Expression of myosin heavy chain transcripts during Xenopus laevis development

Myogenic waves and myogenic programs during Xenopus embryonic myogenesis

Although Xenopus is a key model organism in developmental biology, little is known about the myotome formation in this species. Here, we assessed the expression of myogenic regulatory factors of the Myod family (MRFs) during embryonic development and revealed distinct MRF programs.

RESULTS

The expression pattern of each MRF during embryonic development highlights three successive myogenic waves. We showed that a first median and lateral myogenesis initiates before dermomyotome formation: the median cell population expresses Myf5, Myod, and Mrf4, whereas the lateral one expresses Myod, moderate levels of Myogenin and Mrf4. The second wave of myoblasts arising from the dermomyotome is characterized by the full MRF program expression, with high levels of Myogenin. The third wave is revealed by Myf5 expression in the myotome and could contribute to the formation of plurinucleated fibers at larval stages. Furthermore, Myf5- or Myod-expressing anlagen are identified in craniofacial myogenesis.

CONCLUSIONS

The first median and lateral myogenesis and their associated MRF programs have probably disappeared in mammals. However, some aspects of Xenopus myogenesis have been conserved such as the development of somitic muscles by successive myogenic waves and the existence of Myf5-dependent and -independent lineages.

The genome of the diploid anuran Xenopus tropicalis contains a novel array of sarcoplasmic myosin heavy chain genes expressed in larval muscle and larynx

The sarcomeric myosin heavy chain (MyHC) proteins are a family of molecular motors responsible for the transduction of chemical energy into mechanical work in striated muscle. The vertebrate genome contains multiple copies of the MyHC gene, and expression of different isoforms correlates with differences in the physiological properties of muscle fibers. Most MyHC isoforms are found in two arrays, one containing the "fast-twitch" skeletal muscle isoforms and the other the "slow-twitch" or cardiac isoforms. To extend our understanding of MyHC evolution, we have examined the genome of the anuran Xenopus tropicalis. The X. tropicalis genome includes 15 full-length MyHC genes organized in seven genomic locations. One unique array of MyHC genes is similar to the mammalian fast-skeletal array, but is not found in amniotes. The isoforms in this array are expressed during larval stages and in muscles of the adult larynx. Duplication of the fast-skeletal MyHC array appears to have led to expression divergence of muscle proteins in the larval and adult stages of the anuran life cycle. A striking similarity of gene order between regions flanking X. tropicalis MyHC arrays and human arrays was evident; genomic organization of MyHC isoforms may thus be highly conserved across tetrapods.

Xtn3 is a developmentally expressed cardiac and skeletal muscle-specific novex-3 titin isoform

Titin proteins play an essential role in maintaining muscle function and structure. Recent work has implicated the involvement of the novex-3 titin isoform in sarcomere restructuring and disease. Unlike avian and mammalian systems, Xenopus laevis myogenesis is characterized by a wave of primary myogenesis followed by apoptosis of the primary muscles and formation of new muscles by secondary myogenesis. We show here that the Xenopus laevis novex-3 titin isoform (Xtn3) is developmentally expressed throughout the somites, heart, and primary muscles of the developing embryo. Downregulation of Xtn3 expression at tadpole stages appears to coincide with the change in myofiber composition from solely embryonic "fast" fiber types to myofibers containing both "fast" and "slow" fiber types. We demonstrate that Xtn3 is expressed early in the presomitic mesoderm and remains expressed in the somites, ventral myoblasts, and developing jaw muscles through late tailbud stage. Furthermore, we show that Xtn3 is expressed in the cardiac primordia prior to linear heart tube formation and remains expressed in the heart until tadpole stage, at which point it is downregulated in the heart except in discrete patches of cardiac cells. Finally, we demonstrate that Xtn3 transcripts are detectable in adult heart and muscle tissues.

Temperature and the expression of myogenic regulatory factors (MRFs) and myosin heavy chain isoforms during embryogenesis in the common carp Cyprinus carpio L

