Raver2, a new member of the hnRNP family

Raver2 was identified as a novel member of the hnRNP family based on sequence homology within three RNA recognition motifs and its general domain organization reminiscent of the previously described raver1 protein. Like raver1, raver2 contains two putative nuclear localization signals and a potential nuclear export sequence, and also displays nucleo-cytoplasmic shuttling in a heterokaryon assay. In glia cells and neurons, raver2 localizes to the nucleus. Moreover, the protein interacts with polypyrimidine tract binding protein (PTB) suggesting that it may participate in PTB-mediated nuclear functions. In contrast to ubiquitously expressed raver1, raver2 exerts a distinct spatio-temporal expression pattern during embryogenesis and is essentially restricted to brain, lung, and kidney in the adult mouse.

Chick CFC controls Lefty1 expression in the embryonic midline and nodal expression in the lateral plate

Members of the EGF-CFC family of proteins have recently been implicated as essential cofactors for Nodal signaling. Here we report the isolation of chick CFC and describe its expression pattern, which appears to be similar to Cfc1 in mouse. During early gastrulation, chick CFC was asymmetrically expressed on the left side of Hensen's node as well as in the emerging notochord, prechordal plate, and lateral plate mesoderm. Subsequently, its expression became confined to the heart fields, notochord, and posterior mesoderm. Implantation experiments suggest that chick CFC expression in the lateral plate mesoderm is dependent on BMP signaling, while in the midline its expression depends on an Activin-like signal. The asymmetric expression domain within Hensen's node was not affected by application of FGF8, Noggin, or Shh antibody. Implantation of cells expressing human or mouse CFC2, or chick CFC on the right side of Hensen's node randomized heart looping without affecting expression of genes involved in left-right axis formation, including SnR, Nodal, Car, or Pitx2. Application of antisense oligodeoxynucleotides to the midline of Hamburger-Hamilton stage 4-5 embryos also randomized heart looping, but in contrast to the overexpression experiments, antisense oligodeoxynucleotide treatment resulted in bilateral expression of Nodal, Car, Pitx2, and NKX3.2, whereas Lefty1 expression in the midline was transiently lost. Application of the antisense oligodeoxynucleotides to the lateral plate mesoderm abolished Nodal expression. Thus, chick CFC seems to have a dual function in left-right axis formation by maintaining Nodal expression in the lateral plate mesoderm and controlling expression of Lefty1 expression in the midline territory.

Activated raf kinase inhibits muscle cell differentiation through a MEF2-dependent mechanism

Muscle cell development is dependent on the activity of cell type-specific basic-helix-loop-helix transcription factors, MyoD, Myf-5, myogenin, and MRF4 which collaborate with myocyte enhancer factor 2 proteins to activate muscle-specific gene expression. Growth factors and activated Ras prevent differentiation of myoblasts in culture but the downstream signalling pathways are not well understood. Here, we demonstrate that active Raf kinase (Raf-BxB) completely inhibits myogenic conversion of 10T1/2 cells mediated by Myf-5 and differentiation of L6 myoblasts as indicated by the absence of myotubes, lack of myogenin expression, and markedly reduced expression of myosin heavy chain. However, activated Raf inhibits transcriptional activation by Myf-5 only partially suggesting that other potential targets of Ras/Raf signalling may be involved. Significantly, we observed that elevated Raf kinase activity in L6 muscle cells suppresses the accumulation of MEF2 protein in nuclei, while MEF2 transcription appears unaffected. Moreover, forced expression of MEF2A in 10T1/2 cells rescues MyoD dependent myogenic conversion in the presence of constitutively active Raf kinase and partially restores transactivation of a myogenin promoter-dependent reporter gene in L6 muscle cells containing activated Raf kinase. From these observations we conclude that persistent activation of Raf signalling affects nuclear MEF2 functions which may explain why myogenin expression and myoblast differentiation are inhibited.

Isolation and characterization of the novel popeye gene family expressed in skeletal muscle and heart

We identified a novel gene family in vertebrates which is preferentially expressed in developing and adult striated muscle. Three genes of the Popeye (POP) family were detected in human and mouse and two in chicken. Chromosomal mapping indicates that Pop1 and Pop3 genes are clustered on mouse chromosome 10, whereas Pop2 maps to mouse chromosome 16. We found evidence that POP1 and POP3 in chicken may also be linked and multiple transcript isoforms are generated from this locus. The POP genes encode proteins with three potential transmembrane domains that are conserved in all family members. Individual POP genes exhibit specific expression patterns during development and postnatally. Chicken POP3 and mouse Pop1 are first preferentially expressed in atrium and later also in the subepicardial compact layer of the ventricles. Chicken POP1 and mouse Pop2 are expressed in the entire heart except the outflow tract. All three Pop genes are expressed in heart and skeletal muscle of the adult mouse and lower in lung. Pop1 and Pop2 expression is upregulated in uterus of pregnant mice. Like the mouse genes, human POP genes are predominantly expressed in skeletal and cardiac muscle. The strong conservation of POP genes during evolution and their preferential expression in heart and skeletal muscle suggest that these novel proteins may have an important function in these tissues in vertebrates.

NKX2.3 is required for MAdCAM-1 expression and homing of lymphocytes in spleen and mucosa-associated lymphoid tissue

Targeted disruption of the transcription factor NKX2.3 gene in mice results in anatomical defects of intestine and secondary lymphoid organs. Here, we report that spleen and Peyer's patches of NKX2. 3-deficient mice are considerably reduced in size and lack the ordered tissue architecture. T and B cells are misplaced within the spleen and mesenteric lymph nodes and fail to segregate into the appropriate T and B cell areas. Furthermore, splenic marginal zones, characterized by specific B cells and various types of macrophage-derived cells around the marginal sinus, are absent in mutants. Homozygous NKX2.3 mutants lack the mucosal addressin cell adhesion molecule-1 (MAdCAM-1) that is normally expressed in mucosa-associated lymphoid tissue (MALT) and spleen. We provide evidence that NKX2.3 can activate MAdCAM-1 transcription directly, suggesting that MAdCAM-1 is at least partly responsible for the migration and homing defects of lymphocytes and macrophages in mutants. Therefore, expression of MAdCAM-1 seems to be required for building functional structures in spleen and MALT, a prerequisite for unimpaired migration and segregation of B and T cells to and within these organs.

