Susannah L. Hyatt

Nanoparticles of compacted DNA transfect postmitotic cells.

Charge-neutral DNA nanoparticles have been developed in which single molecules of DNA are compacted to their minimal possible size. We speculated that the small size of these DNA nanoparticles may facilitate gene transfer in postmitotic cells, permitting nuclear uptake across the 25-nm nuclear membrane pore. To determine whether DNA nanoparticles can transfect nondividing cells, growth-arrested neuroblastoma and hepatoma cells were transfected with DNA/liposome mixtures encoding luciferase. In both models, growth-arrested cells were robustly transfected by compacted DNA (6,900–360-fold more than naked DNA). To evaluate mechanisms responsible for enhanced transfection, HuH-7 cells were microinjected with naked or compacted plasmids encoding enhanced green fluorescent protein. Cytoplasmic microinjection of DNA nanoparticles generated a ∼10-fold improvement in transgene expression as compared with naked DNA; this enhancement was reversed by the nuclear pore inhibitor, wheat germ agglutinin. To determine the upper size limit for gene transfer, DNA nanoparticles of various sizes were microinjected into the cytoplasm. A marked decrease in transgene expression was observed as the minor ellipsoidal diameter approached 25 nm. In summary, suitably sized DNA nanoparticles productively transfect growth arrested cells by traversing the nuclear membrane pore.

Adaptive Regulation of the Cationic Amino Acid Transporter-1 (Cat-1) in Fao Cells

The regulation of the high affinity cationic amino acid transporter Cat-1 in Fao rat hepatoma cells by amino acid availability has been studied. Cat-1 mRNA level increased (3-fold) in 4 h in response to amino acid starvation and remained high for at least 24 h. This induction was independent of the presence of serum in the media and transcription and protein synthesis were required for induction to occur. When Fao cells were shifted from amino acid-depleted media to amino acid-fed media, the levels of the induced cat-1 mRNA returned to the basal level. In amino acid-fed cells, accumulation of cat-1mRNA was dependent on protein synthesis, indicating that a labile protein is required to sustain cat-1 mRNA level. No change in the transcription rate of the cat-1 gene during amino acid starvation was observed, indicating thatcat-1 is regulated at a post-transcriptional step. System y+ mediated transport of arginine was reduced by 50% in 1 h and by 70% in 24 h after amino acid starvation. However, when 24-h amino acid-starved Fao cells were preloaded with 2 mmlysine or arginine for 1 h prior to the transport assays, arginine uptake was trans-stimulated by 5-fold. This stimulation was specific for cationic amino acids, since alanine, proline, or leucine had no effect. These data lead to the hypothesis that amino acid starvation results in an increased cat-1 mRNA level to support synthesis of additional Cat-1 protein. The following lines of evidence support the hypothesis: (i) the use of inhibitors of protein synthesis in starved cells inhibits the trans-zero transport of arginine; (ii) cells starved for 1–24 h exhibited an increase of trans-stimulated arginine transport activity for the first 6 h and had no loss of activity at 24 h, suggesting that constant replenishment of the transporter protein occurs; (iii) immunofluorescent staining of 24-h fed and starved cells for cat-1 showed similar cell surface distribution; (iv) new protein synthesis is not required for trans-stimulation of arginine transport upon refeeding of 24-h starved cells. We conclude that the increased level of cat-1 mRNA in response to amino acid starvation support the synthesis of Cat-1 protein during starvation and increased amino acid transport upon substrate presentation. Therefore, the cat-1 mRNA content is regulated by a derepression/repression mechanism in response to amino acid availability. We propose that the amino acid-signal transduction pathway consists of a series of steps which include the post-transcriptional regulation of amino acid transporter genes.

