Role of methylation in maintenance of positionally restricted transgene expression in developing muscle

Development and characterization of a new inbred transgenic rat strain expressing DsRed monomeric fluorescent protein

The inbred rat is a suitable model for studying human disease and because of its larger size is more amenable to complex surgical manipulation than the mouse. While the rodent fulfills many of the criteria for transplantation research, an important requirement is the ability to mark and track donors cells and assess organ viability. However, tracking ability is limited by the availability of transgenic (Tg) rats that express suitable luminescent or fluorescent proteins. Red fluorescent protein cloned from Discosoma coral (DsRed) has several advantages over other fluorescent proteins, including in vivo detection in the whole animal and ex vivo visualization in organs as there is no interference with autofluorescence. We generated and characterized a novel inbred Tg Lewis rat strain expressing DsRed monomeric (DsRed mono) fluorescent protein under the control of a ubiquitously expressed ROSA26 promoter. DsRed mono Tg rats ubiquitously expressed the marker gene as detected by RT-PCR but the protein was expressed at varying levels in different organs. Conventional skin grafting experiments showed acceptance of DsRed monomeric Tg rat skin on wild-type rats for more than 30 days. Cardiac transplantation of DsRed monomeric Tg rat hearts into wild-type recipients further showed graft acceptance and long-term organ viability (>6 months). The DsRed monomeric Tg rat provides marked cells and/or organs that can be followed for long periods without immune rejection and therefore is a suitable model to investigate cell tracking and organ transplantation.

Epigenetic control of development and expression of quantitative traits

In recent years, it has become increasingly clear that epigenetic regulation of gene expression is critical during embryo development and subsequently during pre- and post-natal life. The phenotype of an individual is the result of complex interactions between genotype and current, past and ancestral environment leading to a lifelong remodelling of its epigenome. Practically, if the genome was compared with the hardware in a computer, the epigenome would be the software that directs the computer's operation. This review points to the importance of epigenetic processes for genome function in various biological processes, such as embryo development and the expression of quantitative traits.

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.

New lines of GFP transgenic rats relevant for regenerative medicine and gene therapy

Adoptive cell transfer studies in regenerative research and identification of genetically modified cells after gene therapy in vivo require unequivocally identifying and tracking the donor cells in the host tissues, ideally over several days or for up to several months. The use of reporter genes allows identifying the transferred cells but unfortunately most are immunogenic to wild-type hosts and thus trigger rejection in few days. The availability of transgenic animals from the same strain that would express either high levels of the transgene to identify the cells or low levels but that would be tolerant to the transgene would allow performing long-term analysis of labelled cells. Herein, using lentiviral vectors we develop two new lines of GFP-expressing transgenic rats displaying different levels and patterns of GFP-expression. The "high-expresser" line (GFP(high)) displayed high expression in most tissues, including adult neurons and neural precursors, mesenchymal stem cells and in all leukocytes subtypes analysed, including myeloid and plasmacytoid dendritic cells, cells that have not or only poorly characterized in previous GFP-transgenic rats. These GFP(high)-transgenic rats could be useful for transplantation and immunological studies using GFP-positive cells/tissue. The "low-expresser" line expressed very low levels of GFP only in the liver and in less than 5% of lymphoid cells. We demonstrate these animals did not develop detectable humoral and cellular immune responses against both transferred GFP-positive splenocytes and lentivirus-mediated GFP gene transfer. Thus, these GFP-transgenic rats represent useful tools for regenerative medicine and gene therapy.

Mighty is a novel promyogenic factor in skeletal myogenesis

Genetic analysis has revealed an important function in myogenesis for Myostatin, a member of the TGF-beta superfamily. However, the cascade of genes that responds to Myostatin signalling to regulate myogenesis is not well understood. Thus, a suppressive subtraction hybridization to identify such genes was undertaken and here we report the cloning and characterization of a novel gene, Mighty. Mighty is expressed in a variety of different tissues but appears to be specifically regulated by Myostatin in skeletal muscle. Overexpression of Mighty in C2C12 cells results in early withdrawal of myoblasts from the cell cycle, enhanced and accelerated differentiation and hypertrophy of myotubes. Most importantly, Mighty overexpression leads to increased and earlier expression of MyoD and increased secretion of another known differentiation inducing factor, IGF-II. Furthermore, viral expression of Mighty in mdx mice resulted in an increase in the number of larger healthy muscle fibers. Given its role in myogenesis, we propose that Mighty is a critical promyogenic factor which plays a key role in the signalling pathway downstream of Myostatin.