BMP2 is required for early heart development during a distinct time period

BMP2, like its Drosophila homologue dpp, is an important signaling molecule for specification of cardiogenic mesoderm in vertebrates. Here, we analyzed the time-course of BMP2-requirement for early heart formation in whole chick embryos and in explants of antero-lateral plate mesoderm. Addition of Noggin to explants isolated at stage 4 and cultured for 24 h resulted in loss of NKX2.5, GATA4, eHAND, Mef2A and vMHC expression. At stages 5-8 the individual genes showed differential sensitivity to Noggin addition. While expression of eHAND, NKX2.5 and Mef2A was clearly reduced by Noggin vMHC was only marginally affected. In contrast, GATA4 expression was enhanced after Noggin treatment. The developmental period during which cardiac mesoderm required the presence of BMP signaling in vivo was assessed by implantation of Noggin expressing cells into stage 4-8 embryos which were then cultured until stage 10-11. Complete loss of NKX2.5 and eHAND expression was observed in embryos implanted at stages 4-6, and expression was still suppressed in stages 7 and 8 implanted embryos. GATA4 expression was also blocked by Noggin at stage 4, however increased at stages 5, 6 and 7. Explants of central mesendoderm, that normally do not form heart tissue were employed to study the time-course of BMP2-induced cardiac gene expression. The induction of cardiac lineage markers in central mesendoderm of stage 5 embryos was distinct for different genes. While GATA4, -5, -6 and MEF2A were induced to maximal levels within 6 h after BMP2 addition, eHAND and dHAND required 12 h to reach maximum levels of expression. NKX2.5 was induced by 6 h and accumulated over 48 h. vMHC and titin were induced at significant levels only after 48 h of BMP2 addition. These results indicate that cardiac marker genes display distinct expression kinetics after BMP2 addition and differential response to Noggin treatment suggesting complex regulation of myocardial gene expression in the early tubular heart.

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.

NKX2 gene expression in neuroectoderm but not in mesendodermally derived structures depends on sonic hedgehog in mouse embryos

NKX2 genes in vertebrates encode a sub- family of homeodomain-containing transcription factors which regulate morphogenetic events and cell differentiation during embryogenesis. In mouse embryos several NKX2 genes are expressed in the ventral midline domains of the neuroectoderm, while other NKX2 genes are primarily expressed in the mesendoderm and mesendodermally derived organs, such as heart and gut. Within several patterning centers for tissue organization sonic hedgehog (Shh) is an important signal in the formation of ventral midline structures in vertebrate embryos. Here, we investigated the role of Shh in the embryonic expression of six different but closely related NKX2 genes in Shh null mutant mice. We found that expression of NKX2.1, NKX2.2, and NKX2.9 in neural domains requires Shh signaling, whereas NKX2.3, NKX2.5 and NKX2.6 expression in endoderm and mesoderm is independent of Shh.

Transcriptional activation of the myogenin gene by MEF2-mediated recruitment of myf5 is inhibited by adenovirus E1A protein

The basic helix-loop-helix (bHLH) transcription factor myogenin plays a crucial role in terminal differentiation of committed myoblasts into mature myocytes. Transcriptional activation of the myogenin gene requires coordinate action of myocyte enhancer factor 2 (MEF2) proteins and the myogenic bHLH regulators, MyoD or Myf5. Here we show that transcription of the myogenin gene in differentiated cells correlates with MEF2 and NF1 binding to their cognate sites in the proximal myogenin promoter but not with binding of Myf5 or MyoD to the E-box. The importance of MEF2 activity was further demonstrated by expression of antisense MEF2 RNA which repressed MEF2 and Myf5-mediated MEF2 site-dependent reporter gene activation and the synergistic transactivation of a myogenin CAT reporter by Myf5 and MEF2. Adenovirus E1A which has previously been shown to specifically interfere with myogenin gene transcription also inhibited the cooperative transactivation by Myf5/MEF2 and MEF2. Consistently, coimmunoprecipitation studies revealed impaired MEF2/Myf5 protein-protein interactions. These results support a model of transcriptional activation and stabilization of myogenin expression in which DNA-bound MEF2 recruits myogenic bHLH factors into an active but E1A-sensitive transcription factor complex.

Two highly related homeodomain proteins, Nkx5-1 and Nkx5-2, display different DNA binding specificities

The mouse Nkx5-1 and Nkx5-2 genes are related to NK genes in Drosophila and encode proteins with very similar homeodomains. In higher vertebrates Nkx5 genes are specifically expressed in the inner ear. Inactivation of the mouse Nkx5-1 gene by homologous recombination revealed a critical role for the formation of vestibular inner ear structures. Here, we investigated biochemical properties of the proteins encoded by the Nkx5 genes. A similar consensus binding sequence was isolated for both Nkx5 proteins using binding site selection. This sequence is related to target sequences previously identified for other Nkx proteins and contains the conserved homeodomain binding core TAAT. An additional, novel and unrelated high affinity binding sequence could be identified for the Nkx5-2 protein.

The homeobox gene NKX3.2 is a target of left-right signalling and is expressed on opposite sides in chick and mouse embryos

Vertebrate internal organs display invariant left-right (L-R) asymmetry. A signalling cascade that sets up L-R asymmetry has recently been identified (reviewed in [1]). On the right side of Hensen's node, activin represses Sonic hedgehog (Shh) expression and induces expression of the genes for the activin receptor (ActRIIa) and fibroblast growth factor-8 (FGF8) [2] [3]. On the left side, Shh induces nodal expression in lateral plate mesoderm (LPM); nodal in turn upregulates left-sided expression of the bicoid-like homeobox gene Pitx2 [4] [5] [6]. Here, we found that the homeobox gene NKX3.2 is asymmetrically expressed in the anterior left LPM and in head mesoderm in the chick embryo. Misexpression of the normally left-sided signals Nodal, Lefty2 and Shh on the right side, or ectopic application of retinoic acid (RA), resulted in upregulation of NKX3.2 contralateral to its normal expression in left LPM. Ectopic application of FGF8 on the left side blocked NKX3.2 expression, whereas the FGF receptor-1 (FGFR-1) antagonist SU5402, implanted on the right side, resulted in bilateral NKX3.2 expression in the LPM, suggesting that FGF8 is an important negative determinant of asymmetric NKX3.2 expression. NKX3.2 expression was also found to be asymmetric in the mouse LPM but, unlike in the chick, it was expressed in the right LPM. In the inversion of embryonic turning (inv) mouse mutant, which has aberrant L-R development, NKX3.2 was expressed predominantly on the left side. Thus, NKX3.2 transcripts accumulate on opposite sides of mouse and chick embryos although, in both the mouse and chick, NKX3.2 expression is controlled by the L-R signalling pathways.

The MADS domain containing transcription factor cMef2a is expressed in heart and skeletal muscle during embryonic chick development

Muscle enhancer factor 2 (MEF2) proteins are important transcription factors for muscle-specific gene activation. Four family members are known in mammals, referred to as MEF2A, MEF2B, MEF2C, and MEF2D. Here we report the isolation and expression pattern of the chick Mef2a gene (cMef2a). cMef2a expression starts in precardiac mesoderm of HH stage 8 embryos. During further embryonic development expression continues in the heart tube and later in atrium and ventricle. A second cMef2a expression domain appears in somites of stage 13 embryos. Somitic cMef2a expression is limited to the myotome and is not found in newly formed somites until the muscle-specific transcription factors MyoD and myogenin are present. This suggests that activation of the cMef2a gene in skeletal muscle is dependent on these basic helix-loop-helix transcription factors. cMef2a expression in heart and skeletal muscle continues into adulthood when it is also seen in intestinal mesenchyme and in brain.

