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

Parameters for DNA vaccination using adaptive constant-current electroporation in mouse and pig models

Enhancing the expression of DNA vaccines requires that specific conditions of delivery are optimized. We describe experiments using adaptive constant-current electroporation (EP) in mice and pigs examining parameters such as target muscle, delay between plasmid delivery and onset of EP pulses and DNA vaccine formulation; our studies show that concentrated formulations result in better expression and immunogenicity. Furthermore, various conditions of EP that limit the amount of muscle damage were measured. The results of these studies will help to advance the success of DNA vaccines in animals into success in human clinical trials.

Muscle electrotransfer as a tool for studying muscle fiber-specific and nerve-dependent activity of promoters

Muscle electrotransfer has recently become a promising tool for efficient delivery of plasmids and transgene expression in skeletal muscle. This technology has been mainly applied to use of muscle as a bioreactor for production of therapeutic proteins. However, it remains to be determined whether muscle electrotransfer may also be accurately used as an alternative tool to transgenesis for studying aspects of muscle-specific gene control that must be explored in fully mature muscle fibers in vivo, such as fiber specificity and nerve dependence. It was also not known to what extent the initial electrical stimulations alter muscle physiology and gene expression. Therefore, optimized conditions of skeletal muscle electroporation were first tested for their effects on muscles of transgenic mice harboring a pM310-CAT transgene in which the CAT reporter gene was under control of the fast IIB fiber-specific and nerve-dependent aldolase A pM promoter. Surprisingly, electrostimulation led to a drastic but transient shutdown of pM310-CAT transgene expression concomitant with very transient activation of MyoD and, mostly, with activation of myogenin, suggesting profound alterations in transcriptional status of the electroporated muscle. Return to a normal transcriptional state was observed 7-10 days after electroporation. Therefore, we investigated whether a reporter construct placed under control of pM could exhibit fiber-specific expression 10 days after electrotransfer in either fast tibialis anterior or slow soleus muscle. We show that not only fiber specificity, but also nerve dependence, of a pM-driven construct can be reproduced. However, after electrotransfer, pM displayed a less tight control than previously observed for the same promoter when integrated in a chromatin context.

Studies of myosin isoforms in muscle cells: single cell mechanics and gene transfer

Myosin, the motor protein in skeletal muscle, is composed of two subunits, myosin heavy chain and myosin light chain. All vertebrates express a family of myosin heavy chain and myosin light chain isoforms that together are primary determinants of force, velocity, and power in muscle fibers. Therefore, appropriate expression of myosin isoforms in skeletal muscle is critical to proper motor function. Myosin isoform expression is highly plastic and undergoes significant changes in response to muscular injury, muscle disuse, and disease. Therefore, myosin isoform function and plasticity are highly relevant to clinical orthopaedic research, musculoskeletal surgery, and sports medicine. Muscle from frogs offers a special opportunity to study the structural basis of contractile protein function because single intact fibers can be isolated that maintain excellent mechanical stability, allowing for high-resolution studies of contractile performance in intact cells. The current authors summarize recent studies defining the myosin isoforms in muscle from frogs and the relationship between myosin isoforms and mechanical performance of intact single muscle cells. Preliminary studies also are described that show the potential for simple plasmid-based in vivo gene transfer approaches as a model system to elucidate the structural basis of muscle protein function in intact cells.

Delivery of FGF genes to wound repair cells enhances arteriogenesis and myogenesis in skeletal muscle

Tissue repair is driven by migratory macrophages and fibroblasts that infiltrate injury sites and secrete growth factors. We now report the enhancement of skeletal muscle repair by targeting transgene delivery to these repair cells using matrix-immobilized gene vectors. Plasmid and adenovirus vectors immobilized in collagen-gelatin admixtures were delivered to excisional muscle wounds, and when encoding either fibroblast growth factor-2 (FGF2) or FGF6 transgenes, produced early angiogenic responses that subsequently remodeled into arteriogenesis. FGF2 gene delivery enhanced the number of CD31(+) endothelial cells present at treatment sites > 6-fold by day 14, and muscular arteriole density up to 11-fold by day 21 (P<0.0001). Muscle repair was also enhanced, as FGF gene-treated wounds filled with regenerating myotubes expressing the marker CD56 (an average 20-fold increase in CD56 expression versus controls, P<0.0001). These responses required transfection of a threshold level of repair cells, achievable only in injured muscles, and were transgene-driven, as neither platelet-derived growth factor-B (PDGFB) gene nor FGF2 protein delivery produced equivalent responses. In conclusion, using biomatrices to direct gene delivery to repair cells allows for relatively complex regenerative processes such as arteriogenesis and myogenesis, and therefore represents a promising approach to treating injured and ischemic muscle.

High-efficiency adenovirus-mediated in vivo gene transfer into neonatal and adult rodent skeletal muscle

Several methodological limitations have emerged in the use of viral gene transfer into skeletal muscle. First, because the nuclei of mature muscle fibers do not undergo division, the use of strategies involving replicative integration of exogenous DNA is greatly limited. Another important limitation concerns the maturation-dependent loss in muscle fiber infectivity with adenoviral vectors. In this study, we investigated the possibility that high-titer infections with recombinant adenovirus, expressing a foreign marker gene under the control of a strong viral promoter, can significantly improve the efficiency of gene transfer in vivo into neonatal and adult rat skeletal muscle. High-titer (2 x 10(10) plaque forming units) intramuscular injection of replication-defective adenovirus vector, expressing green fluorescent protein (GFP) under the control of cytomegalovirus promoter, resulted in GFP expression in 99 +/- 0.34% of fibers in the adult soleus muscle and in approximately 85 +/- 1.44% of fibers in the adult tibialis anterior muscle. Interestingly, reduction in injected adenoviral dose significantly reduced the number of GFP-positive fibers in the adult tibialis anterior muscle, but not in the soleus muscle. However, in neonates, adenoviral infection resulted in GFP expression in 96-99% of the fibers in the tibialis anterior and the gastrocnemius muscles regardless of administered adenoviral dose.