A versatile method for the coupling of protein to DNA: synthesis of alpha 2-macroglobulin-DNA conjugates

Targeting of Polyplexes: Toward Synthetic Virus Vector Systems

Dominating issues in gene vector optimization are specific in recognizing the target cells and exploiting the proper intracellular trafficking routes. Any progress in this area will result in improved specific gene transfer, reduce the required therapeutic vector doses and, in consequence, lower the overall toxicity to the host. To provide polyplexes with the ability to distinguish between non-target and target cells, cell-binding ligands have been incorporated which recognize target-specific cellular receptors. In addition, polyplex domains with unspecific binding capacity (such as surface charges) have to be shielded or removed. Cell-binding ligands can be small molecules, vitamins, carbohydrates, peptides or proteins such as growth factors or antibodies. Such ligands have been incorporated into polyplexes after chemical conjugation to cationic polymers. The choice of the ligand and physical properties of the DNA formulation strongly influence extracellular routing (circulation in blood, tissue distribution), uptake and intracellular delivery of polyplexes. Recent efforts are discussed that aim at the development of polyplexes into virus-like supramolecular complexes; such particles should undergo structural changes compatible with extracellular and intracellular targeting.

Systemic genetic transfer of p21WAF-1 and GM-CSF utilizing of a novel oligopeptide-based EGF receptor targeting polyplex

Based on the fact that aberrant overexpression of some growth factor receptors was observed in a variety of human cancer cells, a novel nonviral gene delivery system GE7, which contains a 16-amino-acid ligand for identifying EGF receptor was constructed for tumor-targeted gene therapy. Intravenous administration of GE7 system revealed that it has the ability to target beta-galactosidase (beta-gal) reporter gene into murine hepatoma (Hepa) cells. Owing to the limited antitumor effects elicited by a single-gene transfer, recent efforts to treat malignancy using combined gene therapy have been accomplished with varying degrees of success. In this study, the human cyclin-dependent kinase inhibitor gene p21(WAF-1) and the murine cytokine gene granulocyte-macrophage colony-stimulating factor (GM-CSF) were used simultaneously for in vivo gene therapy through systemic injection of the EGF R targeted GE7/DNA complex into murine hepatoma-bearing mice. The results demonstrated that combined administration of p21(WAF-1) and GM-CSF could remarkably inhibit the growth of subcutaneously transplanted hepatoma Hepa cells, and significantly increase the survival rate of tumor-bearing mice. The activities of natural killer (NK) cells and specific cytotoxic T lymphocytes (CTL) were clearly enhanced after combined gene therapy. In vitro experiments showed that p21(WAF-1) gene transfer exhibited a suppressive function on the growth of Hepa cells and the expression of H-2K(b) and B7-1 molecules on Hepa cells increased significantly after combined genes delivery. All these results suggested that the GE7 system was able to target therapeutic genes efficiently to cancer cells, which showed high EGF R expression. The cotransfer of p21(WAF-1) and GM-CSF genes apparently inhibited the growth of tumors through (a) the arrest of tumor cell growth and (b) the enhancement of systemic antitumor immunity.

Biological properties of poly-L-lysine-DNA complexes generated by cooperative binding of the polycation

We have evaluated the effect of NaCl concentration on the mode of binding of poly-L-lysine to DNA and the resulting structural and functional features of the condensed DNA particles using DNA precipitation, DNase I resistance, electron microscopy, and receptor-mediated gene transfer assays. At a high concentration of NaCl and in the presence of excess DNA, poly-L-lysine interacted with DNA cooperatively, fully condensing some of the DNA and leaving the rest of the DNA unbound. At low NaCl concentrations, poly-L-lysine molecules interacted with DNA in a noncooperative fashion, i.e. they bind randomly to the whole population of DNA molecules. Cooperative binding of poly-L-lysine to DNA occurred over a narrow range of NaCl concentrations, and the specific salt concentration depended on the length of the poly-L-lysine. The ability of condensed DNA to withstand digestion by DNase I was correlated with the structural features of the condensed DNA as determined by electron microscopy. Using our condensation procedure, cooperative binding of poly-L-lysine to DNA is a necessary prerequisite for the preparation of condensed DNA having a spherical shape and a diameter of 15-30 nm. Condensed DNA, containing galactosylated poly-L-lysine, was evaluated further for the extent and specificity of receptor-mediated gene transfer into HuH-7 human hepatoma cells via the asialoglycoprotein receptor. Efficient receptor-mediated transfection occurred only when condensed DNA complexes had a spherical shape with a diameter of 15-30 nm; asialofetuin, a natural ligand for the asialoglycoprotein receptor, inhibited this process by up to 90%. Our results support the importance of appropriate DNA condensation for the uptake and ultimate expression of DNA in hepatic cells.

