NAKED PLASMIDS: MUSCLING INTO GENE TRANSFER

The development of DNA vaccines and plasmid vectors for gene therapy in recent years is based on the capacity of mammalian cells and tissues to take up exogenous DNA spontaneously. Naked DNA uptake is a universal property of mammalian cells, having been demonstrated in several different mammalian cell lines, primary cells in culture, solid tumours as well as in normal tissues (e.g. skeletal muscle and liver) in vivo. While mechanisms of uptake of exogenous DNA by bacteria are understood in some detail, almost nothing is known about this process in mammalian cells. It is unlikely that the extremely poor efficiency of naked plasmid entry into mammalian cells and tissues can be overcome until eukaryotic mechanisms are elucidated. We review the current state of knowledge of naked plasmid entry into mammalian cells, and present preliminary evidence for saturable plasmid binding sites on the plasma membrane of C2C12 myoblasts followed by time-dependent translocation of internalized plasmid into the nuclear compartment. Myoblasts can be engineered for insulin secretion via plasmid-mediated gene transfer. Thus, muscle and myoblast implants may be a suitable platform for developing genetic treatment of systemic metabolic disorders.