Transfecting Mammalian Cells: Optimization of Critical Parameters Affecting Calcium-Phosphate Precipitate Formation

TLDR

DNA‑calcium phosphate co‑precipitates form spontaneously in supersaturated solutions, but effective transfection‑grade precipitates require a narrow range of physico‑chemical conditions, primarily governed by calcium and phosphate concentrations and also influenced by temperature, DNA concentration, and reaction time. Rapid (<1 min) precipitation captures nearly all soluble DNA, but extending the reaction to 20 min reduces expression; optimized protocols improved reproducibility and increased transient transfection efficiency up to 60 % β‑gal positivity and 5–10‑fold protein production, with comparable gains in stable transfections and a strong correlation between early transient expression and later stable line titers.

Abstract

DNA-calcium phosphate co-precipitates arise spontaneously in supersaturated solutions. Highly effective precipitates for transfection purposes, however, can be generated only in a very narrow range of physico-chemical conditions that control the initiation and growth of precipitate complexes. The concentrations of calcium and phosphate are the main factors influencing characteristics of the precipitate complex, but other parameters, such as temperature, DNA concentration and reaction time are important as well. An example for this is the finding that almost all of the soluble DNA in the reaction mix can be bound into an insoluble complex with calcium phosphate in <1 min. Extending the reaction time to 20 min results in aggregation and/or growth of particles and reduces the level of expression. With improved protocols we gained better reproducibility and higher efficiencies both for transient and for stable transfections. Up to 60% of cells stained positive for β-gal and transient production of secreted proteins was improved 5- to 10-fold over results seen with transfections using standard procedures. Similar improvements in efficiency (number of recombinant cell colonies) were observed with stable transfections, using co-transfected marker plasmids for selection. Transient expression levels 2 days after DNA transfer and titers obtained from stable cell lines, emerging weeks later, showed strong correlation.