Gene transfer mediated by alpha2-macroglobulin

alpha2-Macroglobulin covalently linked to poly(L)-lysine can be used as a vehicle for receptor-mediated gene transfer. This modified alpha2-macroglobulin maintains its ability to bind to the alpha2-macroglobulin receptor, and was shown to introduce a luciferase reporter gene plasmid into HepG2 human hepatoma cells in vitro. The alpha2-macroglobulin receptor is a very large and multifunctional cell surface receptor, whose rapid and efficient internalization rate makes it attractive for gene therapy, e.g. for hepatic gene targeting via injection into the portal vein.

An evaluation of receptor-mediated gene transfer using synthetic DNA-ligand complexes

Receptor-mediated gene transfer is an attractive method for therapeutically correcting human genetic diseases since it permits the targeting of DNA to cellular receptors in specific tissues of adult animals. Genes introduced by this technique have been shown to be expressed in the target tissue for varying periods. However, to be useful for gene therapy, it is critical that both the chemical properties and physical interactions of the reagents involved in the design of the DNA delivery vehicle be rigorously characterized. In this review, we discuss the critical steps in the preparation of the DNA-ligand complex and the factors involved in the delivery and regulated expression of a transgene in animal tissues. The feasibility of using this technique for the therapeutic delivery of genes to mammalian tissues will also be evaluated.

Evidence for targeted gene transfer by receptor-mediated endocytosis. Stable expression following insulin-directed entry of NEO into HepG2 cells

Evidence is presented for targeted gene delivery to HepG2 cells via the endocytotic pathway under the direction of insulin. Serum albumin was treated with the water-soluble carbodiimide N-ethyl-N'-(3-dimethylaminopropyl) carbodiimide hydrochloride and the resultant positively charged N-acylurea albumin covalently conjugated to insulin by glutaraldehyde cross-linkage. The conjugated protein is shown by nitrocellulose filter binding and gel band shift assays to bind DNA, and by competitive displacement of [125I]insulin to bind to the insulin receptor. When the expression vectors ptkNEO and pAL-8 which incorporate the neo gene were complexed to the conjugate in an in vitro system of transfection, G418 resistant clones developed at frequencies of 2.0-5.5 x 10(-5). Subsequently, a 923bp PstI fragment within the neo sequence was identified by Southern transfer in genomic DNA from transfected cell populations which had been maintained on a G418 regime for 44 days.

The preparation of poly (dT)-5'-transferrin conjugates and hybridisation studies with poly (dA)-tailed linearised pBR322 plasmid DNA

The formation of transferrin-DNA complexes intended for ligand-directed transfection studies has been achieved through a hybridisation technique involving complementary homodeoxypolynucleotide chains attached to the participating protein and DNA species. Oligothymidylate residues (pT)n obtained by dicyclohexylcarbodiimide (CDI) polymerisation of thymidine-5'-monophosphate (5'-TMP) were activated to the 5'-imidazolides which on incubation with transferrin yielded the 5'linked phosphoramidates (pT)n-5'-transferrin. Homopolymeric chain extension of (pT)5-5'-transferrin by terminal transferase and dTTP at 30 degrees for 30 min yielded (pT) 300-5'-transferrin. Cleavage of the phosphoramide link in the polymer modified transferrin at 37 degrees was pronounced after 30 min although at 25 degrees hydrolysis was less than 5% after 4 hr. Poly(dT)-5'-transferrin readily hybridised with [3H]poly(dA)-tailed Pst 1 linearised pBR322 DNA. Resultant complexes were demonstrated by nitrocellulose filter binding and immunoprecipitation with anti-transferrin antibody. In contrast with poly(dT)-5'-transferrin, poly(dT)-5'-transferrin-poly(dA)-tailed pBR322 DNA complexes were stable at 37 degrees suggesting that annealing is followed by further stabilising interactions between the DNA and protein components.

Ligation of oligonucleotides to nucleic acids or proteins via disulfide bonds

We have developed general methods for joining together, via cleavable disulfide bonds, either two unprotected polynucleotides or a polynucleotide and a peptide or protein. To join two oligonucleotides, each is first converted to an adduct in which cystamine is joined to the 5'-terminal phosphate of the oligonucleotide by a phosphoramidate bond. The adducts are mixed and reduced with dithiothreitol. The dithiothreitol is then removed by dialysis. Oxidation by atmospheric oxygen occurs to yield the required dimer. To join an oligonucleotide to a cysteine-containing peptide or protein, the 5'-cystamine oligomer is first converted to a 2'-pyridyldisulfide adduct and then reacted with an excess of the peptide or protein. If the peptide does not contain a free cysteine residue, it is first treated with iminothiolane to introduce one or more sulfhydryl groups. We have used these procedures to join a 16 mer deoxynucleotide probe and MDV-1 RNA, a substrate of Q beta RNA polymerase. This adduct hybridizes with a complementary target DNA. We have also joined a 16mer probe to peroxidase and MDV-1 RNA to human IgG. The probe-peroxidase adduct maintains enzymatic activity and the MDV-1 RNA-IgG adduct binds to a complementary anti-IgG.