Faithful Expression of GFP from the PLCβ2 Promoter in a Functional Class of Taste Receptor Cells

Phospholipase C-type beta2 (PLCbeta2) is expressed in a subset of cells within mammalian taste buds. This enzyme is involved in the transduction of sweet, bitter, and umami stimuli and thus is believed to be a marker for gustatory sensory receptor cells. We have developed transgenic mice expressing green fluorescent protein (GFP) under the control of the PLCbeta2 promoter to enable one to identify these cells and record their physiological activity in living preparations. Expression of GFP (especially in lines with more than one copy integrated) is strong enough to be detected in intact tissue preparations using epifluorescence microscopy. By immunohistochemistry, we confirmed that the overwhelming majority of cells expressing GFP are those that endogenously express PLCbeta2. Expression of the GFP transgene in circumvallate papillae occurs at about the same time during development as endogenous PLCbeta2 expression. When loaded with a calcium-sensitive dye in situ, GFP-positive taste cells produce typical Ca2+ responses to a taste stimulus, the bitter compound cycloheximide. These PLCbeta2 promoter-GFP transgenic lines promise to be useful for studying taste transduction, sensory signal processing, and taste bud development.

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.

A new view of patterning domains in the vertebrate mesoderm

The musculoskeletal system of vertebrates is derived from the embryonic mesoderm. Its structures are categorized as epaxial or hypaxial based on their adult position and innervation. The epaxial/hypaxial terminology is also used to describe regions of the embryonic somites based on fate mapping of somitic derivatives. However, the adult, functional distinctions are not fully consistent with the changing embryonic environments of mesodermal populations during morphogenesis, and the traditional terminology loses accuracy when used to describe certain mutant phenotypes. Here we describe a new terminology naming two mesodermal environments defined by the lineage of the included cells. We discuss how mutant phenotypes may be better explained by consideration of the embryonic context in which genes take their effect and argue that the recognition of these embryonic territories clarifies description and discussion of the morphogenesis and patterning of the musculoskeletal system.

Targeted deletion of the MLC1f/3f downstream enhancer results in precocious MLC expression and mesoderm ablation

The expression of skeletal muscle contractile proteins is tightly regulated during embryonic development. In the mouse, the myosin light chain (MLC) 1f/3f gene locus is not activated until E9.5, exclusively in skeletal muscle precursor cells. A potent enhancer downstream of the MLC1f/3f locus confers correct temporal and spatial activation of linked reporter gene in transgenic mouse embryos. To examine roles of the MLC downstream enhancer (MLCE) in its native context of the MLC1f/3f gene locus, we eliminated a 1.5-kb DNA segment containing the enhancer from the mouse genome by targeted deletion, leaving no exogenous sequences at the deletion site. Mouse embryos homozygous for the MLCE deletion were smaller and developmentally delayed, formed no mesoderm by E7.5, and were resorbed almost completely at E8.5. In situ hybridization and RT-PCR analyses of affected mutant embryos at E7.5 revealed ectopic MLC transcripts, whose products would be predicted to interfere with a variety of nonmuscle cell functions determining differentiation of mesoderm. These results suggest that the MLC downstream enhancer and its flanking sequences include negative regulatory elements which block precocious activation of MLC expression in mesodermal precursors during a critical window of development, as well as positive elements which subsequently permit tissue-restricted MLC transcription in differentiating skeletal muscles.

