Spatial-temporal patterns of gene expression in mouse skeletal muscle after injection of lacZ plasmid DNA

Gene therapy for muscular diseases requires the efficient transfection of a large proportion of myofiber cells within a given muscle. In the present experiments, patterns of beta-galactosidase expression were examined in mouse rectus femoris muscles at various time-points after a single injection of lacZ encoded plasmid DNA. beta-Galactosidase expression was detected 3 h after injection and rose to peak levels at 3-14 days, and then stabilized at lower levels. beta-Galactosidase staining was detected in an average of about 6% (up to 15%) of the total 4000 myofiber cells, and in about 70% of those myofibers located in the discrete area containing the greatest proportion of transfected cells. Soon after injection of DNA encoding cytoplasmic or nuclear-targeted beta-galactosidase, expression was noted predominantly in the myotendinous junction areas, after which beta-galactosidase activity progressed toward the central parts of the myofibers. This preferential transgene expression at the myotendinous junction may result from some unique, local property of the myofiber cells and/or from a restricted diffusion or binding of the injected plasmid DNA at tendinous surfaces. A better understanding of the reasons for this pattern of reporter gene expression in muscle may suggest procedures for increasing the number of myofiber cells transfected by direct DNA injections.