Targeted disruption of the homeobox transcription factor Nkx2-3 in mice results in postnatal lethality and abnormal development of small intestine and spleen

The homeodomain transcription factor Nkx2-3 is expressed in gut mesenchyme and spleen of embryonic and adult mice. Targeted inactivation of the Nkx2-3 gene results in severe morphological alterations of both organs and early postnatal lethality in the majority of homozygous mutants. Villus formation in the small intestine appears considerably delayed in Nkx2-3(−)/- foetuses due to reduced proliferation of the epithelium, while massively increased growth of crypt cells ensues in surviving adult mutants. Interestingly, differentiated cell types of the intestinal epithelium are present in homozygous mutants, suggesting that Nkx2-3 is not required for their cell lineage allocation or migration-dependent differentiation. Hyperproliferation of the gut epithelium in adult mutants is associated with markedly reduced expression of BMP-2 and BMP-4, suggesting that these signalling molecules may be involved in mediating non-cell-autonomous control of intestinal cell growth. Spleens of Nkx2-3 mutants are generally smaller and contain drastically reduced numbers of lymphatic cells. The white pulp appears anatomically disorganized, possibly owing to a homing defect in the spleen parenchyme. Moreover, some of the Nkx2-3 mutants exhibit asplenia. Taken together these observations indicate that Nkx2-3 is essential for normal development and functions of the small intestine and spleen.

Muscle differentiation: more complexity to the network of myogenic regulators

Recent genetic and biochemical approaches have advanced our understanding of control mechanisms underlying myogenesis in vertebrate organisms. In particular, systematic combinations of targeted gene disruptions in mice have revealed unique and overlapping functions of members of the MyoD family of transcription factors within the regulatory network that establishes skeletal muscle cell lineages. Moreover, Pax3 has been identified as a key regulator of myogenesis which seems to act genetically upstream of MyoD. In addition, novel genes have been discovered that modulate myogenesis and the activity of myogenic basic helix-loop-helix (bHLH) proteins in positive or negative ways. The molecular mechanisms of these interactions and cooperativity are being elucidated, most notably between the myogenic bHLH factors and MEF2 transcription factors.

cMeso-1, a novel bHLH transcription factor, is involved in somite formation in chicken embryos

The segmentation of somites from the paraxial mesoderm is a crucial event in vertebrate embryonic development; however, the mechanisms underlying this process are not well understood. In a yeast two-hybrid screen we have identified the novel basic-helix-loop-helix (bHLH) protein cMeso-1 which is expressed in the presomitic mesoderm of early chicken embryos. Initially the gene is activated in the epiblast and transcripts concentrate later in and around the primitive streak. When the segmental plate is laid down the cMeso-1 expression domain successively retracts toward the caudal end but a second domain appears in bilateral stripes in the anterior paraxial mesoderm. This highly dynamic domain of cMeso-1 transcripts demarcates the area immediately posterior to the next prospective pair of somites in cyclic waves which apparently correspond to the formation of new somites. Loss of cMeso-1 function by antisense RNA or oligonucleotides results in severe attenuation of somitogenesis suggesting that it plays an important role in setting up the segmentation process. The dynamic and periodically reiterated expression of cMeso-1 along the anteroposterior axis is not dependent on anterior structures or the propagation of a signal along the anteroposterior axis but seems to follow an intrinsic patterning program which is already set up in the segmental plate.

Chicken winged-helix transcription factor cFKH-1 prefigures axial and appendicular skeletal structures during chicken embryogenesis

The cDNA cFKH-1 encodes a chicken winged helix/forkhead domain transcription factor that presents a dynamic expression pattern during chicken embryogenesis. Transcripts accumulate predominantly in early paraxial mesoderm, developing somites, and within mesenchymal precursors of skeletal structures. cFKH-1 RNA is first detected in the developing mesoderm of HH stage 6 embryos. During subsequent development cFKH-1 RNA accumulates in a dorsal domain of the anterior presomitic mesoderm and later in all cells of the epithelial somites before it becomes limited to the sclerotome when somites compartmentalise. cFKH-1 expression persists in the sclerotome, forming the vertebrae and in mesenchymal condensations in limb buds that will give rise later to the appendicular bones. In differentiated chondrocytes and definitive bone structures, however, cFKH-1 expression is down-regulated. Additional expression domains are found in mesenchyme of branchial arches and the head, in the dorsal aorta, and weakly in the endocardium. Based on its expression pattern and the structure of the forkhead DNA-binding domain cFKH-1 constitutes a chicken relative to the murine family of fkh-1/MF1 and MFH-1 factors. The embryonic expression of the cFKH-1 gene defines distinct mesodermal domains and suggests that it may regulate gene expression in mesenchymal cell lineages that will form cartilage in trunk and limb buds.

Nkx2-9 is a novel homeobox transcription factor which demarcates ventral domains in the developing mouse CNS

Nkx homeobox transcription factors are expressed in diverse embryonic cells and presumably control cell-type specification and morphogenetic events. Nkx2-9 is a novel family member of NK2 genes which lacks the conserved TN-domain found in all hitherto known murine Nkx2 genes. The prominent expression of Nkx2-9 in ventral brain and neural tube structures defines a subset of neuronal cells along the entire neuraxis. During embryonic development, Nkx2-9-expressing cells shift from the presumptive floor plate into a more dorsolateral position of the neuroectoderm and later become limited to the ventricular zone. Nkx2-9 expression overlaps with that of Nkx2-2 but is generally broader. While initially Nkx2-9 is expressed in close proximity to sonic hedgehog, its expression domain clearly segregates from sonic hedgehog at later developmental stages. The dynamic expression pattern of Nkx2-9 in ventral domains of the CNS is consistent with a possible role in the specification of a distinct subset of neurons.