Molecular dissection of DNA sequences and factors involved in slow muscle-specific transcription

Transcription is a major regulatory mechanism for the generation of slow- and fast-twitch myofibers. We previously identified an upstream region of the slow TnI gene (slow upstream regulatory element [SURE]) and an intronic region of the fast TnI gene (fast intronic regulatory element [FIRE]) that are sufficient to direct fiber type-specific transcription in transgenic mice. Here we demonstrate that the downstream half of TnI SURE, containing E box, NFAT, MEF-2, and CACC motifs, is sufficient to confer pan-skeletal muscle-specific expression in transgenic mice. However, upstream regions of SURE and FIRE are required for slow and fast fiber type specificity, respectively. By adding back upstream SURE sequences to the pan-muscle-specific enhancer, we delineated a 15-bp region necessary for slow muscle specificity. Using this sequence in a yeast one-hybrid screen, we isolated cDNAs for general transcription factor 3 (GTF3)/muscle TFII-I repeat domain-containing protein 1 (MusTRD1). GTF3 is a multidomain nuclear protein related to initiator element-binding transcription factor TF II-I; the genes for both proteins are deleted in persons with Williams-Beuren syndrome, who often manifest muscle weakness. Gel retardation assays revealed that full-length GTF3, as well as its carboxy-terminal half, specifically bind the bicoid-like motif of SURE (GTTAATCCG). GTF3 expression is neither muscle nor fiber type specific. Its levels are highest during a period of fetal development that coincides with the emergence of specific fiber types and transiently increases in regenerating muscles damaged by bupivacaine. We further show that transcription from TnI SURE is repressed by GTF3 when overexpressed in electroporated adult soleus muscles. These results suggest a role for GTF3 as a regulator of slow TnI expression during early stages of muscle development and suggest how it could contribute to Williams-Beuren syndrome.

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

Isoform diversity in striated muscle is largely controlled at the level of transcription. In this review we will concentrate on studies concerning transcriptional regulation of the alkali myosin light chain 1F/3F gene. Uncoupled activity of the MLC1F and 3F promoters, together with complex patterns of transcription in developing skeletal and cardiac muscle, combine to make analysis of this gene particularly intriguing. In vitro and transgenic studies of MLC1F/3F regulatory elements have revealed an array of cis-acting modules that each drive a subset of the expression pattern of the two promoters. These cis-acting regulatory modules, including the MLC1F and 3F promoter regions and two skeletal muscle enhancers, control tissue-specificity, cell or fibre-type specificity, and the spatiotemporal regulation of gene expression, including positional information. How each of these regulatory modules acts and how their individual activites are integrated to coordinate transcription at this locus are discussed.

An E box comprises a positional sensor for regional differences in skeletal muscle gene expression and methylation

To dissect the molecular mechanisms conferring positional information in skeletal muscles, we characterized the control elements responsible for the positionally restricted expression patterns of a muscle-specific transgene reporter, driven by regulatory sequences from the MLC1/3 locus. These sequences have previously been shown to generate graded transgene expression in the segmented axial muscles and their myotomal precursors, fortuitously marking their positional address. An evolutionarily conserved E box in the MLC enhancer core, not recognized by MyoD, is a target for a nuclear protein complex, present in a variety of tissues, which includes Hox proteins and Zbu1, a DNA-binding member of the SW12/SNF2 gene family. Mutation of this E box in the MLC enhancer has only a modest positive effect on linked CAT gene expression in transfected muscle cells, but when introduced into transgenic mice the same mutation elevates CAT transgene expression in skeletal muscles, specifically releasing the rostral restriction on MLC-CAT transgene expression in the segmented axial musculature. Increased transgene activity resulting from the E box mutation in the MLC enhancer correlates with reduced DNA methylation of the distal transgenic MLC1 promoter as well as in the enhancer itself. These results identify an E box and the proteins that bind to it as a positional sensor responsible for regional differences in axial skeletal muscle gene expression and accessibility.

Seeking muscle stem cells

Skeletal muscle development requires the formation of myoblasts that can fuse with each other to form multinucleate myofibers. Distinct primary and secondary, slow and fast, populations of myofibers form by the time of birth. At embryonic, fetal, and perinatal stages of development, temporally distinct lineages of myogenic cells arise and contribute to the formation of these multiple types of myofibers. In addition, spatially distinct lineages of myogenic cells arise and form the anterior head muscles, limb (hypaxial) muscles, and dorsal (epaxial) muscles. There is strong evidence that myoblasts are produced from muscle stem cells, which are self-renewing cells that do not themselves terminally differentiate but produce progeny that are capable of becoming myoblasts and myofibers. Muscle stem cells, which may be multipotent, appear to be distinguishable from myoblasts by a number of indirect and direct criteria. Muscle stem cells arise either in unsegmented paraxial mesoderm (anterior head muscle progenitors) or in segmented mesoderm of the somites (epaxial and hypaxial muscle progenitors). These initial stages of myogenesis are regulated by positive and negative signals, including Wnt, BMP, and Shh family members, from nearby notochord, neural tube, ectoderm, and lateral mesoderm tissues. The formation of skeletal muscles, therefore, depends on the generation of spatially and temporally distinct lineages of myogenic cells. Myogenic cell lineages begin with muscle stem cells which produce the myoblasts that fuse to form myofibers.

