Human cytomegalovirus IE1 promoter/enhancer drives variable gene expression in all fiber types in transgenic mouse skeletal muscle

摘要

Background Versatile transgenic manipulation of skeletal muscle requires knowledge of the expression profiles of diverse promoter/enhancer elements in the transcriptionally specialized fiber types of which muscle is composed. "Universal" viral promoters/enhancers, e.g., cytomegalovirus IE1 (CMV IE1), are of interest as reagents that may drive broad expression. However, a previous study noted a marked heterogeneity of CMV IE1-driven transgene expression among muscle fibers, raising the possibility of fiber-type-restricted expression. The purpose of the present study was to characterize CMV IE1-driven expression in terms of fiber type. Results We produced two lines of transgenic mice carrying the CMV IE1/ β-galactosidase construct CMVLacZ, and analyzed transgene expression and fiber type by histochemical analysis of hindlimb muscle sections. In both lines CMVLacZ was expressed in all four major fiber types: type I (slow) and types IIA, IIB and IIX (fast). There was no unique pattern of fiber-type-preferential expression; fiber-type quantitative differences were observed but details varied between muscle regions and between lines. Both lines showed similar fiber-type-independent regional differences in overall expression levels, and a high level of within-fiber-type variability of expression, even among nearby fibers. The soleus muscle showed strong expression and comparatively little within-fiber-type or between-fiber-type variability. Conclusions The CMV IE1 promoter/enhancer is not fiber-type-restricted and can be useful for driving germ-line transgene expression in all four fiber types. However, not all fibers express the gene at high levels due in part to regional differences in overall expression levels, and to a high level of within-fiber-type variability. Given the multinucleate syncitial nature of muscle fibers, it is not likely that this variability is due to variegating heterochromatinization. The soleus muscle would make a suitable subject for near-uniform experimental gene expression driven by CMV IE1 elements.