Low molecular weight disulfide cross-linking peptides as nonviral gene delivery carriers.

Cross-linking peptides have been developed by inserting multiple Cys residues into a 20 amino acid condensing peptide that polymerizes through disulfide bond formation when bound to DNA resulting in small, highly stable DNA condensates that mediate efficient in vitro gene transfer [McKenzie et al. (2000) J. Biol. Chem. 275, 9970-9977]. In the present study, a minimal peptide of four Lys and two terminal Cys residues was found to substitute for Cys-Trp-(Lys)(17)-Cys, resulting in DNA condensates with similar particle size and gene expression in HepG2 cells. Substitution of His for Lys residues resulted in an optimal peptide of Cys-His-(Lys)(6)-His-Cys that, in addition to the attributes described above, also provided buffering capacity to enhance in vitro gene expression in the absence of chloroquine. The reported structure-activity relationships systematically explore peptides with combinations of Lys, Cys, and His residues resulting in low molecular weight peptides with improved gene transfer properties.

Synthesis of homogeneous glycopeptides and their utility as DNA condensing agents.

Two glycopeptides were synthesized by attaching purified glycosylamines (N-glycans) to a 20 amino acid peptide. Triantennary and Man9 Boc-tyrosinamide N-glycans were treated with trifluoroacetic acid to remove the Boc group and expose a tyrosinamide amine. The amine group was coupled with iodoacetic acid to produce N-iodoacetyl-oligosaccharides. These were reacted with the sulfhydryl group of a cysteine-containing peptide (CWK18), resulting in the formation of glycopeptides in good yield that were characterized by 1H NMR and ESIMS. Both glycopeptides were able to bind to plasmid DNA and form DNA condensates of approximately 110 nm mean diameter with zeta potential of +31 mV. The resulting homogeneous glycopeptide DNA condensates will be valuable as receptor-mediated gene-delivery agents.

A potent new class of reductively activated peptide gene delivery agents.

A new class of peptide gene delivery agents were developed by inserting multiple cysteine residues into short (dp 20) synthetic peptides. Substitution of one to four cysteine residues for lysine residues in Cys-Trp-Lys(18) resulted in low molecular weight DNA condensing peptides that spontaneously oxidize after binding to plasmid DNA to form interpeptide disulfide bonds. The stability of cross-linked peptide DNA condensates increased in proportion to the number of cysteines incorporated into the peptide. Disulfide bond formation led to a decrease in particle size relative to control peptide DNA condensates and prevented dissociation of peptide DNA condensates in concentrated sodium chloride. Cross-linked peptide DNA condensates were 5-60-fold more potent at mediating gene expression in HepG2 and COS 7 cells relative to uncross-linked peptide DNA condensates. The enhanced gene expression was dependent on the number of cysteine residues incorporated, with a peptide containing two cysteines mediating maximal gene expression. Cross-linking peptides caused elevated gene expression without increasing DNA uptake by cells, suggesting a mechanism involving intracellular release of DNA triggered by disulfide bond reduction. The results establish cross-linking peptides as a novel class of potent gene delivery agents that enhance gene expression through a new mechanism of action.

Formulation of highly soluble poly(ethylene glycol)-peptide DNA condensates.

Two poly(ethylene glycol) (PEG)-peptides were synthesized and tested for their ability to bind to plasmid DNA and form soluble DNA condensates with reduced spontaneous gene expression. PEG-vinyl sulfone or PEG-orthopyridyl disulfide were reacted with the sulfhydryl of Cys-Trp-Lys(18) (CWK(18)) resulting in the formation of nonreducible (PEG-VS-CWK(18)) and reducible (PEG-SS-CWK(18)) PEG-peptides. Both PEG-peptides were prepared on a micromole scale, purified by RP-HPLC in >80% yield, and characterized by (1)H NMR and MALDI-TOF. PEG-peptides bound to plasmid DNA with an apparent affinity that was equivalent to alkylated (Alk)CWK(18), resulting in DNA condensates with a mean diameter of 80-90 nm and zeta (zeta) potential of +10 mV. The particle size of PEG-peptide DNA condensates was constant throughout the DNA concentration range of 0. 05-2 mg/mL, indicating these to be approximately 20-fold more soluble than AlkCWK(18) DNA condensates. The spontaneous gene transfer to HepG2 cells mediated by PEG-VS-CWK(18) DNA condensates was over two orders of magnitude lower than PEG-SS-CWK(18) DNA condensates and three orders of magnitude lower than AlkCWK(18) DNA condensates. PEG-VS-CWK(18) efficiently blocked in vitro gene transfer by reducing cell uptake. The results indicate that a high loading density of PEG on DNA is necessary to achieve highly soluble DNA condensates that reduce spontaneous in vitro gene transfer by blocking nonspecific uptake by HepG2 cells. These two properties are important for developing targeted gene delivery systems to be used in vivo.

