En rAAV6:hPLAP is directly transducing, and activating resident inflammatory cells in skeletal muscle. To test this hypothesis, we administered 109 genomes of rAAV vectors carrying the hPLAP expression cassette after substituting the CMV promoter with a muscle-specific CK6 promoter, which does not Title Loaded From File express in tissues other than skeletal muscle [20] (Fig. 3a), and compared the effects of this vector to those observed following administration of rAAV6:CMV-hPLAP (Fig. 3b). Whilst the deleterious effects of rAAV6:CMV-hPLAP upon TA muscle morphology were recapitulated 14 days after vector administration, the injection of rAAV6:CK6-hPLAP did not appear to affect TA skeletal muscle architecture at the same time point. However, by 28 days, inflammation and tissue destruction was evident in TA muscles that had been injected with rAAV6:CK6-hPLAP (Fig. 3b). When we examined macrophage and inflammatory marker gene expression, we found that injection of rAAV6:CMV-hPLAP vectors had marked effects on the induction of EMR, IL-6 and IL1b expression at 14 days, whilst injection of rAAV6:CK6-hPLAP did not. However, by 28 days post treatment, when the proinflammatory signature had Title Loaded From File diminished in muscles administered rAAV6:CMV-hPLAP vectors, a definite, albeit reduced increase in these markers was observed in muscles administered rAAV6:CK6-hPLAP vectors. The phosphorylation of inflammatory mediators IKKb, JNK and Stat3 was also increased in muscles examined 28 days, but not 14 days, after administration of rAAV6:CK6-hPLAP vectors (Fig. 3d). We also confirmed that the cellular disruption observed after administration of rAAV6:CK6hPLAP also coincided with increased expression of the regenerative markers MyoD and micro-RNA-206 (Fig. 3e). Changes in MyoD and miR-206 expression were comparable between muscles treated with rAAV6:CK6-hPLAP and rAAV6CMV:hPLAP. These data demonstrate that although expression of hPLAP under the control of the CK6 promoter/enhancer is restricted to skeletal muscle, the level of transgene expression afforded in muscle can also result in inflammation and damage to muscle fibers.DiscussionWhen using recombinant AAV vectors to manipulate gene expression in skeletal musculature, parallel cohorts are often treated with vectors carrying reporter genes as experimental controls. While reporter genes may be regarded as “nonfunctional” compared with experimental constructs of interest, it is important to consider the effects of the reporter gene when contemplating experimental design, and the relative interpretation of experimental interventions. In this study, we have shown that genes commonly delivered in reporter constructs can promote dose-dependent inflammation and breakdown of murine skeletal musculature. The findings demonstrate that the choice of reporter gene and degree of expression are important considerations when designing studies to examine the impact of a vector-based intervention upon cellular processes implicated in muscle adaptation, and the morphological attributes of experimentally manipulated muscles. Intramuscular inflammation and degeneration of transduced musculature may be caused by priming the immune system to eliminate an introduced antigen, such as the capsid proteins comprising a viral vector particle [27]. Prior exposure of humans and other mammals to wildtype adeno-associated viruses or rAAV vectors can sensitize a host’s immune system to reaction against subsequently administered vectors [28,29]. However we and ot.En rAAV6:hPLAP is directly transducing, and activating resident inflammatory cells in skeletal muscle. To test this hypothesis, we administered 109 genomes of rAAV vectors carrying the hPLAP expression cassette after substituting the CMV promoter with a muscle-specific CK6 promoter, which does not express in tissues other than skeletal muscle [20] (Fig. 3a), and compared the effects of this vector to those observed following administration of rAAV6:CMV-hPLAP (Fig. 3b). Whilst the deleterious effects of rAAV6:CMV-hPLAP upon TA muscle morphology were recapitulated 14 days after vector administration, the injection of rAAV6:CK6-hPLAP did not appear to affect TA skeletal muscle architecture at the same time point. However, by 28 days, inflammation and tissue destruction was evident in TA muscles that had been injected with rAAV6:CK6-hPLAP (Fig. 3b). When we examined macrophage and inflammatory marker gene expression, we found that injection of rAAV6:CMV-hPLAP vectors had marked effects on the induction of EMR, IL-6 and IL1b expression at 14 days, whilst injection of rAAV6:CK6-hPLAP did not. However, by 28 days post treatment, when the proinflammatory signature had diminished in muscles administered rAAV6:CMV-hPLAP vectors, a definite, albeit reduced increase in these markers was observed in muscles administered rAAV6:CK6-hPLAP vectors. The phosphorylation of inflammatory mediators IKKb, JNK and Stat3 was also increased in muscles examined 28 days, but not 14 days, after administration of rAAV6:CK6-hPLAP vectors (Fig. 3d). We also confirmed that the cellular disruption observed after administration of rAAV6:CK6hPLAP also coincided with increased expression of the regenerative markers MyoD and micro-RNA-206 (Fig. 3e). Changes in MyoD and miR-206 expression were comparable between muscles treated with rAAV6:CK6-hPLAP and rAAV6CMV:hPLAP. These data demonstrate that although expression of hPLAP under the control of the CK6 promoter/enhancer is restricted to skeletal muscle, the level of transgene expression afforded in muscle can also result in inflammation and damage to muscle fibers.DiscussionWhen using recombinant AAV vectors to manipulate gene expression in skeletal musculature, parallel cohorts are often treated with vectors carrying reporter genes as experimental controls. While reporter genes may be regarded as “nonfunctional” compared with experimental constructs of interest, it is important to consider the effects of the reporter gene when contemplating experimental design, and the relative interpretation of experimental interventions. In this study, we have shown that genes commonly delivered in reporter constructs can promote dose-dependent inflammation and breakdown of murine skeletal musculature. The findings demonstrate that the choice of reporter gene and degree of expression are important considerations when designing studies to examine the impact of a vector-based intervention upon cellular processes implicated in muscle adaptation, and the morphological attributes of experimentally manipulated muscles. Intramuscular inflammation and degeneration of transduced musculature may be caused by priming the immune system to eliminate an introduced antigen, such as the capsid proteins comprising a viral vector particle [27]. Prior exposure of humans and other mammals to wildtype adeno-associated viruses or rAAV vectors can sensitize a host’s immune system to reaction against subsequently administered vectors [28,29]. However we and ot.
