Myf-5(m1)/Myf-6(m1) compound heterozygous mouse mutants down-regulate Myf-5 expression and exert rib defects: evidence for long-range cis effects on Myf-5 transcription

Assessing Myf5 and Lbx1 contribution to carapace development by reproducing their turtle-specific signatures in mouse embryos

BACKGROUND

The turtle carapace is an evolutionary novelty resulting from changes in the processes that build ribs and their associated muscles in most tetrapod species. Turtle embryos have several unique features that might play a role in this process, including the carapacial ridge, a Myf5 gene with shorter coding region that generates an alternative splice variant lacking exon 2, and unusual expression patterns of Lbx1 and HGF.

RESULTS

We investigated these turtle-specific expression differences using genetic approaches in mouse embryos. At mid gestation, mouse embryos producing Myf5 transcripts lacking exon 2 replicated some early properties of turtle somites, but still developed into viable and fertile mice. Extending Lbx1 expression into the hypaxial dermomyotomal lip of trunk somites to mimic the turtle Lbx1 expression pattern, produced fusions in the distal part of the ribs.

CONCLUSIONS

Turtle-like Myf5 activity might generate a plastic state in developing trunk somites under which they can either enter carapace morphogenetic routes, possibly triggered by signals from the carapacial ridge, or still engage in the development of a standard tetrapod ribcage in the absence of those signals. In addition, trunk Lbx1 expression might play a later role in the formation of the lateral border of the carapace. This article is protected by copyright. All rights reserved.

Development and patterning of rib primordia are dependent on associated musculature

The importance of skeletal muscle for rib development and patterning in the mouse embryo has not been resolved, largely because different experimental approaches have yielded disparate results. In this study, we utilize both gene knockouts and muscle cell ablation approaches to re-visit the extent to which rib growth and patterning are dependent on developing musculature. Consistent with previous studies, we show that rib formation is highly dependent on the MYOD family of myogenic regulatory factors (MRFs), and demonstrate that the extent of rib formation is gene-, allele-, and dosage-dependent. In the absence of Myf5 and MyoD, one allele of Mrf4 is sufficient for extensive rib growth, although patterning is abnormal. Under conditions of limiting MRF dosage, MyoD is identified as a positive regulator of rib patterning, presumably due to improved intercostal muscle development. In contrast to previous muscle ablation studies, we show that diphtheria toxin subunit A (DTA)-mediated ablation of muscle progenitors or differentiated muscle, using MyoD^iCre or HSA-Cre drivers, respectively, profoundly disrupts rib development. Further, a comparison of three independently derived Rosa26-based DTA knockin alleles demonstrates that the degree of rib perturbations in MyoD^iCre/+/DTA embryos is markedly dependent on the DTA allele used, and may in part explain discrepancies with previous findings. The results support the conclusion that the extent and quality of rib formation is largely dependent on the dosage of Myf5 and Mrf4, and that both early myotome-sclerotome interactions, as well as later muscle-rib interactions, are important for proper rib growth and patterning.

Function of the myogenic regulatory factors Myf5, MyoD, Myogenin and MRF4 in skeletal muscle, satellite cells and regenerative myogenesis

Discovery of the myogenic regulatory factor family of transcription factors MYF5, MYOD, Myogenin and MRF4 was a seminal step in understanding specification of the skeletal muscle lineage and control of myogenic differentiation during development. These factors are also involved in specification of the muscle satellite cell lineage, which becomes the resident stem cell compartment inadult skeletal muscle. While MYF5, MYOD, Myogenin and MRF4 have subtle roles in mature muscle, they again play a crucial role in directing satellite cell function to regenerate skeletal muscle: linking the genetic control of developmental and regenerative myogenesis. Here, I review the role of the myogenic regulatory factors in developing and mature skeletal muscle, satellite cell specification and muscle regeneration.

Conditional gene trapping using the FLEx system

The knowledge about the complete genome sequences of mouse, human, and other organisms is only the first step toward the functional annotation of all genes. It facilitates the recognition of sequence conservation, which helps to distinguish between important and not important and also coding from noncoding sequence. Nevertheless, approximately only 50% of all mouse genes have been entirely annotated to date. In the postgenomic era, large-scale projects have been initiated to describe also the expression (Emap, Eurexpress) and the function (International Gene Trap Consortium, Eucomm, Norcomm, Komp) of all mouse genes. By building up on these resources, the average amount of time starting from a gene-coding sequence to finally studying its function in a living organism or embryo, has shortened significantly within the last decade. Several recent developments, namely, in bioinformatics and gene synthesis but also in targeted and random mutagenesis have contributed to the current status. This chapter will highlight the milestones that have been undertaken in order to saturate the mouse genome with gene trap mutations. We have no intention to cover the entire field but will instead focus on most recent vectors and protocols, which have turned out to be most useful in order to promote the technology. Therefore, we apologize upfront to the many studies that could not be mentioned here solely owing to space limitations but which nevertheless made significant contributions to our current understanding. This chapter will finally provide guidance on possible uses of conditional gene trap alleles as well as detailed protocols for the application of this recent technology.