Transcriptional regulation of smooth muscle contractile apparatus expression

The transcriptional regulatory mechanisms that control gene expression during differentiation and contractile protein accumulation are becoming well understood in skeletal and cardiac muscle lineages. Current understanding of smooth muscle-specific gene transcription is much more limited, though recent studies have begun to shed light on this topic. In this review, we summarize some of the themes emerging from these studies and identify transcriptional regulatory elements common to several smooth muscle genes. These include potential binding sites for serum response factor, Sp1, AP2, Mhox, and YY1, as well as a potential transforming growth factor-beta control element. We speculate that it may be possible to manipulate smooth muscle-specific gene expression in asthma or pulmonary arterial hypertension as an eventual therapy.

The chicken GATA-6 locus contains multiple control regions that confer distinct patterns of heart region-specific expression in transgenic mouse embryos

The GATA-6 transcription factor is expressed in cardiogenic cells and during subsequent stages of heart development in diverse vertebrate species. To gain insights into the molecular events that govern this heart-restricted expression, we isolated the chicken GATA-6 gene and used several approaches to screen for associated control regions. Our analysis of two chicken GATA-6/lacZ constructs in transgenic mouse embryos was particularly revealing. One GATA-6/lacZ construct, which has 1.5 kilobase pairs of upstream sequences along with the promoter and first intron, was expressed exclusively in the atrioventricular canal region of the heart. This expression pattern is novel and appears to mark specialized myocardial cells that induce underlying endocardial cells to initiate valve formation. The other GATA-6/lacZ construct, which has an additional 7.7 kilobase pairs of upstream sequences, was expressed in the ventricle and outflow tract in addition to the atrioventricular canal. The failure of these GATA-6 control regions to function as enhancers in transfected cardiac myocyte cultures underscores the importance of using transgenic approaches to elucidate transcriptional controls that function in the developing heart. Although the endogenous GATA-6 gene is expressed throughout the heart, our results indicate that this is effected in a heart region-specific manner.

Regionalized expression of myosin isoforms in heterotypic myotubes formed from embryonic and fetal rat myoblasts in vitro

The development of mammalian limb muscles involves the appearance and fusion of at least two separate populations of muscle precursor cells. These two populations, termed embryonic and fetal myoblasts, are first detected within the limb bud at different stages of development. We have previously demonstrated that, in the rat, each myoblast population expresses a unique pattern of myosin heavy chains (MyHCs) during differentiation in vitro (Pin and Merrifield [1993] Dev. Genet. 14:356-368). Embryonic myoblasts accumulate embryonic and slow MyHCs, whereas fetal myoblasts accumulate embryonic, neonatal, and adult fast MyHCs but not slow MyHC. To determine if the two populations can fuse with each other and whether the pattern of MyHC expression is altered in the resulting heterokaryons, embryonic and fetal myoblasts were labelled with the lipophilic dye PKH26, [3H]-thymidine, or 5-bromodeoxyuridine (BRDU) and cocultured for 24-48 hr. Our results demonstrate that fusion occurs between embryonic and fetal myoblasts in vitro. Moreover, analysis of the resulting heterokaryons revealed regionalized accumulations of MyHC around individual nuclei. Interestingly, these accumulations were typical of the default pattern of expression that individual nuclei would have normally expressed in single culture. Nuclei contributed by embryonic myoblasts were surrounded by localized accumulations of slow MyHC, whereas nuclei from fetal myoblasts were surrounded by neonatal/fast MyHC. The occurrence of such nuclear domains indicates that the myoblast-specific expression of MyHC isoforms is dictated by cis-acting factors established prior to fusion.