Embryos of the common carp, Cyprinus carpio L., were reared from fertilization of the eggs to inflation of the swim bladder in the larval stage at 18 and 25 degrees C. cRNA probes were used to detect transcripts of the myogenic regulatory factors MyoD, Myf-5 and myogenin, and five myosin heavy chain (MyHC) isoforms during development. The genes encoding Myf-5 and MyoD were switched on first in the unsegmented mesoderm, followed by myogenin as the somites developed. Myf-5 and MyoD transcripts were initially limited to the adaxial cells, but Myf-5 expression spread laterally into the presomitic mesoderm before somite formation. Two distinct bands of staining could be seen corresponding to the cellular fields of the forming somites, but as each furrow delineated, Myf-5 mRNA levels declined. Upon somite formation, MyoD expression spread laterally to encompass the full somite width. Expression of the myogenin gene was also switched on during somite formation, and expression of both transcripts persisted until the somites became chevron-shaped. Expression of MyoD was then downregulated shortly before myogenin. The expression patterns of the carp myogenic regulatory factor (MRF) genes most-closely resembled that seen in the zebrafish rather than the rainbow trout (where expression of MyoD remains restricted to the adaxial domain of the somite for a prolonged period) or the herring (where expression of MyoD persists longer than that of myogenin). Expression of two embryonic forms of MyHC began simultaneously at the 25-30 somite stage and continued until approximately two weeks post-hatch. However, the three adult isoforms of fast muscle MyHC were not detected in any stage examined, emphasizing a developmental gap that must be filled by other, as yet uncharacterised, MyHC isoform(s). No differences in the timing of expression of any mRNA transcripts were seen between temperature groups. A phylogenetic analysis of the MRFs was conducted using all available full-length amino acid sequences. A neighbour-joining tree indicated that all four members evolved from a common ancestral gene, which first duplicated into two lineages, each of which underwent a further duplication to produce Myf-5 and MyoD, and myogenin and MRF4. Parologous copies of MyoD from trout and Xenopus clustered closely together within clades, indicating recent duplications. By contrast, MyoD paralogues from gilthead seabream were more divergent, indicating a more-ancient duplication.

Pattern and morphogenesis of presumptive superficial mesoderm in two closely related species, Xenopus laevis and Xenopus tropicalis

The mesoderm, comprising the tissues that come to lie entirely in the deep layer, originates in both the superficial epithelial and the deep mesenchymal layers of the early amphibian embryo. Here, we characterize the mechanisms by which the superficial component of the presumptive mesoderm ingresses into the underlying deep mesenchymal layer in Xenopus tropicalis and extend our previous findings for Xenopus laevis. Fate mapping the superficial epithelium of pregastrula stage embryos demonstrates ingression of surface cells into both paraxial and axial mesoderm (including hypochord), in similar patterns and amounts in both species. Superficial presumptive notochord lies medially, flanked by presumptive hypochord and both overlie the deep region of the presumptive notochord. These tissues are flanked laterally by superficial presumptive somitic mesoderm, the anterior tip of which also appears to overlay the presumptive deep notochord. Time-lapse recordings show that presumptive somitic and notochordal cells move out of the roof of the gastrocoel and into the deep region during neurulation, whereas hypochordal cells ingress after neurulation. Scanning electron microscopy at the stage and position where ingression occurs suggests that superficial presumptive somitic cells in X. laevis ingress into the deep region as bottle cells whereas those in X. tropicalis ingress by "relamination" (e.g., [Dev. Biol. 174 (1996) 92]). In both species, the superficially derived presumptive somitic cells come to lie in the medial region of the presumptive somites during neurulation. By the early tailbud stages, these cells lie at the horizontal myoseptum of the somites. The morphogenic pathway of these cells strongly resembles that of the primary slow muscle pioneer cells of the zebrafish. We present a revised fate map of Xenopus, and we discuss the conservation of superficial mesoderm within amphibians and across the chordates and its implications for the role of this tissue in patterning the mesoderm.

Inhibition of the cell cycle is required for convergent extension of the paraxial mesoderm during Xenopus neurulation

Coordination of morphogenesis and cell proliferation is essential during development. In Xenopus, cell divisions are rapid and synchronous early in development but then slow and become spatially restricted during gastrulation and neurulation. One tissue that transiently stops dividing is the paraxial mesoderm, a dynamically mobile tissue that forms the somites and body musculature of the embryo. We have found that cessation of cell proliferation is required for the proper positioning and segmentation of the paraxial mesoderm as well as the complete elongation of the Xenopusembryo. Instrumental in this cell cycle arrest is Wee2, a Cdk inhibitory kinase that is expressed in the paraxial mesoderm from mid-gastrula stages onwards. Morpholino-mediated depletion of Wee2 increases the mitotic index of the paraxial mesoderm and this results in the failure of convergent extension and somitogenesis in this tissue. Similar defects are observed if the cell cycle is inappropriately advanced by other mechanisms. Thus, the low mitotic index of the paraxial mesoderm plays an essential function in the integrated cell movements and patterning of this tissue.