Somite development without influence of the surface ectoderm in the chick embryo: the compartments of a somite responsible for distal rib development

In the development of the somite, signals from neighboring tissues have been suggested to play critical roles. We have found that when interaction between the ectoderm and the somite is blocked by inserting a piece of polyethylene terephatalate film between them in 2-day-chicken embryo, one of the derivatives of somite, the distal rib, did not form. We examined somite development after the operation, to know the correlation between somite development and distal rib formation. In the operated embryo, the dermomyotome was medio-laterally shorter than in the normal embryo, and Pax3 and Sim1 expressions that are seen in the lateral part of normal dermomyotomes were not found, suggesting that the lateral part of the dermomyotome was missing. Although the sclerotome appeared to be normal in its histology and Pax1 expression pattern in the operated embryo, we could not detect the expression of either Scleraxis nor gamma-FBP that are expressed in the cells around the boundaries between the adjacent dermomyotomes in normal embryos. Thus, under the influence of surface ectoderm, the lateral part of dermomyotome and/or the mesenchyme around rostral and caudal edges of dermomyotomes are suggested to play an important role in the distal rib development.

Effects of Wnt1 signaling on proliferation in the developing mid-/hindbrain region

The secreted glycoprotein WNT1 is expressed in the caudal midbrain and is essential for proper development of the entire mid-/hindbrain region. To get better insights into Wnt1 function in the mid-/hindbrain region, we ectopically expressed Wnt1 under the control of the endogenous En1 promoter, thereby extending Wnt1 expression rostrally into the anterior midbrain and caudally into rhombomere 1. In these transgenic mice, the position of the mid-/hindbrain organizer is not altered and pattern formation is not changed. During midgestation, ectopic Wnt1 induced strong overproliferation of precursor cells only in the caudal midbrain in a gene dosage-dependent manner. Enhanced proliferation is at least in part mediated by shortening of the cell cycle length. In adults, Wnt1 exhibited a cell size promoting effect specifically on neurons. We suggest that Wnt1 acts as a regulator of proliferation of specific precursor populations in the developing mid-/hindbrain region and is only secondarily involved in maintenance of the mid-/hindbrain organizer.

IL-7Ralpha and E47: independent pathways required for development of multipotent lymphoid progenitors

Mice that lack the transcription factors encoded by the E2A gene or the receptor for interleukin 7 (IL-7R) have severe overlapping defects in lymphocyte development. Here, we show that E2A proteins are required for the survival of early T-lineage cells; however, they function through a pathway that is distinct from the survival pathway initiated by IL-7R signaling. While E2A proteins are required to suppress caspase 3 activation, ectopic expression of the anti-apoptotic protein Bcl-2 is not sufficient to overcome the lymphopoietic defects observed in the absence of E2A. Remarkably, mice that lack both IL-7Ralpha and E47 display a synergistic decrease in the number of T-cell, NK-cell and multipotent progenitors in the thymus, indicating that these distinct survival pathways converge to promote the development of multipotent lymphoid progenitors.

Thoracic skeletal defects in myogenin- and MRF4-deficient mice correlate with early defects in myotome and intercostal musculature

Myogenin and MRF4 are skeletal muscle-specific bHLH transcription factors critical for muscle development. In addition to a variety of skeletal muscle defects, embryos homozygous for mutations in myogenin or MRF4 display phenotypes in the thoracic skeleton, including rib fusions and sternal defects. These skeletal defects are likely to be secondary because myogenin and MRF4 are not expressed in the rib cartilage or sternum. In this study, the requirement for myogenin and MRF4 in thoracic skeletal development was further examined. When a hypomorphic allele of myogenin and an MRF4-null mutation were placed together, the severity of the thoracic skeletal defects was greatly increased and included extensive rib cartilage fusion and fused sternebrae. Additionally, new rib defects were observed in myogenin/MRF4 compound mutants, including a failure of the rib cartilage to contact the sternum. These results suggested that myogenin and MRF4 share overlapping functions in thoracic skeletal formation. Spatial expression patterns of skeletal muscle-specific markers in myogenin- and MRF4-mutant embryos revealed early skeletal muscle defects not previously reported. MRF4-/- mice displayed abnormal intercostal muscle morphology, including bifurcation and fusion of adjacent intercostals. myogenin/MRF4-mutant combinations displayed ventral myotome defects, including a failure to express normal levels of myf5. The results suggested that the early muscle defects observed in myogenin and MRF4 mutants may cause subsequent thoracic skeletal defects, and that myogenin and MRF4 have overlapping functions in ventral myotome differentiation and intercostal muscle morphogenesis.

