IIx myosin heavy chain promoter regulation cannot be characterized in vivo by direct gene transfer

CORP: Gene delivery into murine skeletal muscle using in vivo electroporation

The strategy of gene delivery into skeletal muscles has provided exciting avenues in identifying new potential therapeutics towards muscular disorders and addressing basic research questions in muscle physiology through overexpression and knockdown studies. In vivo electroporation methodology offers a simple, rapidly effective technique for the delivery of plasmid DNA into post-mitotic skeletal muscle fibers and the ability to easily explore the molecular mechanisms of skeletal muscle plasticity. The purpose of this review is to describe how to robustly electroporate plasmid DNA into different hindlimb muscles of rodent models. Further, key parameters (e.g., voltage, hyaluronidase, plasmid concentration) which contribute to the successful introduction of plasmid DNA into skeletal muscle fibers will be discussed. In addition, details on processing tissue for immunohistochemistry and fiber cross-sectional area (CSA) analysis will be outlined. The overall goal of this review is to provide the basic and necessary information needed for successful implementation of in vivo electroporation of plasmid DNA and thus open new avenues of discovery research in skeletal muscle physiology.

Epigenetic control of skeletal muscle fibre type

Adult muscle is extremely plastic. However, the muscle precursor cells associated with those fibres show stable and heritable differences in gene expression indicative of epigenetic imprinting. Epigenetic processes in the development of skeletal muscle have been appreciated for over a decade; however, there are a paucity of studies looking at whether epigenetics determines the phenotype of adult and/or ageing skeletal muscle. This review presents the evidence that epigenetics plays a role in determining adult muscle function and a series of unanswered questions that would greatly increase our understanding of how epigenetics works in adult muscle. With the increased interest in epigenetics, over the next few years this field will begin to unfold in unimaginable directions.

Transcriptional regulation of the myosin heavy chain IIb gene in inactive rat soleus

The myosin heavy chain (MHC) isoform composition of skeletal muscle is dependent, in part, on the functional demands of the muscle. The rat soleus muscle primarily expresses the slow-contracting type I MHC; however, chronic inactivity increases expression of the faster-contracting type II MHC isoforms. The purpose of this study was to identify the type IIb MHC promoter region(s) that regulate de novo transcription during chronic inactivity of the soleus induced by spinal cord isolation (SI; complete mid-thoracic and high sacral spinal cord transections plus deafferentation). Seven days after SI, transcription of IIb MHC was evidenced by increases in IIb pre-mRNA and mRNA. The activity of an approximately 2.2-kb IIb promoter-firefly luciferase reporter plasmid increased in SI soleus over control as compared to that of a promoterless plasmid. Deletion analyses indicated that the regions encompassing -2237 to -1431, -1048 to -461, and -192 to -161 basepairs (bp) each contributed to the increase in transcriptional activity. Moreover, deletions or mutations of AT-rich regions in the proximal -192 bp region abolished the increased promoter activity. These results provide important insights related to how proximal IIb MHC promoter elements regulate the increased expression of the IIb MHC gene in response to inactivity of a predominantly slow postural muscle as it undergoes a remodeling of its phenotype and functional characteristics.

Intergenic bidirectional promoter and cooperative regulation of the IIx and IIb MHC genes in fast skeletal muscle

This study investigated the dynamic regulation of IIx-IIb MHC genes in the fast white medial gastrocnemius (WMG) muscle in response to intermittent resistance exercise training (RE), a model associated with a rapid shift from IIb to IIx expression (11). We investigated the effect of 4 days of RE on the transcriptional activity across the skeletal MHC gene locus in the WMG in female Sprague-Dawley rats. Our results show that RE resulted in significant shifts from IIb to IIx observed at both the pre-mRNA and mRNA levels. An antisense RNA (xII NAT) was detected in the intergenic (IG) region between IIx and IIb, extending across the entire IIx gene and into its promoter. The expression of the xII NAT was positively correlated with IIb pre-mRNA (R = +0.8), and negatively correlated with IIx pre-mRNA (R = -0.8). Transcription mapping of the IIx-IIb IG region revealed the generation of sense IIb and xII NATs from a single promoter region. This bidirectional promoter is highly conserved among species and contains several regulatory elements that may be implicated in its regulation. These results suggest that the IIx and the IIb genes are physically and functionally linked via the bidirectional promoter. In order for the IIx MHC gene to be regulated, a feedback mechanism from the IG xII NAT is needed. In conclusion, the IG bidirectional promoter generating antisense RNA appears to be essential for the coordinated regulation of the skeletal muscle MHC genes during dynamic phenotype shifts.