Function
Liposome transfection (i.e. lipofection) is a technique that results in the transfer of genetic material into a cell by means of liposomes. Liposomes are vesicles that can easily merge with a cell membrane, as both are made of a phospholipid bilayer. Lipofection generally consists of a positively charged (cationic) lipid that forms an aggregate complex with negatively charged (anionic) genetic material such as DNA or RNA. The positively charged lipid molecules form strong associations with the negatively charged nucleic acids. A net positive charge of the formed aggregate increases the effectiveness of transfection through the negatively charged phospholipid bilayer. This technology is similar to other biochemical procedures that utilize polymers, DEAE dextran, calcium phosphate, and electroporation.
Lipid based reagents use cell membrane semi-permeability to deliver foreign nucleic acid molecules into a cell. Typically, the liposome is endocytosed by the cell and the nucleic acid cargo is released into the cytoplasm. This molecular biology technique of transferring foreign DNA or RNA in a host cell is accomplished by using different transfection methods. Common transfection approaches use viral vectors, a chemical carrier or direct methods.
Structure and Relation to Cell Membrane
A cell membrane consists of a phospholipid bilayer with the hydrophobic ends of lipid molecules facing each other and their corresponding hydrophilic ends in opposite directions. These water-soluble and lipid-soluble surfaces make the cell membrane semipermeable, thus restricting certain molecules from entering the cell. Small molecules, like water, oxygen, and carbon dioxide are able to directly pass through the membrane’s phospholipid bilayer. However, large and charged molecules need a specific transport protein to help facilitate them across the membrane. Liposomes, on the other hand, are absorbed by the cell as they are indistinguishable from the cell membrane. This makes liposome encapsulation (coating nucleic acids with liposomes) a feasible way of delivering DNA and RNA into cells. Liposomes are especially helpful in transporting difficult-to-deliver molecules, for instance, siRNAs.
Cationic lipids are either “off-the-shelf” products or can be custom-designed in order to deliver DNA and RNA into the target cell. Specially-designed lipids usually involve choosing positively-charged head groups with a proper hydrocarbon chain for a given experiment. The optimal head group controls the interaction between the lipid and the nucleic acid backbone, enabling the agglomeration of DNA in the nanoparticle. This also brings together the nucleic acid with the cell membrane to fuse the liposome to the membrane during endocytosis. This allows the liposomal complex to bypass the endosomal pathway, which would lead to degradation of the passenger DNA molecule.
The primary advantage of lipid based transfection reagents is their wide spectrum efficacy. Liposomal delivery can be used for transfecting multiple types of nucleic acids across diverse cell types, both in vitro and in vivo. They can be easily used for different types of transfection applications, including sequential, transient and reverse transfection. Liposomes generate a low toxicity response and are used in live subjects as a delivery vehicle.
Considerations
Cationic lipids inhibit entry of foreign DNA into the nucleus, and therefore the rate of stable transfection remains relatively low. Advanced techniques are often required for successful generation of stable cell lines.
There are some disadvantages to lipofection. Liposome solutions are fairly expensive; commercial lipofection systems average around $4.50 per transfection. However, certain transfection services offer competitive pricing for lipofection and other procedures. The advantages of lipofection are high efficiency, ability to transfect multiple types of nucleic acids into a wide range of cell types, reproducibility, low toxicity, and its ease of use. In addition, this method enables all transfection applications (stable, transient, co-transfection, reverse transfection, sequential or multiple transfections).