medium, was employed to examine the relative importance of introns in the expression of this gene. The bGH gene lacking introns is expressed more efficiently than the bGH gene in avian and mammalian cells.

Expression of simian virus 40 early and late genes in mouse oocytes and embryos

Simian virus 40 (SV40) large- and small-tumor antigens (T-Ag, t-Ag) are normally synthesized early after infection of either permissive (monkey) or nonpermissive (mouse) fibroblasts, whereas an equivalent amount of viral coat protein (V-Ag) is observed late after infection of permissive cells and only after viral DNA replication has occurred. To determine whether or not expression of these genes is regulated in the same manner during early mammalian development, SV40 DNA was injected into the nuclei of mouse oocytes and one- and two-cell embryos. In oocytes, about three times more V-Ag was produced than T-Ag, and both were synthesized concomitantly in the same cells. Viral mRNA and proteins synthesized in oocytes comigrated during gel electrophoresis with the same products synthesized in SV40-infected monkey cells. Viral gene expression required circular DNA molecules injected into the nuclei of transcriptionally and translationally active cells. Injected DNA was stable and underwent conformational changes consistent with chromatin assembly. Oocytes did not replicate either polyomavirus or SV40 DNA. Thus, the temporal order of viral gene expression is circumvented in mouse germ cells, allowing these proteins to be expressed concurrently and in equivalent amounts with no requirement for DNA replication. However, in preimplantation embryos, neither T-Ag nor V-Ag was detected by immunoprecipitation although T-Ag synthesis was demonstrated as a specific requirement for SV40 DNA replication. Thus, viral gene expression in mouse embryos as early as the one-cell stage was reduced at least 500-fold relative to that in oocytes. Similarities between SV40 gene expression in mouse oocytes and that in Xenopus oocytes suggest that germ cells in higher animals share common regulatory mechanisms.

Comparison of methods for introducing vectors based on bovine papillomavirus-1 DNA into mammalian cells

The intracellular structure of several vectors based on BPV-1 DNA has been analyzed following transfection into mouse C127 cells by the calcium phosphate method or, for the first time, by microinjection directly into the nucleus. It is shown that the method of introduction markedly affects the fate of a BPV-1 based vector. In general, microinjection appears to do little damage to DNA and is more likely to result in a vector replicating extrachromosomally as a monomeric structure of the same size as the input DNA. The method of selection for transformed cells, e.g., focus formation versus resistance to the neomycin analog G418, can also affect the intracellular state of the BPV-1 vector DNA. The nature of the recipient mammalian cell also influences whether a vector can replicate extrachromosomally or whether it integrates. BPV-1 based vectors, which replicated predominantly as multicopy intact extrachromosomal forms in mouse C127 cells, were always found to have integrated at low copy number in mouse LtAp20 cells.

Significance of the gastrin homology and surrounding sequences in polyomavirus middle T antigen for cell transformation

Deletion of residues 305 to 327 of polyomavirus middle T antigen, including the (Glu)6-Tyr-315 sequence that is a preferred site of phosphorylation in vitro by pp60c-src, markedly altered viral transformation of rat cells. The efficiency of transformation by the deletion mutant depended on how it was introduced into cells, and the resulting transformants displayed limited growth rates in monolayer and in suspension. Substitution of the polyomavirus residues 305 to 327 with a homologous region (containing [Glu]5-Ala-Tyr) from porcine gastrin did not restore wild-type transforming activity. These mutant middle T antigens interacted with pp60c-src and were phosphorylated in vitro. Thus, although a sequence of consecutive glutamic acid residues followed by a tyrosine is a dominant structural element which strongly influences the physical properties of middle T antigen, its presence did not ensure the biological activity of the protein. Other elements in this region of middle T antigen also contributed substantially to the transforming capacity of polyomavirus.