Genetics of muscle determination and development

Skeletal muscles in vertebrates develop from somites as the result of patterning and cell type specification events. Here, we review the current knowledge of genes and signals implicated in these processes. We discuss in particular the role of the myogenic determination genes as deduced from targeted gene disruptions in mice and how their expression may be controlled. We also refer to other transcription factors which collaborate with the myogenic regulators in positive or negative ways to control myogenesis. Moreover, we review experiments that demonstrate the influence of tissues surrounding the somites on the process of muscle formation and provide model views on the underlying mechanisms. Finally, we present recent evidence on genes that play a role in regeneration of muscle in adult organisms.

Overlapping functions of the myogenic bHLH genes MRF4 and MyoD revealed in double mutant mice

The myogenic basic helix-loop-helix (bHLH) genes - MyoD, Myf5, myogenin and MRF4 - exhibit distinct, but overlapping expression patterns during development of the skeletal muscle lineage and loss-of-function mutations in these genes result in different effects on muscle development. MyoD and Myf5 have been shown to act early in the myogenic lineage to establish myoblast identity, whereas myogenin acts later to control myoblast differentiation. In mice lacking myogenin, there is a severe deficiency of skeletal muscle, but some residual muscle fibers are present in mutant mice at birth. Mice lacking MRF4 are viable and have skeletal muscle, but they upregulate myogenin expression, which could potentially compensate for the absence of MRF4. Previous studies in which Myf5 and MRF4 null mutations were combined suggested that these genes do not share overlapping myogenic functions in vivo. To determine whether the functions of MRF4 might overlap with those of myogenin or MyoD, we generated double mutant mice lacking MRF4 and either myogenin or MyoD. MRF4/myogenin double mutant mice contained a comparable number of residual muscle fibers to mice lacking myogenin alone and myoblasts from those double mutant mice formed differentiated multinucleated myotubes in vitro as efficiently as wild-type myoblasts, indicating that neither myogenin nor MRF4 is absolutely essential for myoblast differentiation. Whereas mice lacking either MRF4 or MyoD were viable and did not show defects in muscle development, MRF4/MyoD double mutants displayed a severe muscle deficiency similar to that in myogenin mutants. Myogenin was expressed in MRF4/MyoD double mutants, indicating that myogenin is insufficient to support normal myogenesis in vivo. These results reveal unanticipated compensatory roles for MRF4 and MyoD in the muscle differentiation pathway and suggest that a threshold level of myogenic bHLH factors is required to activate muscle structural genes, with this level normally being achieved by combinations of multiple myogenic bHLH factors.

Faithful expression of the Myf-5 gene during mouse myogenesis requires distant control regions: a transgene approach using yeast artificial chromosomes

Myf-5, a member of the family of four muscle-specific basic helix-loop-helix (bHLH) transcription factors is the first to be expressed in somites, branchial arches, and limb buds during prenatal mouse development. However, little is known about control mechanisms which actually regulate Myf-5 gene activity within these various muscle-forming domains. To identify control regions that contribute to the correct spatiotemporal activity pattern of the Myf-5 gene during mouse embryogenesis, here we report the characterization of yeast artificial chromosomes (YACs) which faithfully direct muscle-specific expression of the gene in chimeric mouse embryos. Forty-five kilobases of sequence 5' to the Myf-5 gene together with 500 kb of 3' flanking DNA drives the correct Myf-5 expression in the mesenchyme of the visceral arches and in somites but not in the hypaxial muscles of limb buds. An additional 50 kb of DNA at the 5' end is required to activate Myf-5 gene expression in developing limbs. These results demonstrate for the first time that unexpectedly distant regions of the Myf-5 gene are necessary to recapitulate its precise developmental expression pattern. We also show that Myf-5 expression in hypaxial muscles and in somites and visceral arches is regulated by separate and distinct far upstream regions. The identification of these remote regulatory elements on YACs carrying the mouse Myf-5 gene constitutes the first important step toward further dissection of the complex mechanisms by which cell-autonomous and external cues control Myf-5 expression during skeletal muscle formation in the mouse embryo.