Comparative gene transfer efficiency of low molecular weight polylysine DNA-condensing peptides.

In a previous report (M.S. Wadhwa et al. (1997) Bioconjugate Chem. 8, 81-88), we synthesized a panel of polylysine-containing peptides and determined that a minimal repeating lysine chain of 18 residues followed by a tryptophan and an alkylated cysteine residue (AlkCWK18) resulted in the formation of optimal size (78 nm diameter) plasmid DNA condensates that mediated efficient in vitro gene transfer. Shorter polylysine chains produced larger DNA condensates and mediated much lower gene expression while longer lysine chains were equivalent to AlkCWK18. Surprisingly, AlkCWK18 (molecular weight 2672) was a much better gene transfer agent than commercially available low molecular weight polylysine (molecular weight 1000-4000), despite its similar molecular weight. Possible explanations were that the cysteine or tryptophan residue in AlkCWK18 contributed to the DNA binding and the formation of small condensates or that the homogeneity of AlkCWK18 relative to low molecular weight polylysine facilitated optimal condensation. To test these hypotheses, the present study prepared AlkCYK18 and K20 and used these to form DNA condensates and conduct in vitro gene transfer. The results established that DNA condensates prepared with either AlkCYK18 or K20 possessed identical particle size and mediated in vitro gene transfer efficiencies that were indistinguishable from AlkCWK18 DNA condensates, eliminating the possibility of contributions from cysteine or tryptophan. However, a detailed chromatographic and electrospray mass spectrometry analysis of low molecular weight polylysine revealed it to possess a much lower than anticipated average chain length of dp 6. Thus, the short chain length of low molecular weight polylysine explains its inability to form small DNA condensates and mediate efficient gene transfer relative to AlkCWK18 DNA condensates. These experiments further emphasize the need to develop homogenous low molecular weight carrier molecules for nonviral gene delivery.

Peptide-mediated gene delivery: influence of peptide structure on gene expression.

Cationic peptides possessing a single cysteine, tryptophan, and lysine repeat were synthesized to define the minimal peptide length needed to mediate transient gene expression in mammalian cells. The N-terminal cysteine in each peptide was either alkylated or oxidatively dimerized to produce peptides possessing lysine chains of 3, 6, 8, 13, 16, 18, 26, and 36 residues. Each synthetic peptide was studied for its ability to condense plasmid DNA and compared to polylysine19 and cationic lipids to establish relative in vitro gene transfer efficiency in HepG2 and COS7 cells. Peptides with lysine repeats of 13 or more bound DNA tightly and produced condensates that decreased in mean diameter from 231 to 53 nm as lysine chain length increased. In contrast, peptides possessing 8 or fewer lysine residues were similar to polylysine19, which bound DNA weakly and produced large (0.7-3 microns) DNA condensates. The luciferase expression was elevated 1000-fold after HepG2 cells were transfected with DNA condensates prepared with alkylated Cys-Trp-Lys18 (AlkCWK18) versus polylysine19. The gene transfer efficiencies of AlkCWK18 and cationic lipids were equivalent in HepG2 cells but different by 10-fold in COS 7 cells. A 40-fold reduction in particle size and a 1000-fold amplification in transfection efficiency for AlkCWK18 DNA condensates relative to polylysine19 DNA condensates suggest a contribution from tryptophan that leads to enhanced gene transfer properties for AlkCWK18. Tryptophan-containing cationic peptides result in the formation of small DNA condensates that mediate efficient nonspecific gene transfer in mammalian cells. Due to their low toxicity, these peptides may find utility as carriers for nonspecific gene delivery or may be developed further as low molecular weight DNA condensing agents used in targeted gene delivery systems.