Xenopus muscle development: from primary to secondary myogenesis

Xenopus myogenesis is characterized by specific features, different from those of mammalian and avian systems both at the cellular level and in gene expression patterns. During early embryogenesis, after the initial molecular signals inducing mesoderm, the myogenic determination factors XMyoD and XMyf-5 are activated in presomitic mesoderm in response to mesoderm-inducing factors. After these first inductions of the myogenic program, forming muscles in Xenopus can have different destinies, some of these resulting in cell death before adulthood. In particular, it is quite characteristic of this species that, during metamorphosis, the primary myotomal myofibers completely die and are progressively replaced by secondary "adult" multinucleated myofibers. This feature offers the unique opportunity to totally separate the molecular analysis of these two distinct types of myogenesis. The aim of this review is to summarize our knowledge on the cellular and molecular events as well as the epigenetic regulations involved in the construction of Xenopus muscles during development.

Xenopus bagpipe-related gene, koza, may play a role in regulation of cell proliferation

The homeobox gene koza is a new member of the vertebrate bagpipe-related gene family. Embryonic expression of koza is observed at highest levels in the muscle layer of the somites and, during later development, is restricted to the lateral somitic cells, which correspond to slow twitch muscle tissue. Expression of koza is also observed in the myocardial layer of the heart and in the cement gland. In each of these tissues, koza transcription commences only after the expression of terminal differentiation markers. By injection of synthetic mRNA, we show that overexpression of koza leads to an apparent decrease in the number of cells in the somites. No reduction in cell number is observed when koza is present in neural tissues, suggesting that koza exhibits some tissue specificity in regulation of cell proliferation. Embryonic manipulations show that restriction of koza expression to the slow twitch muscle layer is independent of axial structures but is, at least partly, regulated by signals arising in ectodermal tissue. Finally, in Drosophila, bagpipe expression is regulated by the hedgehog signaling pathway. By using ectopic expression, we show that koza transcription is positively regulated by banded hedgehog. This result indicates that regulation of bagpipe expression by hedgehog signaling is evolutionarily conserved.

Two myogenin-related genes are differentially expressed in Xenopus laevis myogenesis and differ in their ability to transactivate muscle structural genes

Among the myogenic regulatory factors, myogenin is a transcriptional activator situated at a crucial position for terminal differentiation in muscle development. It is unclear at present whether myogenin exhibits unique specificities to transactivate late muscular markers. During Xenopus development, the accumulation of myogenin mRNA is restricted to secondary myogenesis, at the onset of the appearance of adult isoforms of beta-tropomyosin and myosin heavy chain. To determine the role of myogenin in the isoform switch of these contractile proteins, we characterized and directly compared the functional properties of myogenin with other myogenic regulatory factors in Xenopus embryos. Two distinct cDNAs related to myogenin, XmyogU1 and XmyogU2, were differentially expressed during myogenesis and in adult tissues, in which they preferentially accumulated in oxidative myofibers. Animal cap assays in Xenopus embryos revealed that myogenin, but not the other myogenic regulatory factors, induced expression of embryonic/larval isoforms of the beta-tropomyosin and myosin heavy chain genes. Only XmyogU1 induced expression of the adult fast isoform of the myosin heavy chain gene. This is the first demonstration of a specific transactivation of one set of muscle structural genes by myogenin.

Myosin isoforms in anuran skeletal muscle: their influence on contractile properties and in vivo muscle function

Functional studies on isolated single anuran skeletal muscle cells represent classic experiments from which much of our understanding of muscle contraction mechanisms have been derived. Because of their superb mechanical stability when isolated, single anuran fibers provide a uniquely powerful model system that can be exploited to understand the relationship between myosin heavy chain (MHC) and myosin light chain (MLC) composition and muscle fiber function. In this review, we summarize historic and recent studies of MHC and MLC expression patterns in the fiber types of anuran species. We extend the traditional classification scheme, using data from recent reports in which frog MHCs have been cloned, to reveal the molecular basis of frog muscle fiber types. The influence of MHC and MLC isoforms on contractile kinetics of single intact fibers is reviewed. In addition, we discuss more subtle questions such as variability of myosin coexpression along a single cell, and its potential influence on contractile function. The frog jump is used as a model system to elucidate principles of muscular system design, including the role of MHC isoforms on in vivo muscle function. Sequence information is used from cloned frog MHCs to understand the role of specific regions of the myosin motor domain in regulating contractile function and the evolutionary origins of fast and slow amphibian MHCs. Finally, we offer promising future possibilities that combine molecular methods (such as recombinant gene transfer) with single cell contractile measurements to address questions regarding myosin structure/function and gene regulation.

