Cationic liposome-mediated RNA transfection

Purpose: RNA transfection was inefficient and restricted because intact RNA was difficult to obtain and rapidly degraded. In vitro transcription could now supply large amounts of RNA, creating a need for a reliable method to introduce RNA into cells and study translational efficiency and mRNA stability.

Hypothesis: Cationic liposome-mediated delivery using DOTMA/lipofectin can efficiently transfect in vitro-transcribed mRNA into cultured cells, and sequence elements such as the 5′ cap and β-globin 5′/3′ untranslated regions will enhance translation of the delivered mRNA.

Aims: Develop an efficient and reproducible RNA transfection method using DOTMA-containing liposomes (lipofectin). - Optimize the RNA-to-lipofectin ratio and characterize transfection kinetics and efficiency. - Test the method across a wide variety of cell types and compare it with DEAE-dextran. - Use the method to analyze the role of capping and β-globin 5′ and 3′ untranslated sequences on translation efficiency.

Delivery system: Platform: Cationic liposome — lipofectin, composed of DOTMA and DOPE at 1:1 mol/mol. - Payload: In vitro T7-transcribed mRNAs, primarily Photinus pyralis luciferase mRNA; capped or uncapped; with or without β-globin 5′ and 3′ UTRs and poly(A) or A₂₃C₃₀ tracts. - Formulation: RNA mixed with lipofectin in Opti-MEM. Optimized ratio: 2.5 µg lipofectin per 1 µg RNA; standard condition used 50 µg lipofectin with 20 µg total RNA. - Targeting ligand: None. - Carrier RNA: Uncapped carrier RNA used to bring total RNA to 20 µg in most transfections.

Approach: Model system: In vitro cultured cells only; no in vivo disease model. - Primary cell line: NIH 3T3 mouse fibroblasts. - Additional cell types: Human, rat, mouse, Xenopus, and Drosophila cells, including fibroblasts, hematopoietic lines, F9 teratocarcinoma, JEG choriocarcinoma, PC12 pheochromocytoma, and suspension cells. - Dosing: 10 ng to 5 µg specific mRNA, with carrier RNA to 20 µg total; 50 µg lipofectin; 4 mL Opti-MEM; standard 8 h incubation. - Controls: Mock transfection, DEAE-dextran transfection, and in vitro rabbit reticulocyte translation. - Replication: Key transfection curves performed in duplicate; luciferase assays corrected for background.

Key methods: Luciferase activity assay for protein expression. - ³²P-labeled RNA cell-association and RNase-resistance assays. - Agarose gel electrophoresis for transcript integrity. - In vitro rabbit reticulocyte translation. - Linear regression of dose–response data.

Key results: Transfection of 10 ng to 5 µg luciferase mRNA gave a linear luciferase response. - Within 1 h, at least 60% of labeled RNA was tightly cell-associated; 20–30% was RNase-resistant. - RNA/lipofectin was at least 100- to 1000-fold more efficient than DEAE-dextran; in one comparison, 122 pg versus 0.01 pg luciferase. - Luciferase synthesis continued for at least 5 h after transfection, with an initial lag of about 30 min. - Capped mRNA was nearly 40-fold more efficiently translated than uncapped mRNA. - β-globin 5′ UTR imparted nearly 9-fold greater translational efficiency. - β-globin 3′ UTR plus A₂₃C₃₀ conferred at least 6-fold advantage over a 30-residue poly(A) tail alone. - Maximal protein synthesis occurred with cap plus both β-globin 5′ and 3′ UTRs. - In reticulocyte lysates, all transcripts translated efficiently within 2–3-fold regardless of cap/UTRs, and at least 1000-fold more luciferase was produced than with RNA/lipofectin. - Efficient transfection was observed across many cell types.

Interpretation: The authors conclude that DOTMA-containing liposomes provide a simple, reliable, high-efficiency RNA transfection method, at least 100- to 1000-fold better than DEAE-dextran. The method enables direct study of mRNA translation and stability, works across diverse cell types, and may extend to antisense RNA delivery, tissues, and embryos, supporting the concept of RNA as a drug.

Limitations: Lipofectin toxicity from positively charged lipids; optimal RNA-to-lipofectin ratio must be determined for each cell type. - Only transient RNA expression is achieved; no genome integration. - No in vivo disease model was evaluated in this paper. - Efficiency remains much lower than in vitro reticulocyte translation. - Some broader tissue/embryo applications are mentioned only as unpublished results.