Modular regulation of the MLC1F/3F gene and striated muscle diversity

Design and testing of regulatory cassettes for optimal activity in skeletal and cardiac muscles

Gene therapy for muscular dystrophies requires efficient gene delivery to the striated musculature and specific, high-level expression of the therapeutic gene in a physiologically diverse array of muscles. This can be achieved by the use of recombinant adeno-associated virus vectors in conjunction with muscle-specific regulatory cassettes. We have constructed several generations of regulatory cassettes based on the enhancer and promoter of the muscle creatine kinase gene, some of which include heterologous enhancers and individual elements from other muscle genes. Since the relative importance of many control elements varies among different anatomical muscles, we are aiming to tailor these cassettes for high-level expression in cardiac muscle, and in fast and slow skeletal muscles. With the achievement of efficient intravascular gene delivery to isolated limbs, selected muscle groups, and heart in large animal models, the design of cassettes optimized for activity in different muscle types is now a practical goal. In this protocol, we outline the key steps involved in the design of regulatory cassettes for optimal activity in skeletal and cardiac muscle, and testing in mature muscle fiber cultures. The basic principles described here can also be applied to engineering tissue-specific regulatory cassettes for other cell types.

Identification of novel transcripts from the porcine MYL1 gene and initial characterization of its promoters

The fast skeletal alkali myosin light polypeptide 1 (MYL1) gene is one of three mammalian alkali MLC genes and encodes two isoforms, 1f and 3f, which play a vital role in embryonic, fetal, and adult skeletal muscle development. We isolated the MYL1 gene from a pig BAC library with the goal of characterizing its promoter and identifying its transcripts. Genes and isoforms were identified by reverse transcriptase-PCR, northern blot and RACE (Rapid Amplification of cDNA Ends). Potential MYL1 gene promoters were characterized using a luciferase reporter assay and electrophoretic mobility shift assays (EMSA). MLC1f, MLC3f, and three additional isoforms of porcine MYL1, MLC5f-A, -B, and -C were identified. Up to now, the three novel isoforms had not been reported in human or mouse. Northern blot analysis indicated that MLC1f, MLC3f, and MLC5fs were expressed only in longissimus dorsi muscles. Two transcription initiation and termination sites were identified by RACE. Promoter analysis and EMSA demonstrated the presence of a MEF3 (skeletal muscle-specific transcriptional enhancer) binding site (+384 to +481), which might be essential for porcine MYL1 transcription. Our results suggested that five transcript variants were generated using alternative promoters, two transcription start sites, and polyA sites, as well as variable splicing of the pig MYL1 exon 5. The identification of alternative promoters and splice variants, the expression of the splice variants in different muscle tissues, and the definition of regulatory elements provide important molecular genetic knowledge concerning the MYL1 gene.

An intragenic MEF2-dependent enhancer directs muscle-specific expression of microRNAs 1 and 133

The muscle-specific microRNAs, miR-1 and miR-133, play important roles in muscle growth and differentiation. Here, we show that the MEF2 transcription factor, an essential regulator of muscle development, directly activates transcription of a bicistronic primary transcript encoding miR-1-2 and 133a-1 via an intragenic muscle-specific enhancer located between the miR-1-2 and 133a-1 coding regions. This MEF2-dependent enhancer is activated in the linear heart tube during mouse embryogenesis and thereafter controls transcription throughout the atrial and ventricular chambers of the heart. MEF2 together with MyoD also regulates the miR-1-2/-133a-1 intragenic enhancer in the somite myotomes and in all skeletal muscle fibers during embryogenesis and adulthood. A similar muscle-specific intragenic enhancer controls transcription of the miR-1-1/-133a-2 locus. These findings reveal a common architecture of regulatory elements associated with the miR-1/-133 genes and underscore the central role of MEF2 as a regulator of the transcriptional and posttranscriptional pathways that control cardiac and skeletal muscle development.

Integration of embryonic and fetal skeletal myogenic programs at the myosin light chain 1f/3f locus

The genetic control of skeletal muscle differentiation at the onset of myogenesis in the embryo is relatively well understood compared to the formation of muscle during the fetal period giving rise to the bulk of skeletal muscle fibers at birth. The Mlc1f/3f (Myl1) locus encodes two alkali myosin light chains, Mlc1f and Mlc3f, from two promoters that are differentially regulated during development. The Mlc1f promoter is active in embryonic, fetal and adult fast skeletal muscle whereas the Mlc3f promoter is upregulated during fetal development and remains on in adult fast skeletal muscle. Two enhancer elements have been identified at the mammalian Mlc1f/3f locus, a 3' element active at all developmental stages and an intronic enhancer activated during fetal development. Here, using transgenesis, we demonstrate that these enhancers act combinatorially to confer the spatial, temporal and quantitative expression profile of the endogenous Mlc3f promoter. Using double reporter transgenes we demonstrate that each enhancer can activate both Mlc1f and Mlc3f promoters in vivo, revealing enhancer sharing rather than exclusive enhancer-promoter interactions. Finally, we demonstrate that the fetal activated enhancer contains critical E-box myogenic regulatory factor binding sites and that enhancer activation is impaired in vivo in the absence of myogenin but not in the absence of innervation. Together our observations provide insights into the regulation of fetal myogenesis and the mechanisms by which temporally distinct genetic programs are integrated at a single locus.