Postembryonic expression of the myosin heavy chain genes in the limb, tail, and heart muscles of metamorphosing amphibian tadpoles

Thyroid hormone is presumed to play a role in initiating and/or orchestrating the postembryonic expression of the genes encoding isoforms of the myosin heavy chains (MHCs) that characterize the muscle fibres in an adult organism. The fact that the postembryonic development of a free-living amphibian tadpole takes place during its thyroid hormone-dependent metamorphosis has made the metamorphosing tadpole an ideal system for elucidating the molecular mechanism(s) by which this hormone affects these postembryonic changes. In this review, we summarize the results from recent studies focused on the postembryonic expression of the MHC genes in the skeletal muscles and hearts of metamorphosing anuran (Rana catesbeiana) tadpoles. The demonstration that mRNAs encoding at least five of the MHC isoforms present in the tadpole tail muscles are also present in the adult hind-limb muscles and that an mRNA encoding a cardiac-specific MHC isoform is present in the heart of both the tadpole and adult organism, rules out the possibility that thyroid hormone initiates the expression of these MHC genes. Instead, it seems more likely that this hormone acts by modulating the expression of one or more of the genes encoding these particular MHC isoforms. Whatever the case, the fact that sequence homology suggests that the five distinct skeletal muscle-specific MHCs are all "fast" isoforms raises the question of how these MHCs are distributed among the three different fibre types described for Rana. On the other hand, the possibility exists that the mRNAs for one or more of these fast MHC isoforms encode developmental isoforms that are present but not translated in the muscles of the tadpole and/or adult frog. Finally, an evaluation of the evolutionary relatedness of the R. catesbeiana MHCs to the MHCs in another species of Rana and to the MHCs in other vertebrates discloses, among other things, that the nucleotide sequence in the R. catesbeiana cardiac MHC isoform is more closely related to the chicken ventricular MHC isoform than it is to any of the other MHC isoforms examined.

Expression of the myosin heavy chain genes in the tail muscle of thyroid hormone-induced metamorphosing Rana catesbeiana tadpoles

In tadpoles of the North American bullfrog, Rana catesbeiana, spontaneous and thyroid hormone (T3)-induced metamorphosis is characterized by regression of the tail, which is preceded by a decrease in total protein synthesis in tail tissues. We have demonstrated that thyroid hormone treatment of a tadpole does not affect the synthesis of all proteins equally in the tadpole tail muscle. For example, the synthesis of myosin heavy chains (MHCs) is depressed within 1 day and decreases to 45% of control values after 5 days of T3 treatment, whereas the decreased synthesis of soluble muscle proteins is transient and returns to above control levels by day 5. To determine whether the hormone-induced decrease in MHC synthesis is the result of changes in the transcription of translation of MHC mRNAs, we isolated cDNAs complementary to five different MHC mRNAs from a tail muscle cDNA library and used them to examine the levels of each MHC mRNA in the tail muscle of T3-treated tadpoles. mRNAs that recognize the cDNAs for these five different MHCs are all expressed in the tadpole tail and limb muscles, as well as in the adult leg muscles. MHC mRNAs unique to tadpole tail were not detected. Interestingly, the relative amounts of mRNA for four of the five MHCs increase in tail muscle after T3 treatment of the tadpole, suggesting that repression of MHC gene expression at the protein level does not result from a decrease in the amount of MHC mRNAs. Rather, these results support the contention that the decreased synthesis of MHCs in the tail muscle of T3-treated tadpoles is caused by this hormone, either directly or indirectly, depressing the translation of the MHC mRNAs in this tissue. These results, coupled with the observation that the synthesis of soluble muscle proteins is depressed only in a transient fashion, suggest that T3 may be initiating the expression of a gene(s) that encodes a protein(s) responsible for inhibiting the translation of the MHCs and, perhaps, other structural proteins in the tadpole tail muscle. Whatever the case, the translational regulation of MHC synthesis occurs well before any degradation of the tail tissue is evident and appears to be one of the earliest events in the hormone-induced cell death program of the tadpole tail muscle.

