Purpose: Receptor-mediated gene transfer using ligand–poly-L-lysine–DNA complexes requires DNA to be condensed into small particles for efficient cellular uptake and expression. However, the relationship between NaCl concentration, the mode of poly-L-lysine binding to DNA, the resulting condensed DNA structure, and transfection efficacy was not well defined.
Hypothesis: If poly-L-lysine binds DNA cooperatively at specific NaCl concentrations, then the resulting condensed DNA particles will have a spherical shape of 15–30 nm, resist DNase I digestion, and mediate efficient receptor-mediated gene transfer. Noncooperative binding will produce aggregated or rodlike structures with poor transfection.
Aims: Determine how NaCl concentration and poly-L-lysine length affect the mode of binding to DNA and DNA precipitation. - Characterize the structural features of condensed DNA complexes using DNase I resistance and electron microscopy. - Correlate condensed DNA structure with receptor-mediated gene transfer into HuH-7 human hepatoma cells via the asialoglycoprotein receptor.
Delivery system:
Component: Cationic polymer; Details: Poly-L-lysine (PLL) and galactosylated poly-L-lysine (GalPLL; ~1% galactosylated)
Component: Payload; Details: Plasmid DNA: pCMV-Luc, pPEPCK-hFIX, pPEPCK-hOTC
Component: Complex formation; Details: Dropwise titration at fixed NaCl concentration; input ratio r = moles lysine/nucleotide (NH₄⁺/PO₃⁻)
Component: Cooperative binding condition; Details: High NaCl (e.g., ~1.0 M for GalPLL₂₅₆) with excess DNA; 1:1 charge precipitation
Component: Targeting ligand; Details: Galactose residues for asialoglycoprotein receptor on hepatoma cells
Component: Key feature; Details: Condensed DNA particles; spherical 15–30 nm structures required for efficient receptor-mediated transfer
Approach: In vitro only. Human hepatoma cell line HuH-7. - Transfection: GalPLL₂₅₆–DNA complexes prepared at 1 M NaCl, diluted to 0.15 M NaCl, added to cells at ~30% confluence; 1.5 h exposure; luciferase assay 2 days later. - Competition control: Asialofetuin (ASF, 5 µg/mL) to compete for asialoglycoprotein receptor binding. - Groups: r = 0, 0.25, 0.5, 0.75; 4 µg DNA per plate; n ≥ 3; no in vivo studies. - Controls: Free DNA, blank medium, ASF alone.
Key methods: DNA precipitation assays: GalPLL lengths 256, 92, 57, 26 amino acids at varying NaCl; absorbance at 260 nm. - DNase I digestion assay: 800 units DNase I, trypsin release, agarose gel electrophoresis. - Electron microscopy (EM): Uranyl acetate staining; SigmaScan analysis of major/minor diameters, area, perimeter. - Reporter gene expression: Luciferase activity in HuH-7 cells; ASF inhibition as measure of receptor specificity.
Key results: Cooperative binding: 1:1 molar equivalent precipitation occurred near 1.0 M NaCl for GalPLL₂₅₆, 0.7 M for GalPLL₉₂, 0.6 M for GalPLL₅₇, and 0.3 M for GalPLL₂₆. - Structural outcomes: At 1 M NaCl with excess DNA, cooperative binding produced two populations: fully condensed DNA and free DNA. EM showed spherical particles 15–30 nm (Experiments 1, 7, 9) or rods ~100 nm (other experiments). At 0 M NaCl, noncooperative binding produced large aggregates, rods, and toroids. - DNase I resistance: Protection correlated with spherical condensed DNA; full protection only at higher r values. - Transfection: Efficient receptor-mediated gene transfer occurred only when condensed DNA was spherical, 15–30 nm. ASF inhibited luciferase activity by up to ~90% (r = 0.5) and ~70% (r = 0.75). Luciferase activity at r = 0.5 and 0.75 was ~400-fold and ~1300-fold above background, respectively. Rodlike ~100 nm particles did not express via receptor-mediated transfer.
Interpretation: The authors claim that appropriate DNA condensation via cooperative binding of poly-L-lysine is a necessary prerequisite for forming spherical 15–30 nm particles, and that this size/shape is required for efficient receptor-mediated uptake and gene expression in hepatic cells. These results support the importance of controlled DNA condensation for non-viral gene delivery.
Limitations: In vitro only: No in vivo validation. - Single cell line: HuH-7 hepatoma cells only. - Variability: Nine independent condensation experiments produced either spherical or rodlike particles; factors beyond NaCl and r (e.g., polymer length distribution, trace counterions) may affect structure. - Limited polymer characterization: Most structural and transfection studies used GalPLL₂₅₆; other lengths mainly assessed by precipitation. - Short-term expression: Luciferase measured at 2 days. - No cytotoxicity data. - Nuclear translocation data mentioned but not fully shown; mechanism of nuclear entry not resolved.