Modulation of morphogenesis by noncanonical Wnt signaling requires ATF/CREB family-mediated transcriptional activation of TGFbeta2

Transcriptional readout downstream of canonical Wnt signaling is known to be mediated by beta-catenin activation of well-described targets, but potential transcriptional readout in response to noncanonical Wnt signaling remains poorly understood. Here, we define a transcriptional pathway important in noncanonical Wnt signaling. We have found that Wnt11 is a direct target of a canonical beta-catenin pathway in developing heart and that Wnt11 mutants show cardiac outflow tract defects. We provide genetic and biochemical evidence thatWnt11 signaling affects extracellular matrix composition, cytoskeletal rearrangements and polarized cell movement required for morphogenesis of the cardiac outflow tract. Notably, transforming growth factor beta2 (TGFbeta2), a key effector of organ morphogenesis, is regulated by Wnt11-mediated noncanonical signaling in developing heart and somites via one or more activating transcription factor (ATF)/cyclic AMP response element binding protein (CREB) family members. Thus, we propose that transcriptional readout mediated at least in part by a Wnt11 --> ATF/CREB --> TGFbeta2 pathway is critical in regulating morphogenesis in response to noncanonical Wnt signaling.

Mouse muscle identity: the position-dependent and fast fiber-specific expression of a transgene in limb muscles is methylation-independent and cell-autonomous

We previously characterised transgenic mice in which fast-muscle-specific regulatory sequences from the human aldolase A pM promoter drive the chloramphenicol acetyltransferase gene expression. Mutation of a NF1/MEF2 binding site (M2 motif) in this promoter does not affect fibre-type specificity of the transgene but modifies its expression in a subset of fast-twitch fibres at the limb level, preferentially affecting distal limb muscles. We investigated the molecular and cellular bases of this peculiar expression pattern that provided an adequate model to characterise the mechanisms responsible for muscle positional information. By direct electrotransfer of mutated M2 construct in adult muscle, we demonstrate that positional differences in mutated M2 transgene expression are not observed when the transgene is not integrated into chromatin. Also, this transgene expression pattern does not seem to be correlated with the extent of CpG methylation in its promoter sequence. Finally, we show that positional values reflected by CAT levels are maintained in primary cultures established from different adult limb muscles, as well as in heterotopically transplanted muscles. Our results suggest that mutation of the M2 site contributes to reveal a molecular memory of fibre fate that would be set up on pM promoter during development and persist into adulthood possibly through a chromatin imprint maintained in satellite cells associated with various limb muscles.

Myosin light chain genes in the turkey (Meleagris gallopavo)

Myosin light chains associate with the motor protein myosin and are believed to play a role in the regulation of its actin-based ATPase activity. Myosin light chain cDNA clones from the turkey (Meleagris gallopavo) were isolated and sequenced. One sequence corresponded to an alternative transcript, the skeletal muscle essential light chain (MYL1 isoform 1) and a second to the smooth muscle isoform of myosin light chain (MYL6). The DNA and predicted amino acid sequences of both light chain genes were compared to that of the chicken. Based on the cDNA sequence, oligonucleotide primers were designed to amplify genomic DNA from six of the seven introns of the MYL1 gene. Approximately 5 kb of DNA was sequenced (introns and 3' UTR) and evaluated for the presence of single nucleotide polymorphisms (SNPs). SNPs were verified by sequencing common intron regions from multiple individuals and three polymorphisms were used to genotype pedigreed families. MYL1 is assigned to a turkey linkage group that corresponds to a region of chicken chromosome 7 (GGA7). The results of this study provide genomic reagents for comparative studies of avian muscle components and muscle biology.

Molecular and cellular mechanisms involved in the generation of fiber diversity during myogenesis

Skeletal muscles have a characteristic proportion and distribution of fiber types, a pattern which is set up early in development. It is becoming clear that different mechanisms produce this pattern during early and late stages of myogenesis. In addition, there are significant differences between the formation of muscles in head and those found in rest of the body. Early fiber type differentiation is dependent upon an interplay between patterning systems which include the Wnt and Hox gene families and different myoblast populations. During later stages, innervation, hormones, and functional demand increasingly act to determine fiber type, but individual muscles still retain an intrinsic commitment to form particular fiber types. Head muscle is the only muscle not derived from the somites and follows a different development pathway which leads to the formation of particular fiber types not found elsewhere. This review discusses the formation of fiber types in both head and other muscles using results from both chick and mammalian systems.