Identification and genomic cloning of CMHC1. A unique myosin heavy chain expressed exclusively in the developing chicken heart

We report the identification and cloning of a unique chick myosin heavy chain (CMHC1) that is expressed exclusively in the heart during embryogenesis. Using primers specific to myosin heavy chains, we used reverse transcriptase-polymerase chain reaction to clone and isolate CMHC1 from embryonic day 10 chicken heart RNA. Sequence analysis indicated that CMHC1 was a novel member of the myosin heavy chain family. Expression of the CMHC1 transcripts was detected in Hamburger Hamilton stage 10 chick embryos in the fusing myocardium. Expression of CMHC1 was maintained at high levels throughout the tubular heart of later stage embryos. Reverse transcriptase-polymerase chain reaction and in situ hybridizations failed to detect CMHC1 transcripts in the developing somites, limb buds, or skeletal musculature at any stage of chick development. Genomic CMHC1 clones have been isolated that contain sequences approximately 5.2 kilobase upstream of the presumptive CMHC1 transcription start site. Portions of the upstream regulatory region induced a 21-fold increase in reporter gene expression in primary cardiomyocytes. Because of its unique cardiac-restricted expression, CMHC1 will provide an excellent model system to study the molecular mechanisms required for the early developmental regulation of heart-specific genes.

Long-term denervation modulates differentially the accumulation of myogenin and MRF4 mRNA in adult Xenopus muscle

In adult Xenopus laevis, we analyzed, using reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization, the influence of long-term muscle denervation on the accumulation of MRF4 and myogenin transcripts. The brachial muscle was denervated by cutting the brachial nerve and was examined after 4 months. MRF4 mRNA levels decreased about two-fold in denervated muscle as compared with contralateral muscle. Myogenin mRNA levels, by contrast, were induced about five-fold by denervation. This report shows that muscle denervation persistently reduces the levels of MRF4 transcripts suggesting that MRF4 expression may be induced by innervation and hence may be involved in mediating transcriptional responses to innervation. The up-regulation of myogenin by denervation suggests that myogenin expression may compensate for the down-regulation of MRF4 gene.

Identification and expression analysis of two developmentally regulated myosin heavy chain gene transcripts in carp (Cyprinus carpio)

Whilst developmentally regulated genes for the myosin heavy chain (MyoHC) have been characterised in mammalian, avian and amphibian species, no developmental MyoHC gene has previously been characterised in a species of fish. In this study, we identify two developmentally regulated MyoHC gene transcripts (named Eggs22 and Eggs24) in carp (Cyprinus carpio) and characterise their expression patterns during embryonic and larval development. The transcripts showed an identical temporal pattern of expression commencing 22 h post-fertilisation (18 degrees C incubation temperature), coincident with the switch from exclusive expression of genes for beta-actin to expression of genes for both beta- and alpha-actin, and continuing for 2 weeks post-hatching. No expression of these myosin transcripts was detected in juvenile or adult carp. Wholemount in situ hybridisation showed that both transcripts are expressed initially in the rostral region of the developing trunk and progress caudally. Both are expressed in the developing pectoral fin and protractor hyoideus muscles. However, the muscles of the lower jaw express only the Eggs22 transcript. No expression of either transcript was detected in cardiac or smooth muscle. A distinct chevron pattern of expression was observed in the myotomal muscle. This was shown to be caused by localisation of the mRNAs to the myoseptal regions of the fibres, the sites of new sarcomere addition during muscle growth, suggesting transport of MyoHC mRNA transcripts. The 3′ untranslated region of the Eggs24 transcript contains a 10 base pair motif (AAAATGTGAA) which is shown to be also present in the 3′ untranslated regions of MyoHC genes from a wide range of species. Possible reasons for the need for developmental isoforms of myosin heavy chain isoforms are discussed.

Expression of myogenic regulatory factors during muscle development of Xenopus: myogenin mRNA accumulation is limited strictly to secondary myogenesis

To clarify the acquisition of the adult muscle pattern in Xenopus laevis, in situ hybridization and reverse transcriptase-polymerase chain reaction were used to correlate the time course of gene expression for myogenic regulatory factors (Myf-5, MyoD, and myogenin) with the expression of contractile protein (myosin heavy chain; MHC) genes during hindlimb formation compared with their expression in dorsal body muscles. After the precocious expression of Myf-5 and MyoD mRNA in limb bud (stage 50), myogenin mRNA strongly accumulated later at paddle stages (stages 52/53) concomitantly with the accumulation of both the larval and the adult MHC mRNAs. In dorsal body muscles, as early as stage 52, myogenin transcripts accumulated in a few small, secondary myofibers expressing the adult MHC mRNA that were located along the dorsomedial edge, but they were never detected in the large, primary myofibers of the body expressing the larval MHC mRNA. During metamorphosis, the areas expressing both the adult MHC and the myogenin transcripts gradually expanded from the dorsomedial edge to the ventral side of the dorsal body muscles, accounting for the progression of the secondary "adult" myogenesis described previously (Nishikawa and Hayashi [1994] Dev. Biol. 165:86-94). This work shows that, in Xenopus, the accumulation of myogenin mRNA is restricted to secondary myogenesis, including the formation of new muscles in developing limbs as well as in dorsal muscles during body remodeling. This shows that myogenin is not required for primary myogenesis, and it suggests a crucial role for myogenin in the terminal differentiation program, including myoblast fusion and the activation of adult-type muscle genes.