Two regulatory genes, cNkx5-1 and cPax2, show different responses to local signals during otic placode and vesicle formation in the chick embryo

The early stages of otic placode development depend on signals from neighbouring tissues including the hindbrain. The identity of these signals and of the responding placodal genes, however, is not known. We have identified a chick homeobox gene cNkx5-1, which is expressed in the otic placode beginning at stage 10 and exhibits a dynamic expression pattern during formation and further differentiation of the otic vesicle. In a series of heterotopic transplantation experiments, we demonstrate that cNkx5-1 can be activated in ectopic positions. However, significant differences in otic development and cNkx5-1 gene activity were observed when placodes were transplanted into the more rostral positions within the head mesenchyme or into the wing buds of older hosts. These results indicate that only the rostral tissues were able to induce and/or maintain ear development. Ectopically induced cNkx5-1 expression always reproduced the endogenous pattern within the lateral wall of the otocyst that is destined to form vestibular structures. In contrast, cPax2 which is expressed in the medial wall of the early otic vesicle later forming the cochlea never resumed its correct expression pattern after transplantation. Our experiments illustrate that only some aspects of gene expression and presumably pattern formation during inner ear development can be established and maintained ectopically. In particular, the dorsal vestibular structures seem to be programmed earlier and differently from the ventral cochlear part.

Nkx5-1 controls semicircular canal formation in the mouse inner ear

The inner ear develops from the otic vesicle, a one-cell-thick epithelium, which eventually transforms into highly complex structures including the sensory organs for balance (vestibulum) and hearing (cochlea). Several mouse inner ear mutations with hearing and balance defects have been described but for most the underlying genes have not been identified, for example, the genes controlling the development of the vestibular organs. Here, we report the inactivation of the homeobox gene, Nkx5-1, by homologous recombination in mice. This gene is expressed in vestibular structures throughout inner ear development. Mice carrying the Nkx5-1 null mutation exhibit behavioural abnormalities that resemble the typical hyperactivity and circling movements of the shaker/waltzer type mutants. The balance defect correlates with severe malformations of the vestibular organ in Nkx5-1(−/−) mutants, which fail to develop the semicircular canals. Nkx5-1 is the first ear-specific molecule identified to play a crucial role in the formation of the mammalian vestibular system.

BMP-2 induces ectopic expression of cardiac lineage markers and interferes with somite formation in chicken embryos

In Drosophila induction of the homeobox gene tinman and subsequent heart formation are dependent on dpp signaling from overlying ectoderm. In order to define vertebrate heart-inducing signals we screened for dpp-homologues expressed in HH stage 4 chicken embryos. The majority of transcripts were found to be BMP-2 among several other members of the BMP family. From embryonic HH stage 4 onwards cardiogenic mesoderm appeared to be in close contact to BMP-2 expressing cells which initially were present in lateral mesoderm and subsequently after headfold formation in the pharyngeal endoderm. In order to assess the role of BMP-2 for heart formation, gastrulating chick embryos in New culture were implanted with BMP-2 producing cells. BMP-2 implantation resulted in ectopic cardiac mesoderm specification. BMP-2 was able to induce Nkx2-5 expression ectopically within the anterior head domain, while GATA-4 was also induced more caudally. Cardiogenic induction by BMP-2, however remained incomplete, since neither Nkx2-8 nor the cardiac-restricted structural gene VMHC-1 became ectopically induced. BMP-2 expressing cells implanted adjacent to paraxial mesoderm resulted in impaired somite formation and blocked the expression of marker genes, such as paraxis, Pax-3, and the forkhead gene cFKH-1. These results suggest that BMP-2 is part of the complex of cardiogenic signals and is involved in the patterning of early mesoderm similar to the role of dpp in Drosophila.

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.

Two putative protein kinase CK2 phosphorylation sites are important for Myf-5 activity

Myf-5, a member of a family of muscle-specific transcription factors, is important for myogenic cell determination and differentiation. Here, we report that Myf-5 protein constitutes a substrate for phosphorylation in vitro by protein kinase CK2. We identified two potential phosphorylation sites at serine49 and serine133, both of which seem to be necessary for Myf-5 activity. Mutants which can no longer be phosphorylated fail to transactivate E-box-dependent reporter genes and act as trans-dominant repressors of wild-type Myf-5. Normal activity can be restored by replacing the serine residues with glutamate suggesting that a negative charge at these sites is obligatory for Myf-5 activity. Although serine133 is part of helix 2 which mediates dimerization, we find no evidence for impaired DNA-binding or heterodimerization of the Ser-Ala133 mutant. Some serine49 mutations exhibit reduced nuclear localization and/or protein stability. Our data suggest that CK2-mediated phosphorylation of Myf-5 is required for Myf-5 activity.

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.

A role for FGF-6 in skeletal muscle regeneration

Fibroblast growth factor-6 (FGF-6) belongs to a family of cytokines that control cell proliferation, cell differentiation, and morphogenetic events. Individual FGFs are either expressed widely or in a restricted pattern during embryonic, fetal, and adult life. FGF-6 exhibits a restricted expression profile predominantly in the myogenic lineage. Important functions in wound healing and tissue regeneration have been proposed for various FGFs in the past, although data from knockout mice have not supported this view. We have inactivated the FGF-6 gene in mice to investigate the role of FGF-6 in skeletal muscle development and regeneration. Wild-type mice up-regulate FGF-6 after skeletal muscle injuries and completely restore experimentally damaged skeletal muscle. In contrast, FGF-6(-/-) mutant mice show a severe regeneration defect with fibrosis and myotube degeneration. The number of MyoD- and Myogenin-expressing activated satellite cells after injury were significantly reduced in mutants. This reduction was not caused by a reduced pool of quiescent satellite cells but presumably by a lack of activation or proliferation. Interbreeding of FGF-6(-/-) mutants with mdx mice leads to striking dystrophic changes in skeletal muscles of double homozygous mice characterized by myotube degeneration, the presence of large amounts of mononuclear cells, and deposition of collagen. RNA analysis revealed an up-regulation of MyoD mRNA in mdx but not in FGF-6(-/-)/mdx double mutant mice. We conclude that FGF-6 is a critical component of the muscle regeneration machinery in mammals, possibly by stimulating or activating satellite cells.

Chicken NKx2-8, a novel homeobox gene expressed during early heart and foregut development

cNkx2-8 represents a novel member of the NK2-family transcription factors. The gene contains three highly conserved regions, the TN-, NK2-, and homeodomains which are diagnostic for this group of proteins. cNkx2-8 is expressed during chick embryogenesis in ventral foregut endoderm, myocardial mesoderm, epithelium of the branchial arches and the dorsal mesocardium. While cNkx2-8 expression partially overlaps with other NK genes, such as Nkx2-5 and Nkx2-3, its onset and aspects of its expression domains are specific. Thus, structural data and the expression profile suggest that cNkx2-8 constitutes a new homeobox protein which may cooperate with its known relatives in defining an antero-ventral field including the developing heart and pharyngeal endoderm.

