Overcoming the Specific Toxicity of Large Plasmids Electrotransfer in Primary Cells In Vitro

Abstract

Gene electrotransfer is a safe and efficient nonviral technique for the transfer of nucleic acids of all sizes. Using a small reporter plasmid (3.5 kbp), electrotransfer of more than 90% of the cells, with ~70% viability, can be routinely achieved even in primary cells like mesenchymal stem cells. However, under the same experimental conditions, electrotransfer of larger plasmids (from 6 to 16 kbp) results in very low viability and transfection efficacy. Here, we show that these strong decreases are directly linked to the physical size of the plasmid molecule. Moreover, large plasmids are toxic only when the cells are exposed to electrotransfer pulses. This specific toxicity of large plasmids during electrotransfer is not due to transgene expression and occurs within less than 45 minutes. Indeed, postpulses recovery times of up to 45 minutes are able to entirely abolish the specific toxicity of large plasmid electrotransfer, resulting in a survival and transfection efficacy identical to that of small plasmids. Finally, electrotransfer of small and large plasmids can reach 90-99% of transfection with 60-90% survival considering the findings here reported.

Figure 1

Effect of plasmid size on electrotransfer toxicity and efficacy. Adipose tissue-derived mesenchymal stem cells (AT-MSCs) were analyzed by flow cytometry 24 hours after treatment. Equimass experiment (a–c): AT-MSCs were electrotransferred (eight pulses, 1,500 V/cm, 100 µs, 1 Hz) with 50 µg of plasmids of various sizes. Equimolar experiment (d–f): AT-MSCs were electrotransferred with ~6.7 pmol of plasmids of various sizes (equivalent to 50 µg of the 11.4 kbp plasmid pCAGMKOSiE). In panels b and e, the open circles represent the percentage of treated cells expressing green fluorescent protein (GFP) and the filled triangles represent the percentage of surviving cells expressing GFP. In panels c and f, the dotted line represent the background fluorescence intensity of the controls. Data are representative of 4–10 independent experiments. The survival, percentage of transfection, and expression per cell are inversely correlated to the plasmid size in both equimolar and equimass experiments (P < 0.001, Kendall's rank correlation).

Figure 2

Specific toxicity of each component of the electrotransfer protocol for a small and a large plasmid. Adipose tissue-derived mesenchymal stem cells were exposed to each component of the electrotransfer alone or in combination: pulsing buffer (S-MEM), improved pulsing buffer (S-MEM with 50% H₂O), small (pCMV-GFP, 3.5 kbp, 50 µg) or large plasmid (pCAGMKOSiE, 11.4 kbp, 50 µg), eight electric pulses (1,500 V/cm, 100 µs, 1 Hz), complete EGT (S-MEM + 50% H₂O + plasmid + pulses). For each condition, cells were counted with the flow cytometer 24 hours after treatment and survival expressed as the percentage of cells counted in the control (S-MEM). Data are representative of three to five independent experiments. (*P < 0.05 and ****P < 0.0001, one-way analysis of variance with Holm-Šídák multiple comparison test). CMV, cytomegalovirus; EGT, electrogene transfer; GFP, green fluorescent protein; ns, nonsignificant; S-MEM, minimum essential medium modified for suspension cultures.

Figure 3

Effect of the quantity of large plasmid on electrotransfer toxicity and efficacy. Adipose tissue-derived mesenchymal stem cells were electrotransferred (eight pulses, 1,500 V/cm, 100 µs, 1 Hz) with various quantities of the large plasmid pCAGMKOSiE (11.4 kbp) and analyzed by flow cytometry 24 hours after treatment. In panel (a) the percentage of dead cells was assessed by comparing the number of cells recovered in the treated groups versus the number of cells recovered in the control group. In panel (b), the open circles and dotted lines represent the percentage of treated cells expressing green fluorescent protein (GFP) and the filled triangles and solid lines represent the percentage of surviving cells expressing GFP. Panel (c) represent the median fluorescence intensity per cell. In panel (d), the 95th percentile of fluorescence is represented instead of the maximum fluorescence to limit the effects of possible outliers. Data are representative of six independent experiments. The toxicity, percentage of surviving transfected cells, and expression per cell are all positively correlated to the plasmid quantity (P < 0.001, Kendall's rank correlation).

Figure 4

Toxicity of small and large plasmid 2 and 4 hours after electrotransfer. Adipose tissue-derived mesenchymal stem cells were electrotransferred (eight pulses, 1,500 V/cm, 100 µs, 1 Hz) with 50 µg of either the small pCMV-GFP plasmid (3.5 kbp) (a,b) or the large pCAGMKOSiE plasmid (11.4 kbp) (c,d) and observed 2 hours (a,c) and 4 hours (b,d) after electrotransfer using a phase contrast objective. Within less than 2 hours, the cells were attaching and at 4 hours, they were well spread on the surface. For large plasmid, a significant amount of cells did not attach and some were blebbing. Bar = 400 µm. Pictures are representative of three experiments. CMV, cytomegalovirus; GFP, green fluorescent protein.

Figure 5

Effect of an electrophoretic pulse on large plasmid electrotransfer toxicity and efficacy. Adipose tissue-derived mesenchymal stem cells were electrotransferred (eight pulses, 1,500 V/cm, 100 µs, 1 Hz) with 50 µg of the large plasmid pCAGMKOSiE (11.4 kbp). A single electrophoretic pulse (LV) of 200 V/cm and a duration of 1, 5, 10, or 100 ms was applied 1–1,000 ms (Δ) after the EGT. Panel (a) represent the number of cells recovered in the treated groups versus the number of cells recovered in the control group. In panel (b), the grey columns represent the percentage of surviving cells expressing GFP and the white columns represent the percentage of treated cells expressing GFP. Data are representative of three independent experiments. The electrophoretic pulses improved neither the survival nor the transfection efficiency (one-way analysis of variance with Dunnett's multiple comparison test). EGT, electrogene transfer; GFP, green fluorescent protein; LV, low voltage.

Figure 6

Effect of a resting time on electrotransfer efficacy and toxicity in presence of small or large plasmids. Adipose tissue-derived mesenchymal stem cells were electrotransferred (eight pulses, 1,500 V/cm, 100 µs, 1 Hz) with 50 µg of either pCAGMKOSiE (11.4 kbp, 6.7 pmol) (filed squares and solid lines), pCXLE-eGFP (10.9 kbp, 7 pmol) (crosses and dashed line), or pCMV-GFP (3.5 kbp, 22 pmol) (open circles and dotted lines). After electrotransfer, cells were allowed to recover for 45 minutes (b,e) or for different amounts of time (a,c,d) before being put back in culture. Survival and transfection efficiency were assessed by flow cytometry 24 hours after treatment (a,c,d) or at several different days (b,e). In panel e, the dotted line represents the background fluorescence intensity of the controls. Data are representative of 3–11 independent experiments. CMV, cytomegalovirus; GFP, green fluorescent protein.