Four novel myosin heavy chain transcripts define a molecular basis for muscle fibre types in Rana pipiens

1. Differential expression of myosin heavy chain (MHC) isoforms dramatically affects mechanical and energetic properties of skeletal muscle fibre types. As many as five different fibre types, each with different mechanical properties, have been reported in frog hindlimb muscles. However, only two frog MHC isoforms have previously been detected by SDS-PAGE and only one adult hindlimb MHC isoform has been cloned. 2. In the present study, four different fibre types (type 1, type 2, type 3 and tonic) were initially identified in adult Rana pipiens anterior tibialis muscle based on myosin ATPase histochemistry, size and location. Each fibre type exhibited unique reactivity to a panel of MHC monoclonal antibodies. Single fibre analysis using SDS-PAGE revealed that MHCs from immunohistochemically defined type 1, type 2 and type 3 fibres ran as three distinct isoform bands, while MHC of tonic fibres co-migrated with type 1 MHC. The combined data from immunohistochemistry and SDS-PAGE suggests that Rana fibre types are composed of four different MHCs. 3. Four novel MHC cDNAs were cloned and expression of the corresponding transcripts was measured in single immuno-identified fibres using specific polymerase chain reaction (PCR) primer pairs. Each of the four transcripts was found to be primarily expressed in a different one of the four fibre types. 4. Coexpression of MHC isoforms was observed only between types 1/2 and types 2/3 at both the protein and mRNA level. 5. These data provide a molecular basis for differentiation between frog fibre types and permit future molecular studies of MHC structure/function and gene regulation in this classic physiological system. 6. Comparison of sequence homology among amphibian, avian and mammalian MHC families supports the concept of independent evolution of fast MHC genes within vertebrate classes subsequent to the amphibian/avian/mammalian radiation.

Localization of Myf-5, MRF4 and alpha cardiac actin mRNAs in regenerating Xenopus skeletal muscle

We have analysed the spatial and temporal expression patterns of Myf-5, MRF4 and alpha cardiac actin mRNAs during muscle regeneration following cardiotoxin injury in adult Xenopus laevis using in situ hybridization. Myf-5 transcripts began to be detected in the activated satellite cells as early as the beginning of the regeneration process, then dramatically decreased in young plurinucleated myotubes. MRF4 mRNA was detected later, just before the young myotube stage, and was strongly expressed during the different stages of the maturation of myotubes. Like Myf-5, alpha cardiac actin mRNA began to accumulate early in activated satellite cells. These results, which contribute to an overview of the expression of the genes coding for myogenic bHLH proteins during muscle regeneration, are discussed in relation to the expression of these factors during development.

Ultrastructure of the larval tentacle and its skeletal muscle in Xenopus laevis

During premetamorphic development, tadpoles of Xenopus laevis possess a transitory pair of long, slender, mobile tentacles situated at the corners of the mouth. Microscopic examination of the larval tentacle typically reveals three distinct compartments: a central core of cartilage, a laterally situated skeletal muscle, and a nerve supply medially. Along the length of each tentacle, the epidermis is supplied by many unmyelinated nerve fibers, presumably sensory in nature, which terminate as naked axons in close association with the epidermal cells. The striated tentacular muscle, in the proximal region of the lateral compartment, consists of extrafusal muscle fibers of varying size which range in number from 36 to 48 per tentacle (n = 10). Using morphometric criteria, we have classified the skeletal muscle fibers of the larval tentacular muscle into three types: large (30-50 microns), intermediate (20-30 microns), and small (10-20 microns). By electron microscopy, each type displays characteristic sarcomeric banding patterns, sarcotubular and mitochondrial disposition, and motor endplate ultrastructure. Our morphological observations indicate that the tentacles of the Xenopus tadpole are complex mobile facial extensions which may play roles in mechanoreception and/or chemoreception during the waterborne stages of development. Because of its transitory nature, the Xenopus tentacle may be a useful experimental model in future studies of neuromuscular development and subsequent regression in a relatively short period of time.