Studies on cellular tandemization of herpes simplex virus thymidine kinase DNA

The cellular tandemization of the herpes simplex virus (HSV) thymidine kinase (tk) gene was studied in tk- mouse fibroblasts after gene transfer by microinjection into the nucleus or by calcium phosphate-mediated transfection. Three different DNA substrates, designed to yield simple integration patterns, were used: a gel-purified 3.6-kb Bam HI fragment containing the HSV tk gene; the same fragment self-ligated; and the 3.6-kb fragment ligated to a Bam HI-cleaved subset of genomic mouse DNA. The genomic DNA of six independently isolated transformed cell lines was analyzed by Southern blotting and the structure of the tk-specific DNA was studied. The data suggest that modifications (mutations, deletions, recombination events, and recircularization, etc.) of the input DNA fragment occur early after its introduction into the cell. Subsequently these structures are multiplied in a directional manner, generating larger arrays of DNA with distinct and regularly repeated areas. These concatemers can eventually be integrated into the host genome.

Circular and linear simian virus 40 DNAs differ in recombination

Linear forms of simian virus 40 (SV40) DNA, when added to transfection mixtures containing circular SV40 and phi X174 RFI DNAs, enhanced the frequency of SV40/phi X174 recombination, as measured by infectious center in situ plaque hybridization in monkey BSC-1 cells. The sequences required for the enhancement of recombination by linear DNA reside within the SV40 replication origin/regulatory region (nucleotides 5,171 to 5,243/0 to 128). Linearization of phi X174 RFI DNA did not increase the recombination frequency. The SV40/phi X174 recombinant structures arising from transfections supplemented with linear forms of origin-containing SV40 DNA contained phi X174 DNA sequences interspersed within tandem head-to-tail repeats derived from the recombination-enhancing linear DNA. Evidence is presented that the tandem repeats are not formed by homologous recombination and that linear forms of SV40 DNA must compete with circular SV40 DNA for the available T antigen to enhance recombination. We propose that the enhancement of recombination by linear SV40 DNA results from the entry of that DNA into a rolling circle type of replication pathway which generates highly recombinogenic intermediates.

Use of avian retroviral-bovine growth hormone DNA recombinants to direct expression of biologically active growth hormone by cultured fibroblasts

A variety of recombinant DNA molecules were constructed in which an avian retroviral long terminal repeat (LTR) was ligated to the bovine growth hormone (bGH) gene. The retroviral LTR was derived from a plasmid clone of a Schmidt Ruppin B strain of Rous sarcoma virus while the bGH gene was subcloned from a lambda bacteriophage genomic library. Using a transient eukaryotic expression assay system, recombinant plasmid constructs were screened for their ability to direct expression and secretion of bGH. One such plasmid DNA construct, termed pBGH-4, was found to be active in the production of bGH. Stable mouse fibroblast cell lines were generated containing pBGH-4 DNA integrated into the mouse cell genome. Many of these mouse cell lines express and secrete bGH. One line, L-Pd lambda-BGH4-13, was found to secrete bGH at a rate of 75 micrograms per 5 X 10(6) cells per 24 hr. Bovine growth hormone derived from this cell line is biologically active.

Growth hormone gene expression in eukaryotic cells directed by the Rous sarcoma virus long terminal repeat or cytomegalovirus immediate-early promoter

The cytomegalovirus (CMV) immediate-early (IE) gene-regulatory region was found to be three- to fourfold more efficient than the Rous sarcoma retroviral long terminal repeat (LTR) in promoting expression of the bovine growth hormone (bGH) gene by rat GH3 cells.

Effect of cell cycle position on transformation by microinjection

We have investigated the effect of cell cycle position on the efficiency of transformation by microinjection. Linear recombinant plasmids transform synchronized cells with similar frequencies following injections at all cell cycle stages, whereas supercoiled molecules show a decreased ability to generate transformants in early S phase. This inhibition is not due to an inability to transiently express a transferred gene, since cells at all stages of the cycle efficiently expressed a hamster adenine phosphoribosyltransferase gene introduced on a supercoiled plasmid. Southern transfer analyses of the cell cycle specific transformants revealed that tandem arrays of plasmids, integrated into the host chromosomes, could be generated at all cell cycle stages.