Mechanisms of gene transfer mediated by lipoplexes associated with targeting ligands or pH-sensitive peptides

Purpose: Cationic liposome–DNA complexes (lipoplexes) are safer than viral vectors but have limited transfection efficiency, toxicity at high doses, and sensitivity to serum/extracellular negatively charged macromolecules. The study asks whether targeting ligands or pH-sensitive fusogenic peptides can improve lipoplex-mediated gene delivery, particularly when the ternary complex has a net negative or neutral charge rather than a high positive charge.

Hypothesis: If transferrin or pH-sensitive fusogenic peptides are associated with cationic liposome/DNA complexes, then transfection will be enhanced via receptor-mediated uptake and/or endosomal membrane destabilization. Net negatively charged ternary complexes may be especially effective, remain active in serum, and avoid the drawbacks of highly positively charged complexes.

Aims: Determine whether transferrin enhances transfection of cationic liposome/DNA complexes at different lipid/DNA (+/-) charge ratios. - Test whether pH-sensitive peptides GALA and influenza HA-2 N-terminal peptide enhance transfection and whether endosomal acidification is involved. - Measure zeta potential of plain and ternary complexes to relate surface charge to transfection activity. - Evaluate serum compatibility, cytotoxicity, and transfection efficiency in HeLa and COS-7 cells.

Delivery system: Core carrier: Cationic liposomes composed of DOTAP or DOTAP:DOPE (1:1 weight ratio). - Payload: Plasmid DNA encoding luciferase (`pCMVluc`) or β-galactosidase (`pCMV.SPORT-β-gal`). - Targeting/functional ligands: Iron-saturated human transferrin; pH-sensitive fusogenic peptides GALA and influenza HA-2 N-terminal peptide. - Complex type: Ternary complexes of DNA, cationic liposomes, and transferrin or peptide. - Charge ratios tested: Theoretical lipid/DNA (+/-) ratios of 1/2, 1/1, 2/1, and 4/1. - Key formulation observation: Transferrin or peptide addition reduced zeta potential, producing net negatively charged complexes at some ratios.

Approach: In vitro cell lines: HeLa cells and COS-7 cells. - Transfection protocol: Cells incubated with complexes for 4 h, then medium replaced with 10% FBS medium, and assayed 48 h later. - Serum test: Complexes added in medium containing 10% FBS to assess serum sensitivity. - Controls: Plain lipoplexes; succinylated human serum albumin; VM-Acp copolymer; untreated cells. - No in vivo experiments are reported.

Key methods: Luciferase reporter assay for transfection activity. - β-galactosidase staining to determine percentage of transfected cells. - Alamar Blue assay for cell viability/cytotoxicity. - Zeta potential measurements using a Coulter DELSA 440. - Bafilomycin A1 treatment to test dependence on endosomal acidification.

Key results: Transferrin enhanced transfection at all lipid/DNA charge ratios. For the 1/2 charge ratio in HeLa cells, enhancement was >750-fold, increasing expression from essentially null to relatively high levels. - With transferrin, the highest HeLa transfection was obtained at the 1/1 (theoretically neutral) charge ratio; higher positive ratios (2/1, 4/1) decreased expression. - Zeta potential decreased with transferrin: the 1/1 DOTAP:DOPE/DNA complex changed from about +2 mV to −39 mV in the presence of transferrin. - GALA peptide (0.6 µg) with 2/1 complexes gave a 4-fold increase in luciferase expression in HeLa cells. HA-2 peptide also enhanced transfection but less than GALA. - Bafilomycin A1 reduced GALA-mediated enhancement to about 50% of untreated controls, supporting a role for endosomal acidification. - β-gal transfection efficiency in HeLa cells at 1/1 charge ratio: control 2%, +transferrin 25%, +GALA 5%. - Serum (10% FBS) did not appreciably change transfection levels for plain, transferrin-associated, or GALA-associated complexes. - No significant cytotoxicity was observed with DOTAP:DOPE/DNA complexes, with or without transferrin or fusogenic peptides.

Interpretation: The authors conclude that negatively charged ternary complexes of cationic liposomes, DNA, and transferrin or fusogenic peptides can efficiently mediate transfection, remain active in serum, and are nontoxic. They suggest these complexes may overcome limitations of highly positively charged lipoplexes for in vivo gene delivery and could be potential alternatives to viral vectors.

Limitations: No in vivo validation; all experiments are in HeLa and COS-7 cells. - Transfection efficiency was still modest overall: the highest β-gal-positive fraction reported was 25% with transferrin, with lower values for GALA and controls. - The precise mechanisms of enhancement by transferrin and peptides were not fully resolved. - Optimal charge ratio and peptide effects differed between HeLa and COS-7 cells, indicating cell-type dependence. - The study used reporter genes only; no therapeutic gene efficacy or disease model was tested.