Tail regression, apoptosis and thyroid hormone regulation of myosin heavy chain isoforms in Xenopus tadpoles

T3 effects on myosin heavy chain gene expression were analysed in muscles undergoing different fates during metamorphosis. Muscle fate was followed by somatic gene transfer of a constitutively expressed luciferase vector. Persistent expression was found in dorsal muscle which is remodelled during metamorphosis whilst the signal disappeared in apoptosing caudal muscle. RNAse protection assay was used to follow production of myosin heavy chain isoforms: two isoforms identified as embryonic (E3 and E19) and one adult form (A7). The effects of T3 treatment were followed over 120 h. During this time frame E3 and A7 expression patterns were found to be similar in both caudal and dorsal muscles. Most notably, at 48 h E3 expression was significantly down-regulated and production of A7 significantly upregulated in both caudal and dorsal muscle. Thus T3-induced transitions in muscle gene expression are independent of muscle fate during amphibian metamorphosis.

Cloning of a trout fast skeletal myosin heavy chain expressed both in embryo and adult muscles and in myotubes neoformed in vitro

In fish, little is known about the isoforms of myosin heavy chain in developing muscle. Two cDNA libraries from whole skeletal muscle of embryo (eyed stage) (A) and from white muscle of 300 g body weight immature trout (B) were constructed. Three cDNA clones were isolated and characterised as encoding for a fast skeletal myosin heavy chain. Two cDNA clones A1 (1534 bp) and B6 (2203 bp) which were extracted from the two different libraries had the same nucleotide sequence including the 3' untranslated region. The third cDNA B8 (1606 bp) shared 98% identity with the others. The latter could possibly be an allelic isoform of the B6. Northern blot analysis revealed that the fast skeletal MyoHC transcripts were expressed throughout development from myotube appearance to the white muscle present at older stages (adult). These results suggest that this myosin heavy chain is present throughout muscle development in fish and are consistent with the hyperplastic growth of fish muscle. The amino acid sequence of the trout myosin heavy chain diverged from its mammalian and avain counterpart with respect to a higher level of glycine which could be related to an environmental adaptation by increasing thermal instability of the molecule.

Analysis of MyoD, myogenin, and muscle-specific gene mRNAs in regenerating Xenopus skeletal muscle

We have analyzed in adult Xenopus laevis, using in situ hybridization, the spatial and temporal expression patterns of MyoD, myogenin, and alpha-skeletal actin and fast myosin heavy chain mRNAs during muscle regeneration following cardiotoxin injury. MyoD transcripts could be detected in the satellite cells as early as the first stage of regeneration and were expressed persistently throughout the regeneration process. Myogenin mRNAs were transiently expressed in forming myotubes. alpha-Skeletal actin and fast myosin heavy chain mRNAs were detected precociously, before the young myotube stage. This work has shown, for the first time, the presence of myogenin transcripts during Xenopus myogenesis.

Cardiac myosin heavy chain expression during heart development in Xenopus laevis

Muscle-specific gene expression in the heart during Xenopus development was investigated using reverse transcription-polymerase chain reaction (RT-PCR) and whole-mount in situ hybridization to detect transcripts of the gene for the cardiac myosin heavy chain (CMHC). RT-PCR analysis determined that CMHC transcripts are present in the cardiac mesoderm at state 13, demonstrating that muscle-specific gene expression in the primitive myocardium has begun by the early neurula stage, approximately 30 h before the heart beat begins. Xenopus, therefore, is similar to amniotes and mammals in that cardiac precursor cells begin to express muscle-specific gene transcripts soon after commitment to the cardiac myocyte lineage. The earliest CMHC gene transcripts can be detected in the heart using whole-mount in situ hybridization is early tailbud stage 28, which coincides with the onset of heart tube morphogenesis. CMHC gene expression was also detected in skeletal muscle: RT-PCR analysis determined that CMHC transcripts are transiently expressed in the somite during the initial phases of skeletal muscle differentiation. Furthermore, CMHC mRNAs are expressed in a subset of head muscles of the feeding tadpole. CMHC gene expression is induced in ectodermal cells of the animal cap in blastula-stage embryos injected with synthetic MyoD or Myf5 RNA, suggesting that the CMHC gene contains regulatory elements that are responsive to the activity of those skeletal-muscle-specific transcription factors.