The mouse Nkx2-3 homeodomain gene is expressed in gut mesenchyme during pre- and postnatal mouse development

The Nkx homeodomain proteins are members of a growing family of known vertebrate transcription factors that are believed to play a role in cell type specification and/or maintenance of the differentiated phenotype. In this article we report on the identification and developmental expression pattern of the mouse Nkx2-3 gene. The gene is expressed primarily in gut mesoderm, dinstinct regions of the branchial arches, the tongue epithelium, and limited domains in the developing jaws, possibly including tooth anlagen. In contrast to the chicken and Xenopus genes, Nkx2-3 expression in the mouse was not observed in the developing heart or neural tube. Thus, although structurally related to chicken and Xenopus Nkx2-3, the mouse gene exhibits an overlapping but distinct expression pattern that may suggest the existence of additional family members, as yet unidentified, in vertebrate organisms.

A novel E1A domain mediates skeletal-muscle-specific enhancer repression independently of pRB and p300 binding

The adenovirus E1A oncoprotein completely blocks muscle differentiation and specifically inhibits the transactivating function of myogenic basic helix-loop-helix (bHLH) transcription factors. This inhibition is dependent on the conserved region CR1 of E1A, which also constitutes part of the binding sites for the pocket proteins pRB, p107, and p130 and the transcriptional coactivators p300 and CBP. Here we report a detailed mutational analysis of E1A and the identification of a muscle inhibition motif within CR1. This motif encompasses amino acids 38 to 62 and inhibits Myf-5- or MyoD-mediated activation of myogenin and the muscle creatine kinase gene. Overexpression of this E1A region also inhibits the conversion of 10T1/2 fibroblasts to the myogenic lineage. The sequence motif EPDNEE (amino acids 55 to 60) within CR1 appears to be particularly important, because point mutations of this sequence diminish the E1A inhibitory activity. Interactions of E1A with pRB and with p300 do not seem to be necessary for the muscle-specific enhancer repression, because E1A mutants which lack these interactions still inhibit Myf-5- and MyoD-mediated transactivation. Moreover, overexpression of p300 fails to overcome muscle-specific inhibition by wild-type E1A and mutant E1A protein which lacks pRB binding. Since we have no evidence for direct E1A interaction with bHLH proteins, we propose that E1A may target a necessary cofactor of the muscle-specific bHLH transcription complex.

Regionalized expression of Nkx5-1, Nkx5-2, Pax2 and sek genes during mouse inner ear development

Nkx5-1 and Nkx5-2 are two highly related homeobox genes which are expressed during mouse development in the inner ear. Here, we present the detailed expression of both genes within the developing ear and a comparison to the expression of other potential control genes in this organ. Both genes are active between E13.5 and birth in non-sensory epithelium of the semicircular canals, utricle and saccule. Nkx5-1 and Nkx5-2 are also expressed in the cochlea, where the expression is restricted to the stria vascularis. The endolymphatic duct is devoid of any Nkx5 transcripts. Pax2 is expressed in epithelial cells of the ventral part of the membranous labyrinth where it overlaps with the Nkx5 expression domain. sek shows a complementary pattern to Nkx5 in the vestibular epithelium. In the cochlea sek is expressed throughout the mesenchyme and epithelium but not in the stria vascularis. In the vestibulum Pax2 and sek is limited to the ventral part whereas Nkx5 genes are active throughout. These data suggest that Nkx5 genes, Pax2 and sek play different roles in the patterning of inner ear structures.

Chick NKx-2.3 represents a novel family member of vertebrate homologues to the Drosophila homeobox gene tinman: differential expression of cNKx-2.3 and cNKx-2.5 during heart and gut development

NKx homeodomain proteins are members of a growing family of vertebrate transcription factors with strong homology to the NK genes in Drosophila. Here, we describe the cloning of cNKx-2.3 and cNKx-2.5 cDNAs and their expression during chick development. Both genes are expressed in the developing heart with distinct but overlapping spatio-temporal patterns. While cNKx-2.5 is activated in early precardiac mesoderm and continues to be uniformly expressed throughout the mature heart, expression of NKx-2.3 starts later in differentiated myocardial cells with regional differences compared to NKx-2.5. Additionally, both genes are expressed in adjacent domains of the developing mid- and hindgut mesoderm as well as in branchial arches. The highly conserved structure of cNKx-2.5 and its similar expression to mouse and Xenopus NKx-2.5 genes and to the Drosophila gene tinman argue that it constitutes the chick homologue of these genes. Different temporal and spatial activity of cNKx-2.3 in heart and gut as well as in a regionally restricted expression domain in the neural tube suggest that cNKx-2.3 is a member of the NK-2 gene family which may be involved in specifying mesodermally and ectodermally derived cell types in the embryo.

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

Myf-6 and Myf-5, two members of the family of muscle-specific regulatory genes, are located less than 10 kb apart in the mouse and human genomes. We have shown recently that homozygous mutant mice carrying a pgk-neo-cassette in the first exon of the Myf-6 gene display minor alterations of skeletal musculature but develop a severe rib defect, most likely due to a drastic down-regulation of Myf-5 expression. The mechanism by which the Myf-6 mutation affects the Myf-5 gene is unknown. In order to determine whether Myf-5 transcription is inhibited by the Myf-6 mutation in cis or in trans, we generated compound heterozygous mice carrying inactivated Myf-5 and Myf-6 alleles on different chromosomes. Here, we demonstrate that double-heterozygous mutants exhibit truncated ribs and severe depression of Myf-5 transcription, a phenotype similar to the previously described homozygous Myf-6 mutant mice. These results indicate that the Myf-6 mutation inhibits Myf-5 gene expression by a long-range cis effect.

Targeted inactivation of myogenic factor genes reveals their role during mouse myogenesis: a review

The role of the four myogenic regulating genes Myf-5, myogenin, MyoD, and MRF4 (herculin, Myf-6) during mouse embryogenesis has been investigated by targeted gene inactivation. Null mutations for the MyoD gene generate no skeletal muscle phenotype due to a compensatory activation of the Myf-5 gene. Mice carrying a homozygous Myf-5 mutation exert considerably delayed myotome formation with unexpected consequences. While skeletal myogenesis in these mutant mice resumes normally at the onset of MyoD expression, a skeletal defect of the ribs persists. Apparently, Myf-5 and MyoD individually are not absolutely essential for skeletal muscle development, most likely because they have overlapping or redundant functions. In fact, double mutants lacking both, MyoD and Myf-5, fail to develop skeletal musculature and the muscle forming regions seem to be devoid of myoblasts. Homozygous inactivation of the myogenin gene leads to drastically reduced myofiber formation. These mice accumulate apparently normal numbers of myoblasts which are arrested in their terminal differentiation program. Myf-6 null mutant mice exhibit drastically reduced expression of Myf-5 for reasons presently unknown. The phenotype is very similar to Myf-5 mutants with an additional reduction of deep back muscles and minor alterations in sarcomeric protein isoforms. Based on the phenotypes obtained from these various gene "knock-out" mice, we now begin to understand the regulatory network and the homostatic relationship of genes which are critically involved in myogenesis of vertebrates.

