Skeletal muscle gene transfer: regeneration-associated deregulation of fast troponin I fiber type specificity

Direct gene transfer into skeletal muscle in vivo presents a convenient exp erimental approach for studies of adult muscle gene regulatory mechanisms, including fast vs. slow fiber type specificity. Previous studies have repor ted preferential expression of fast myosin heavy chain and slow myosin ligh t chain and troponin I (TnIslow) gene constructs in muscles enriched in the appropriate fiber type. We now report a troponin I fast (TnIfast) direct g ene transfer study. We injected into the mouse soleus muscle plasmid DNA or recombinant adenovirus carrying a TnIfast/beta-galactosidase (beta-gal) re porter construct that had previously been shown to be expressed specificall y in fast fibers in transgenic mice. Surprisingly, microscopic histochemica l analysis 1 and 4 wk postinjection showed similar TnIfast/beta-gal express ion in fast and slow fibers. A low but significant level of muscle fiber se gmental regeneration was evident in muscles 1 wk postinjection, and TnIfast /beta-gal expression was preferentially targeted to regenerating fiber segm ents. This finding can explain why TnIfast constructs are deregulated with regard to fiber type specificity, whereas the myosin constructs previously studied are not. The involvement of regenerating fiber segments in transduc tion by plasmid DNA and recombinant adenoviruses injected into intact norma l adult muscle is an unanticipated factor that should be taken into account in the planning and interpretation of direct gene transfer experiments.