Molecular Sites of Regulation of Expression of the Rat Cationic Amino Acid Transporter Gene

Cat-1 is a protein with a dual function, a high affinity, low capacity cationic amino acid transporter of the y+ system and the receptor for the ecotropic retrovirus. We have suggested that Cat-1 is required in the regenerating liver for the transport of cationic amino acids and polyamines in the late G1 phase, a process that is essential for liver cells to enter mitosis. In our earlier studies we had shown that the cat-1 gene is silent in the quiescent liver but is induced in response to hormones, insulin, and glucocorticoids, and partial hepatectomy. Here we demonstrate that cat-1 is a classic delayed early growth response gene in the regenerating liver, since induction of its expression is sensitive to cycloheximide, indicating that protein synthesis is required. The peak of accumulation of the cat-1 mRNA (9-fold) by 3 h was not associated with increased transcriptional activity of the cat-1 gene in the regenerating liver, indicating post-transcriptional regulation of expression of this gene. Induction of the cat-1 gene results in the accumulation of two mRNA species (7.9 and 3.4 kilobase pairs (kb)). Both mRNAs hybridize with the previously described rat cat-1/2.9-kb cDNA clone. However, the 3′ end of a longer rat cat-1 cDNA (rat cat-1/6.5-kb) hybridizes only to the 7.9-kb mRNA transcript. Sequence analysis of this clone indicated that the two mRNA species result from the use of alternative polyadenylation signals. The 6.5-kb clone contains a number of AT-rich mRNA destabilizing sequences which is reflected in the half-life of the cat-1 mRNAs (90 min for 7.9-kb mRNA and 250 min for 3.4-kb mRNA). Treatment of rats with cycloheximide superinduces the level of the 7.9-kb cat-1 mRNA in the kidney, spleen, and brain, but not in the liver, suggesting that cell type-specific labile factors are involved in its regulation. We conclude that the need for protein synthesis for induction of the cat-1 mRNA, the short lived nature of the mRNAs, and the multiple sites for regulation of gene expression indicate a tight control of expression of the cat-1 gene within the regenerating liver and suggest that y+ cationic amino acid transport in liver cells is regulated at the molecular level.

505. Effective Transgene Expression in the Murine Lung Using Compacted DNA Nanoparticles Formulated with Plasmid DNAs of 5, 10, and 20 kbp

One concern of non-viral gene therapies is the potential limitation on the size of the DNA construct used in formulating an effective therapeutic. Some open reading frames, such as the coding sequence of the cystic fibrosis transmembrane regulator (4.4 kbp), are quite large and, when combined with other required regulatory elements, result in a therapeutic construct of significant size (> 8 kbp). Gene transfer optimization studies are usually performed using reporter gene constructs. Typical reporter genes have much smaller open reading frames (luciferase 1.7 kbp, green fluorescent protein 0.7 kbp) than many therapeutic genes. Therefore, we wanted to determine if our strategies for gene therapy of multiple inherited and acquired diseases will be limited in vivo by the required size of the plasmid DNA payload. To address this aim, we constructed three luciferase reporter plasmids of increasing size—~5, 10, and 20 kbp—using lambda DNA stuffer fragments inserted into the 5 kbp plasmid. These plasmids were compacted to DNA nanoparticles with a 10 kDa polyethylene glycol (PEG) substituted 30-mer lysine polymer containing either trifluoroacetate or acetate counterions at the time of DNA mixing. As evaluated by transmission electron microscopy, these formulations generate compacted DNA that appears as either ellipsoids or rods, respectively. These formulations were delivered to the lungs of Balb/C mice via the intranasal route, and the lungs were harvested and luciferase activity evaluated 2 days after gene transfer. All DNA nanoparticles tested—~5 kbp to 20 kbp plasmids formulated using PEG-lysine polymers with either counterion—had luciferase reporter activities of ~104 RLU/mg protein/pmol DNA above background. Within each formulation group, the reporter gene activity was equivalent (no statistically significant differences) among all three compacted plasmid DNAs. These results indicate that the size of the plasmid used, up to at least 20 kbp, should not be a limitation for lung gene transfer using this non-viral gene therapy platform.