Myf-5 and myoD genes are activated in distinct mesenchymal stem cells and determine different skeletal muscle cell lineages

Targeted inactivation of the myogenic determination genes myf-5 and myoD in mice resulted in moderate (Myf-5) or no muscle phenotypes (MyoD) and double knock-out mutants lacking both genes failed to develop any skeletal muscle. In order to determine the mechanism of this apparent genetic redundancy we investigated the basis of functional overlap between the two genes. Here we demonstrate that Myf-5 and MyoD are not expressed within the same muscle precursor cell, but rather determine different muscle cell lineages arising from independently committed stem cell populations. Selective ablation of Myf-5-expressing muscle precursors from differentiating ES cells does not prevent Myo-D-dependent muscle differentiation. The early muscle progenitor cells which normally express Myf-5 do not develop into later appearing MyoD cells, even when the myf-5 gene has been inactivated. Thus skeletal musculature in vertebrates develops from two separate cell lineages and complementation may occur at the cellular level, but not between different myogenic factor genes within one cell.

Alterations in somite patterning of Myf-5-deficient mice: a possible role for FGF-4 and FGF-6

Mice carrying a targeted mutation in the gene for the myogenic factor Myf-5 fail to form major parts of the ribs, which leads to an unstable thorax and perinatal death. Here, we report that somites of Myf-5-deficient mice lack the expression of FGF-4 and FGF-6 while TGF beta-2 is expressed normally. Early sclerotomal markers, such as Pax-1 reveal no substantial reduction of sclerotome size. At E11.5 the condensing mesenchyme of the rib anlagen is considerably reduced in size in Myf-5 mutant mice. This may be caused by the lack of Myf-5-positive, FGF-expressing cells which normally are in close contact with the lateral sclerotome generating the rib progenitors. The potential role of FGFs and TGF beta on sclerotome formation is demonstrated in micromass cultures of early somites. Combinations of FGF-4 or FGF-6 with TGF beta-2 potentiate chondrogenesis suggesting that these growth factors emanating from early myotomal and dermomyotomal cells may have instructive or permissive effects on differentiation or outgrowth of sclerotomal cells.

Myogenin's functions do not overlap with those of MyoD or Myf-5 during mouse embryogenesis

The four myogenic basic helix-loop-helix proteins, MyoD, myogenin, Myf-5, and MRF4, can each activate skeletal muscle differentiation when introduced into nonmuscle cells. During embryogenesis, each of these genes is expressed in a unique but overlapping pattern in skeletal muscle precursors and their descendants. Gene knockout experiments have shown that MyoD and Myf-5 play seemingly redundant roles in the generation of myoblasts. However, the role of either of these genes during differentiation in vivo has not been determined. In contrast, a myogenin-null mutation blocks differentiation and results in a dramatic decrease in muscle fiber formation, yet the role of myogenin in the generation or maintenance of myoblast populations is not known. Because myogenin possesses the same myogenic activity as MyoD and Myf-5 in vitro and the expression patterns of these three genes overlap in vivo, we sought to determine if myogenin shares certain functions with either MyoD or Myf-5 in vivo. We therefore generated mice with double homozygous null mutations in the genes encoding MyoD and myogenin or Myf-5 and myogenin. These mice showed embryonic and perinatal phenotypes characteristic of the combined defects observed in mice mutant for each gene alone. As shown by histological analysis and expression of muscle-specific genes, the numbers of undifferentiated myoblasts and residual myofibers were comparable between myogenin-mutant homozygotes and the double-mutant homozygotes. Myoblasts isolated from neonates of the combined mutant genotypes underwent myogenesis in tissue culture, indicating that no more than two of the four myogenic factors are required to support muscle differentiation. These results demonstrate that the functions of myogenin do not overlap with those of MyoD or Myf-5 and support the view that myogenin acts in a genetic pathway downstream of MyoD and Myf-5.

Distinct temporal expression of mouse Nkx-5.1 and Nkx-5.2 homeobox genes during brain and ear development

The mouse Nkx-5.1 and Nkx-5.2 genes have been identified by sequence homology to Drosophila NK genes within the homeobox domain. Here, we report the isolation of the Nkx-5.2 cDNA and a detailed comparative analysis of the spatio-temporal expression patterns for Nkx-5.1 and Nkx-5.2 genes. Nkx-5.2 transcripts are first detected in E13.5 embryos where they colocalize with Nkx-5.1 mRNA in the developing central nervous system and the inner ear. However, the onset of Nkx-5.1 transcription begins much earlier in 10 somite stage embryos (E8.5) in the otic placode and the branchial region. Nkx-5.1 expression in the ear persists until birth, whereas in branchial arches it is transient between E8.5 to E11.5. Transcript distribution appears regionalized in the otic vesicle concentrating at the anterior and posterior margin and later at the dorsal side of the otocyst. These domains are distinct from regions expressing Pax-2 and sek, two other early markers for otic development. From E11.5 to birth several Nkx-5.1 expression domains appear in the brain between the ventral diencephalon and the myelencephalon. The same expression domains also exist for Nkx-5.2 beginning at E13.5. The regionally restricted expression pattern of both Nkx-5 genes during mouse development suggests their involvement in cell type specification of neuronal cells.

Inactivation of Myf-6 and Myf-5 genes in mice leads to alterations in skeletal muscle development

Myf-6, alternatively called MRF4 or herculin, is a member of a group of muscle-specific transcription factors which also comprises Myf-5, myogenin and MyoD. All family members show distinct expression patterns during skeletal muscle development and can convert a variety of cell lines to myocytes. We disrupted the Myf-6 gene in mice to investigate its functional role in the network of regulatory factors controlling myogenesis. Homozygous mice carrying the disrupted Myf-6 gene show pronounced down-regulation of Myf-5 transcription for reasons presently unknown. Consequently, these mice represent a double knock-out model for Myf-6 and Myf-5. The mutants resemble most of the Myf-5 phenotype with aberrant and delayed early myotome formation and lack of distal rib structures. In addition, we find a reduction in the size of axial muscles in the back. Apart from changes in the pattern of some contractile protein isoforms, the existing myofibers appear fairly normal. This suggests that Myf-6 has no major role in the maturation of myotubes, as previously proposed. Our results provide evidence that skeletal myogenesis can proceed in the absence of two myogenic factors, Myf-5 and Myf-6, therefore they must exert largely non-redundant functions in vivo.

