Inefficient nuclear delivery of plasmid DNA is thought to activity of apoptotic and lysosomal nucleases; (2) disposal be one of the daunting hurdles to gene transfer, utilizing a of microinjected plasmid DNA was inhibited in cytosolnonviral delivery system such as polycation-DNA complex.depleted cells or following the encapsulation of DNA in Following its internalization by endocytosis, plasmid DNA phospholipid vesicles; (3) generation and subsequent elimhas to be released into the cytosol before its nuclear entry ination of free 3Ј-OH ends could be detected by the tercan occur. However, the stability of plasmid DNA in the minal deoxynucleotidyl transferase-mediated dUTP nick cytoplasm, that may play a determinant role in the transfecend-labeling assay (TUNEL), reflecting the fragmentation tion efficiency, is not known. The turnover of plasmid DNA, of the injected DNA; and finally (4) isolated cytosol, delivered by microinjection into the cytosol, was deterobtained by selective permeabilization of the plasma memmined by fluorescence in situ hybridization (FISH) and brane, exhibits divalent cation-dependent, thermolabile quantitative single-cell fluorescence video-image analysis. nuclease activity, determined by Southern blotting and 32 PBoth single-and double-stranded circular plasmid DNA disrelease from end-labeled DNA. Collectively, these findings appeared with an apparent half-life of 50-90 min from the suggest that the metabolic instability of plasmid DNA, cytoplasm of HeLa and COS cells, while the amount of cocaused by cytosolic nuclease, may constitute a previously injected dextran (MW 70 000) remained unaltered. We prounrecognized impediment for DNA translocation into the pose that cytosolic nuclease(s) are responsible for the nucleus and a possible target to enhance the efficiency of rapid degradation of plasmid DNA, since (1) elimination of gene delivery. plasmid DNA cannot be attributed to cell division or to the Keywords: gene transfer; plasmid DNA; turnover; degradation; DNase; microinjection Introduction Liposome-mediated cellular transfer of plasmid DNA is a promising approach for gene therapy. However, despite the significant amount of lipid/DNA complexes internalized by the target cells, transgene expression remains undesirably low.1 Obstacles to nuclear accumulation of plasmid DNA include: the slow internalization process of the lipid/DNA complex in certain cells; 2 the entrapment of DNA in the endolysosomal compartment; 1,3,4 and the diffusional barrier of the nuclear envelope. 5The underlying mechanism of escape of internalized plasmid DNA from the endo-lysosomes is not fully understood. This process involves the destabilization of the limiting membrane of the endolysosomal compartment, the dissociation of the lipid/DNA complex and the release of plasmid DNA into the cytosol.6-8 Penetration of naked plasmid DNA into the cytosol was verified by using the T7 polymerase transfection system, which Correspondence: GL Lukacs, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canad...

Cationic lipid-based delivery systems such as lipoplexes or aphidicolin exhibited 20-fold lower reporter gene activity stabilized plasmid-lipid particles (SPLP) represent a safer than asynchronous control cells upon incubation with lipoalternative to viral systems for gene therapy applications.plexes. When cells arrested in the G1 phase were allowed We studied the impact of cell cycle status on the efficiency to proceed though the cell cycle in the presence of the lipoof transfection of human ovarian carcinoma tumor cells plex or SPLP, transgene expression was found to coincide using two cationic-lipid based delivery systems. Cells with the transition of cells from the G2/M phase into the arrested in the G1 phase of the cell cycle by treatment with G1 phase of the subsequent cell cycle. In addition, higher aphidicolin were compared with an asynchronous dividing levels of reporter gene expression were observed when the population of cells. Treatment of the cells with aphidicolin cells were incubated with lipoplexes or SPLP during, or just had no effect on the rate of internalization of the lipid forbefore, mitosis. These results suggest that it may be possmulated DNA or on the level of gene expression observible to augment cationic lipid-mediated transfection by able in stably transfected cells. However, cells treated with manipulating the cell cycle status of the target cells.

The inhibitory effect of serum is one of the main obstacles less in primary human myoblasts. The lower transgene to the in vivo use of cationic liposomes as a DNA delivery expression in primary cells does not appear to be a result system. We have found that a novel liposome formulation, of less DNA uptake but might result from differences in DODAC:DOPE (1:1) is totally resistant to the inhibitory intracellular trafficking of the complexes. DODAC-based effects of serum for transfection of cultured myoblasts and liposomes are unique in their resistance to serum inhibition myotubes. Transfection with a lacZ reporter gene in the and may therefore be valuable for the systemic delivery of presence of 95% fetal bovine serum gave up to 25% ␤-genetic information to muscle and other tissues. gal-positive cells in C2C12 myoblasts and about six-fold