The binding of DNA to water soluble carbodiimide modified proteins

Bovine serum albumin and human serum transferrin modified by the water soluble carbodiimides N-ethyl-N'-(3-dimethyl propylamino) carbodiimide (CDI) or N-ethyl-N'-(3-trimethyl propylammonium) carbodiimide (Me+CDI) to yield N-acylurea proteins bind pBR322 DNA reversibly showing electrostatic and non-electrostatic components in the binding energies (delta G overall). It is proposed that initially an electrostatic interaction arises from ion pair formation between the DNA phosphates and the N-acylurea entities. This is consolidated, in single stranded regions, by a second event in which it is suggested that the base guanine interacts with elements of the N-acylurea moieties through hydrogen bonding or a glyoxal-type addition.

Binding of DNA to albumin and transferrin modified by treatment with water-soluble carbodiimides

N-Acylurea derivatives of albumin and transferrin prepared with the water-soluble carbodiimides N-ethyl-N'-(3-dimethylaminopropyl)carbodiimide and N-ethyl-N'-(3-trimethylpropylammonium)carbodiimide iodide have been found to bind different types of DNA. The two proteins were reacted with varying amounts of carbodiimide in water at pH 5.5 for 36-60 hr at 20 degrees, and then purified. In the case of iron-loaded transferrin, reactions with carbodiimides were in phosphate-buffered saline (pH 7.5) to prevent loss of iron from the protein. [3H]N-Ethyl-N'-(3-trimethylpropylammonium)carbodiimide iodide was used for the determination of covalently attached N-acylurea groups in the modified proteins, and gel electrophoresis for changes in charge and possible aggregation through cross-linking. Binding of DNA to N-acylurea proteins was studied by means of gel electrophoresis and nitrocellulose filter binding. N-Acylurea albumin and N-acylurea transferrin at low concentrations retarded the migration of lambda-Pstl restriction fragments, pBR322 plasmid and M13 mp8 single-stranded DNA on agarose gels, while at higher concentrations of modified protein the N-acylurea protein-DNA complexes were unable to enter the gel. Nitrocellulose filter assays showed that binding pBR322 DNA and calf thymus DNA to N-acylurea proteins is rapid and dependent on protein concentration and the ionic strength of the medium. N-Acylurea albumins prepared with each each of the two carbodiimides gave comparable plots for DNA bound versus protein concentration. On the other hand, binding of DNA by N-acylurea transferrins differed according to the carbodiimide used in the synthesis. N-Acylurea CDI-tkransferrin (prepared with tertiary carbodiimide) was less effective than either of the two N-acylurea albumins in binding DNA. In contrast with these results, N-acylurea Me+-CDI-transferrin (prepared with quaternary carbodiimide) was far more effective in binding DNA and in this respect was similar to the N-acylurea albumins. On the basis of experiments in which N-acylurea protein-DNA complexes were treated with heparin, two types of binding could be distinguished. These were a weak binding occurring in the initial stages of interaction and a tight binding which developed on further incubation of the complexes. These studies show that binding of DNA by N-acylurea proteins is a reversible process dependent on ionic strength; interaction appears to be electrostatic in nature, although other forms of binding might be involved.(ABSTRACT TRUNCATED AT 400 WORDS)

Novel chemical method for the preparation of nucleic acids for nonisotopic hybridization

A novel chemical method was used to prepare biotin-labeled nucleic acids for nonisotopic hybridization. The method involves the transamination of unpaired cytosine residues in polynucleotides with sodium bisulfite and ethylenediamine. Primary amino groups on the cytosine derivatives are then reacted with biotinyl-e-aminocaproic acid N-hydroxysuccinimide ester. Biotinylated probes hybridized with 1 to 2 pg of nitrocellulose filter-bound DNA and were visualized with a colorimetric detection technique. This method is simpler and less expensive than other methods for the preparation of nonisotopic probes. In addition, it is more versatile since the chemically modified bases can potentially react with other "indicator" molecules or proteins such as an enzyme. The specificity for unpaired cytosine residues is another advantage which could allow for the selective labeling of a specific region of a double-stranded nucleic acid. This improved labeling method should lead to the wider application of hybridization techniques in diagnostic microbiology and basic research in infectious diseases.

Derivatization of unprotected polynucleotides

A simple and efficient method for attaching amines to the terminal 5'-phosphate of unprotected oligonucleotides or nucleic acids in aqueous solution is described. The method is applicable to low molecular-weight amines, polypeptides, or proteins. The terminal 5'-phosphate of an oligonucleotide or nucleic acid reacts with a water-soluble carbodiimide in imidazole buffer at pH 6 to give good yields of the 5'-phosphorimidazolide. Exposure of the phosphorimidazolide to amine-containing molecules in aqueous solution results in the production of a wide range of stable phosphoramidates in high yield. The exposure of polynucleotides to carbodiimide does not result in significant breakage of phosphodiester bonds or damage to nucleoside bases. The biological activity of a drug resistant plasmid is not affected. The direct condensation of polynucleotides with amines in 1-methylimidazole buffer is also possible. However, it is not a satisfactory preparative method if the ligand is sensitive to carbodiimide.