Different MRF4 knockout alleles differentially disrupt Myf-5 expression: cis-regulatory interactions at the MRF4/Myf-5 locus

Three different null alleles of the myogenic bHLH gene MRF4/herculin/Myf-6 were created recently. The three alleles were similar in design but were surprisingly different in the intensity of their phenotypes, which ranged from complete viability of homozygotes to complete lethality. One possible explanation for these differences is that each mutation altered expression from the nearby Myf-5 gene to a different extent. This possibility was first raised by the observation that the most severe MRF4 knockout allele expresses no Myf-5 RNA and is a developmental phenocopy of the Myf-5 null mutation. Furthermore, initial studies of the two weaker alleles had shown that their differences in viability correlate with the intensity of rib skeletal defects, and the most extreme version of this rib defect is the hallmark phenotype of Myf-5 null animals. In the present study we tested this hypothesis for the two milder MRF4 alleles. By analyzing compound heterozygous animals carrying either the intermediate or the weakest MRF4 knockout allele on one chromosome 10 and a Myf-5 knockout allele on the other chromosome, we found that both of these MRF4 alleles apparently downregulate Myf-5 expression by a cis-acting mechanism. Compound heterozygotes showed increased mortality of the normally viable MRF4 allele, together with intensified rib defects for both MRF4 alleles and increased deficits in myotomal Myf-5 expression. The allele-specific gradation in phenotypes also suggested that rib morphogenesis is profoundly sensitive to quantitative differences in Myf-5 function if Myf-5 products drop below hemizygous levels. The mechanistic basis for cis interactions at the MRF4/Myf-5 locus was further examined by fusing a DNA segment containing the entire MRF4 structural gene, including all sequences deleted in the three MRF knockout alleles, with a basal promoter and a lacZ reporter. Transgenic embryos showed specific LacZ expression in myotomes in a pattern that closely resembles the expression of Myf-5 RNA. cis-acting interactions between Myf-5 and MRF4 may therefore play a significant role in regulating expression of these genes in the early myotomes of wildtype embryos.

Distal regulatory elements control MRF4 gene expression in early and late myogenic cell populations

MRF4 is a muscle-specific transcription factor that belongs to a family of basic helix-loop-helix proteins known as the myogenic regulatory factors (MRFs). In vitro studies have shown that expression of the MRF4 gene is controlled by a proximal promoter element (-336 to +71) that binds the muscle-specific transcription factors MEF2 and myogenin to activate transcription. To examine further the regulatory elements necessary for endogenous MRF4 gene expression during development, transgenic mice were generated that contained either a proximal MRF4 promoter-LacZ reporter gene (-336 MRF4-nLacZ) or a MRF4-LacZ reporter gene containing 8.5 kb of 5' flanking sequence (-8500 MRF4-nLacZ). Characterization of individual transgenic mouse lines throughout development revealed that expression of both transgenes is restricted to skeletal muscle tissue. However, unlike previous in vitro data, the proximal promoter transgene exhibits only limited transcriptional activity at all developmental time points, whereas the -8500 MRF4-nLacZ lines fully recapitulate the later developmental expression patterns and exhibit transcription in myotomal cells during somitic differentiation. Tissue culture analysis of myogenic cells isolated from E12.5, E16.5, and adults confirmed that the -8500 MRF4-nLacZ transgene is expressed in greater than 90% of the myotubes for all myogenic populations. These results indicate that 8.5 kb of MRF4 5' flanking sequence contains all the regulatory elements necessary for late MRF4 expression and that at least some of these elements lie upstream of the -336 proximal promoter. It is also likely that distant upstream regulatory sequences control early somitic MRF4 expression. These findings, coupled with previous in vitro studies, suggest that the early and late developmental expression patterns of the MRF4 gene are controlled by distinct sets of regulatory elements.

Skeletal muscle determination and differentiation: story of a core regulatory network and its context

Regulation of skeletal muscle determination and differentiation in vertebrates centers on a core regulatory network which is composed of two families of transcription factors, the MyoD group basic helix-loop-helix (bHLH) muscle regulatory factors (MRFs) and the myocyte enhancer factor 2 (MEF2) group of MADS-box regulators. Members of this network interact with each other genetically and physically, and together they cooperate to positively regulate transcription of downstream muscle-specific differentiation genes. During development, the myogenic network can be activated or repressed in response to patterning signals, some of which have recently been identified. Once activated, the powerful myogenic activity of the core network can be modulated and held in check by a remarkably large group of negative regulators that operate on network components by diverse mechanisms. Recent discoveries highlight extensive parallels between myogenesis and peripheral neurogenesis in the structures of their respective regulatory networks and in the interaction of their bHLH networks with other regulatory circuits. Comparisons with Drosophila indicate that these ensembles of interacting molecular circuits have been highly conserved during evolution.