Initial steps of myogenesis in somites are independent of influence from axial structures

Formation of paraxial muscles in vertebrate embryos depends upon interactions between early somites and the neural tube and notochord. Removal of both axial structures results in a complete loss of epaxial myotomal muscle, whereas hypaxial and limb muscles develop normally. We report that chicken embryos, after surgical removal of the neural tube at the level of the unsegmented paraxial mesoderm, start to develop myotomal cells that express transcripts for the muscle-specific regulators MyoD and myogenin. These cells also make desmin, indicating that the initial steps of axial skeletal muscle formation can occur in the absence of the neural tube. However, a few days following the extirpation, the expression of MyoD and myogenin transcripts gradually disappears, and becomes almost undetectable after 4 days. From these observations we conclude that the neural tube is not required for the generation of the skeletal muscle cell lineage, but may support the survival or maitenance of further differentiation of the myotomal cell compartment. Notochord transplanted medially or laterally to the unsegmented paraxial mesoderm leads to a ventralization of axial structures but does not entirely prevent the early appearance of myoblasts expressing MyoD transcripts. However, the additional notochord inhibits subsequent development and maturation of myotomes. Taken together, our data suggest that neural tube promotes, and notochord inhibits, the process of myogenesis in axial muscles at a developmental step following the initial expression of myogenic bHLH regulators.

MyoD expression marks the onset of skeletal myogenesis in Myf-5 mutant mice

The expression pattern of myogenic regulatory factors and myotome-specific contractile proteins was studied during embryonic development of Myf-5 mutant mice by in situ hybridization and immunohistochemistry. In contrast to somites in wild-type embryos, no expression of myogenin and Myf-6 (MRF4), or any other myotomal markers was detected in mutant animals at E9.0 and E10.0 indicating that Myf-5 plays a crucial role during this developmental period. Significantly, the onset of MyoD expression in rostral somites of E10.5 embryos was unaffected by the Myf-5 mutation suggesting that the activation of the MyoD gene occurs independently of Myf-5 at the correct developmental time. Immediately after the activation of MyoD myogenin transcripts and protein accumulated within the myotome. The first contractile proteins of the sarcomeric apparatus appeared slightly later. By E11.5 the expression of muscle markers were indistinguishable between wild-type and Myf-5 mutant mice. The migration of muscle precursor cells that leave the somites to form limb musculature was monitored in Myf-5-mutant mice by Pax-3 expression. Pax-3-positive cells were equally found in somites and limbs of E10.0 wild-type and mutant mice indicating that myogenic factor expression at the level of somites is not a prerequisite for determination and subsequent migration of limb precursor cells.

Muscle cell differentiation of embryonic stem cells reflects myogenesis in vivo: developmentally regulated expression of myogenic determination genes and functional expression of ionic currents

The mouse blastocyst-derived embryonic stem cell (ES cell) line BLC6 efficiently differentiates into myosin heavy chain-, desmin- and myogenin-positive skeletal muscle cells when cultivated in embryo-like aggregates (embryoid bodies). Here, we show that the muscle-specific determination genes myf5, myogenin, myoD, and myf6 are expressed in these embryoid bodies in a characteristic temporal pattern which precisely reflects the sequence observed during mouse development in vivo. Myf5 is the first gene to be expressed followed by myogenin, myoD, and myf6, in this order. In situ hybridization demonstrates transcripts for myogenin and myoD accumulating in mono- and multinucleated myogenic cells, while myf5 mRNA is already found in mononucleated myoblasts. The myocytes also express functional nicotinic cholinoceptors and exhibit T-type Ca2+ currents and later L-type Ca2+ currents, demonstrating physiological properties of skeletal muscle cells. During myocyte differentiation the density of L-type Ca2+ channels significantly increases while the density of T-type Ca2+ channels decreases. The effect of external signals on myogenic differentiation of BLC6 cells was demonstrated by cocultivation with visceral endodermal END-2 cells and the activin A-secreting WEHI-3 cells. END-2 cells essentially prevent skeletal muscle differentiation, whereas basic fibroblast growth factor, transforming growth factor-beta, and WEHI-3 cells have no or an attenuating effect, respectively. Our results suggest that ES cells recapitulate closely the early steps of muscle development in vivo and may serve as an excellent in vitro system to study this process.

ES-cells carrying two inactivated myf-5 alleles form skeletal muscle cells: activation of an alternative myf-5-independent differentiation pathway

Both alleles of the myogenic regulatory gene myf-5 have been inactivated in mouse embryonic stem cells by different strategies involving either consecutive gene targeting with neomycin and hygromycin replacement vectors or spontaneous loss of heterozygosity in cells targeted with the neomycin replacement vector alone. Both selection schemes provided homozygous myf-5 mutant ES-cells with normal developmental potential in vitro. Embryoid bodies differentiated into skeletal muscle cells as assessed by their typical morphology and skeletal muscle markers. The extent of differentiation in homozygous mutant myf-5 embryonic stem cells was virtually indistinguishable from control cultures, suggesting that myf-5 is not required for the early steps of myogenic development in vitro. While myocyte populations derived from wild-type and heterozygous myf-5 mutant ES-cells contained myoD-positive and myoD-negative cells, no myoD-negative muscle cells were found among myf-5 homozygous mutants. Differentiated myf-5 double-knockout cells also showed a premature expression of myoD. These results indicate a compensatory role of myoD and myf-5 during early myocyte development and suggest a possible down-regulation of myoD by myf-5 during early myogenesis.

The myogenin gene is activated during myocyte differentiation by pre-existing, not newly synthesized transcription factor MEF-2

The myogenin gene, a member of the gene family encoding muscle-specific basic-helix-loop-helix transcription factors, is activated in myoblasts at the onset of differentiation and can be induced in fibroblasts by forced expression of MyoD or its relatives. Here, we report that a small proximal promoter region of the Myf-4 gene, the human myogenin homolog, suffices to direct muscle-specific expression and up-regulation by MyoD. The minimal promoter contains an E-box and a MEF-2 consensus element. Paradoxically, we find that the MEF-2 binding site but not the E-box is necessary for cell type-specific expression and activation by MyoD in tissue culture cells. This suggests an activating mechanism which is independent of direct protein interactions at the E-box. MEF-2 binding complexes were detected in myoblasts and myotubes, as well as in fibroblasts with no strict correlation to myogenin expression. Moreover, transcription of myogenin could be induced in the presence of potent inhibitors of protein synthesis. From these results we conclude that myogenin expression is not mediated primarily through de novo synthesis of MEF-2 but rather involves a post-translational mode of activation.

Pax-3 is required for the development of limb muscles: a possible role for the migration of dermomyotomal muscle progenitor cells

Limb muscles in vertebrates originate from dermomyotomal cells, which during early development migrate from the ventrolateral region of somites into the limb buds. These progenitor cells do not express any muscle-specific marker genes or myogenic transcription factors until they reach their destination in the limbs. Here, we demonstrate by in situ hybridization that myogenic cells in somites and a population of presumably migratory muscle precursor cells in somatopleural tissue as well as myoblasts in the developing limbs express Pax-3. Significantly, in homozygous splotch mutant mice, which synthesize altered Pax-3 mRNA but make no normal protein, no cells positive for Pax-3 transcripts can be detected in the region of migrating limb muscle precursors or in the limb itself. In contrast, myotomal precursor cells and axial skeletal muscles contain Pax-3 transcripts also in the mutant. Interestingly, these animals fail to develop limb musculature as demonstrated by the lack of hybridization with various probes for myogenic transcription factors (Myf-5, myogenin, MyoD) but make apparently normal axial muscles. These observations suggest that Pax-3 is necessary for the formation of limb muscles, affecting either the generation of myogenic precursors in the somitic dermomyotome or the migration of these cells into the limb field.