Divergence in species and regulatory role of beta -myosin heavy chain proximal promoter muscle-CAT elements

We examined the functional role of distinct muscle-CAT (MCAT) elements during non-weight-bearing (NWB) regulation of a wild-type 293-base pair beta-myosin heavy chain (beta MyHC) transgene. Electrophoretic mobility shift assays (EMSA) revealed decreased NTEF-1, poly(ADP-ribose) polymerase, and Max binding at the human distal MCAT element when using NWB soleus vs. control soleus nuclear extract. Compared with the wild-type transgene, expression assays revealed that distal MCAT element mutation decreased basal transgene expression, which was decreased further in response to NWB. EMSA analysis of the human proximal MCAT (pMCAT) element revealed low levels of NTEF-1 binding that did not differ between control and NWB extract, whereas the rat pMCAT element displayed robust NTEF-1 binding that decreased when using NWB soleus extracts. Differences in binding between human and rat pMCAT elements were consistent whether using rat or mouse nuclear extract or in vitro synthesized human TEF-1 proteins. Our results provide the first evidence that 1) different binding properties and likely regulatory functions are served by the human and rat pMCAT elements, and 2) previously unrecognized beta MyHC proximal promoter elements contribute to NWB regulation.

Fiber-type specific and position-dependent expression of a transgene in limb muscles

We have previously shown that the proximal sequences of the human aldolase A fast-muscle-specific promoter (pM) are sufficient to target the expression of a linked CAT reporter gene to all fast, glycolytic trunk and limb muscles of transgenic mice (pM310CAT lines) in a manner mimicking the activity of the endogenous mouse promoter. When a NF1-binding site (motif M2) in this proximal regulatory region is mutated, the activity of the corresponding mM2 transgene is strongly affected but only in a some fast muscles. Here we show that the mutation of the M2 motif has only mild effects on pM activity in axial and proximal limb, while it drastically reduces this activity in both fore and hind limb distal muscles. At the cellular level, we show that both the pM310CAT and mM2 transgenes are highly expressed in fast glycolytic 2B fibers. However, by contrast to the pM310CAT transgene, whose expression is mainly restricted to fast glycolytic 2B fibers, the mM2 transgene is also active in a high proportion of 2X fibers. This result suggests that the M2 sequence could play a role in restricting the expression of pM to the 2B fibers. The variable expression of the mM2 transgene along the limb axis already exists at post-natal day 10 and seems to result from a change in the proportion of expressing fast fibers per muscle. Altogether, these results suggest that, although considered as phenotypically similar, different populations of fast glycolytic fibers exist, in which the requirement of the NF1 activity for pM expression varies according to the proximal versus distal position of the muscle along the limb axis.

Coregulation of fast contractile protein transgene and glycolytic enzyme expression in mouse skeletal muscle

Little is known of the gene regulatory mechanisms that coordinate the contractile and metabolic specializations of skeletal muscle fibers. Here we report a novel connection between fast isoform contractile protein transgene and glycolytic enzyme expression. In quantitative histochemical studies of transgenic mouse muscle fibers, we found extensive coregulation of the glycolytic enzyme glycerol-3-phosphate dehydrogenase (GPDH) and transgene constructs based on the fast skeletal muscle troponin I (TnIfast) gene. In addition to a common IIB > IIX > IIA fiber type pattern, TnIfast transgenes and GPDH showed correlated fiber-to-fiber variation within each fast fiber type, concerted emergence of high-level expression during early postnatal muscle maturation, and parallel responses to muscle under- or overloading. Regulatory information for GPDH-coregulated expression is carried by the TnIfast first-intron enhancer (IRE). These results identify an unexpected contractile/metabolic gene regulatory link that is amenable to further molecular characterization. They also raise the possibility that the equal expression in all fast fiber types observed for the endogenous TnIfast gene may be driven by different metabolically coordinated mechanisms in glycolytic (IIB) vs. oxidative (IIA) fast fibers.

Targeting gene expression using HSV vectors

Herpes simplex virus (HSV) is an encapsulated DNA virus, with many favourable properties for use as a gene transfer vector. For gene therapy applications, it may be desirable to restrict transgene expression to pre-defined subsets of cells. One potential method for achieving targeted transgene expression using the HSV vector system might involve dictating the cell types to which the vector will transfer the therapeutic transgene of interest. HSV delivers its genetic payload to cells directly through the plasmalemma; the mechanisms are complex and involve multiple viral and cell surface determinants. We have investigated several ways in which each component of the cell entry cascade may be manipulated in order to restrict viral DNA and transgene delivery to particular cellular populations. Our results indicate that targeted transduction may be a viable approach to achieving our goal of targeted HSV-mediated transgene expression.