A novel NK-related mouse homeobox gene: expression in central and peripheral nervous structures during embryonic development

We have identified three novel mouse homeobox genes that are related to the Drosophila NK gene family. Two genes without direct homologues in Drosophila were designated Nkx-5.1 and Nkx-5.2; the third gene Nkx-1.1 constitutes the mouse homologue to NK1.Nkx-5.1 and Nkx-5.2 are closely linked on mouse chromosome 7, whereas Nkx-1.1 is located on a different chromosome. Here, we report the spatiotemporal expression pattern of Nkx-5.1 during prenatal mouse development. Nkx-5.1 gene activity begins at Embryonic Day 10.5 in the developing ear, the neural tube, and dorsal root ganglia. It continues to be active throughout prenatal life in discrete regions of the brain with an anterior border in the ventral diencephalon at the optic chiasma and expression domains in mesencephalon, metencephalon, and myelencephalon. At midgestation, Nkx-5.1 is also expressed in mesenchyme of the head and branchial arches, and in some cranial ganglia, as well as in derivatives of neural crest, such as the truncus sympathicus and myenteric ganglia. The time pattern of Nkx-5.1 expression and its confinement to primarily postmitotic cells of the central and peripheral nervous system suggest that Nkx-5.1 may play a role in the specification of neuronal cell types.

Early skeletal muscle development proceeds normally in parthenogenetic mouse embryos

In mouse chimeras with parthenogenetic cell contribution, the skeletal musculature appears to be largely devoid of parthenogenetically derived cells. To analyze the appearance and early distribution of myotomal cells in parthenotes, we determined the expression of the muscle-specific transcription factors myogenin, MYF-5, and MYF-6 by in situ hybridization in somites of Day 10 and 11 embryos. Here, we report that these myogenic regulatory proteins are expressed in parthenogenetic animals together with desmin, one of the early muscle-specific structural proteins. We also show that parthenogenetic cells contribute equally to dermatome, sclerotome, and myotome in Day 10 and 11 chimeras. These results suggest that early myotomal cells expressing the myogenic control proteins develop and allocate normally in parthenogenetic embryos and in parthenogenetic<==>normal chimeras. The underrepresentation in older chimeras may therefore be due to selective elimination. These data also argue against imprinting of the myogenic factor genes myogenin, Myf-5, and Myf-6.

MyoD or Myf-5 is required for the formation of skeletal muscle

Mice carrying null mutations in the myogenic regulatory factors Myf-5 or MyoD have apparently normal skeletal muscle. To address whether these two factors functionally substitute for one another in myogenesis, mice carrying mutant Myf-5 and MyoD genes were interbred. While mice lacking both MyoD and Myf-5 were born alive, they were immobile and died soon after birth. Northern blot and S1 nuclease analyses indicated that Myf-5(-1-);MyoD(-1-) mice expressed no detectable skeletal muscle-specific mRNAs. Histological examination of these mice revealed a complete absence of skeletal muscle. Immunohistochemical analysis indicated an absence of desmin-expressing myoblast-like cells. These observations suggest that either Myf-5 or MyoD is required for the determination of skeletal myoblasts, their propagation, or both during embryonic development and indicate that these factors play, at least in part, functionally redundant roles in myogenesis.

TPA-induced differentiation of human rhabdomyosarcoma cells: expression of the myogenic regulatory factors

RD cells (a cell line derived from a human rhabdomyosarcoma) undergo a very limited myogenic differentiation despite the fact that they express several myogenic determination genes. Since we have previously shown (Aguanno et al., Cancer Res. 50, 3377, 1990) that the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) induces myogenic differentiation in these cells, in this paper we investigate the mechanism by which TPA interferes with the expression and/or function of the myogenic determination genes. Northern blot analysis revealed that RD cells express the myf3 (the human analog of MyoD) and myf4 (the human analog of myogenin) transcripts, but not myf5 or myf6 transcripts. The myf3 and the myf4 gene products are correctly translated and accumulated in the nuclei as shown by immunofluorescence analysis. The tumor promoter (TPA) does not modify the pattern of expression of the myf factors while it induces the accumulation of muscle-specific transcripts, such as alpha-actin and fast myosin light chain 1, and their corresponding proteins. On the other hand, within 1 day of treatment, TPA inhibits the expression of the Id gene, which is a negative regulator of MyoD activity. However, while the TPA-induced inhibition of Id message accumulation correlates with differentiation, cell confluence also causes a reduction in Id message accumulation, without inducing differentiation. Under our experimental conditions, overexpression of any of the myf cDNAs in RD cells does induce spontaneous differentiation but enhances the effect of TPA treatment independently from the level of the expressed message. These data suggest that differentiation of RD cells is likely to depend upon the activity of complexes containing the various members of the MyoD family, which can be regulated by proteins affecting MyoD dimerization such as Id, but also by other mechanisms induced by TPA, such as phosphorylation.

cAMP-dependent protein kinase represses myogenic differentiation and the activity of the muscle-specific helix-loop-helix transcription factors Myf-5 and MyoD

Myf-5 and MyoD are members of a family of muscle-specific basic helix-loop-helix (bHLH) proteins that are fundamental for myogenic cell differentiation and transcriptional activation of muscle-specific genes. Here we report that elevated levels of the intracellular signaling molecule cAMP and overexpression of cAMP-dependent protein kinase (PKA) inhibit myogenic differentiation. PKA represses the transcriptional activation of muscle-specific genes by the myogenic regulators Myf-5 and MyoD. The repression is directed at the basic HLH domain and is mediated through the E-box DNA consensus motif to which these proteins bind. However, phosphorylation of Myf-5 and MyoD by PKA in vitro does not affect their ability to bind to DNA. PKA specifically inhibits the activity of myogenic bHLH proteins, but not of other HLH proteins, such as the ubiquitously expressed E2A gene products E12 and E47 (E2-5). Our results demonstrate that PKA mediates the cAMP-induced inhibition of muscle cell differentiation by repressing the activity of Myf-5 and MyoD. The inhibition by PKA occurs post-translationally and presumably affects the transactivation process at a step following DNA-binding. The regulation of Myf-5 and MyoD function by a cAMP-dependent pathway may partly explain how external signals generated by serum and certain peptide growth factors can be transduced to the nucleus and inhibit dominant-acting factors that are responsible for myoblast differentiation.