Different sensitivity to amiloride of body and tail skins of Rana catesbeiana tadpoles during metamorphosis

Regional differences in potential difference and short-circuit current between the body (dorsal) and the tail skin during metamorphosis of Rana catesbeiana tadpoles were investigated. In body skin, the potential difference and the short-circuit current across the skin develop in two successive steps. At stage XX, the potential difference and the short-circuit current across the body skins were amiloride-insensitive (1st step). At stage XXII, however, amiloride-sensitive potential difference and the short circuit current appeared (2nd step). By contrast, in tail skin the potential difference and the short-circuit current remained amiloride-insensitive (1st step) even at stage XXIII. Since the tail regresses after stage XXIII, the appearance of the second step could not be followed in vivo. To determine whether or not the second step can be induced in the tail, tail skin was cultured under conditions where the skin survives for a much longer period than it does in normally developing tadpoles. Such cultured tail skin generated the amiloride-sensitive potential difference and the short-circuit current and cultured body skin also generated them. Therefore, development of the 2nd step in the tail skin may be delayed in vivo. To characterize the differences between body and tail skin, skins were mutually grafted between body and tail at stage XIII-XV. The body skin grafted on the tail underwent both the 1st and 2nd steps by stage XXII, whereas the tail skin grafted on the body only showed the 1st step by the same stage. These results suggest that the regional specificity of the skin is already established before the prometamorphic stage.

Cloning of the cDNA encoding a myosin heavy chain B isoform of Xenopus nonmuscle myosin with an insert in the head region

The complete amino acid sequence of Xenopus laevis nonmuscle myosin heavy chain B (MHC-B) has been deduced from overlapping cDNA clones isolated from an XTC cell library. RNA blots of various developmental stages, adult tissues, and XTC cells detect a single transcript of 7.5 kb which is expressed at similar levels throughout development. MHC-B mRNA was detected in XTC cells, heart, lung, spleen, and brain, at lower levels in ovary, testis, pancreas, stomach, liver, and eye, but not in kidney and skeletal muscle. Protein expression in adult tissues, as detected by immunoblot analysis, correlates well with mRNA expression. In chickens and humans, a fraction of the mRNA encoding the MHC-B isoform was found previously to contain a 10-amino acid insert at amino acid 211 near the ATP-binding site. As reported elsewhere, in the chicken this insert-bearing isoform is nervous system-specific. The Xenopus sequence shows a 16-amino acid insertion at the same position; 7 of 16 residues are identical to those in the chicken and human insertion, and these identical residues include a consensus target sequence for cyclin-p34cdc2 kinase. In contrast to chicken, all frog tissues and embryonic stages tested contained the insert-bearing form, and no evidence for a non-insert-bearing MHC-B isoform was found in Xenopus.

The evolutionary relationship of avian and mammalian myosin heavy-chain genes

Sequence comparisons of avian and mammalian skeletal and cardiac myosin heavy-chain isoforms are used to examine the evolutionary relationships of sarcomeric myosin multigene families. Mammalian fast-myosin heavy-chain isoforms from different species, with comparable developmental expression, are more similar to each other than they are to other fast isoforms within the same genome. In contrast, the developmentally regulated chicken fast isoforms are more similar to each other than they are to myosin heavy-chain isoforms in other species. Extensive regions of nucleotide identity among the chicken fast myosin heavy chains and in the mouse and rat alpha- and beta-cardiac myosin heavy-chain sequences suggest that gene-conversion-like mechanisms have played a major role in the concerted evolution of these gene families. We also conclude that the chicken fast myosin heavy-chain multigene family has undergone recent expansion subsequent to the divergence of birds and mammals and that both the developmental regulation and the specialization of myosin isoforms have likely developed independently in birds and mammals.

Contractile protein isoforms in muscle development

The contractile proteins of skeletal muscle are often represented by families of very similar isoforms. Protein isoforms can result from the differential expression of multigene families or from multiple transcripts from a single gene via alternative splicing. In many cases the regulatory mechanisms that determine the accumulation of specific isoforms via alternative splicing or differential gene expression are being unraveled. However, the functional significance of expressing different proteins during muscle development remains a key issue that has not been resolved. It is widely believed that distinct isoforms within a family are uniquely adapted to muscles with different physiological properties, since separate isoform families are often coordinately regulated within functionally distinct muscle fiber types. It is also possible that different isoforms are functionally indistinguishable and represent an inherent genetic redundancy among critically important muscle proteins. The goal of this review is to assess the evidence that muscle proteins which exist as different isoforms in developing and mature skeletal and cardiac muscles are functionally unique. Since regulation of both transcription and alternative splicing within multigene families may also be an important factor determining the accumulation of specific protein isoforms, evidence that genetic regulation rather than protein coding information provides the functional basis of isoform diversity is also examined.

Sexually dimorphic expression of a laryngeal-specific, androgen-regulated myosin heavy chain gene during Xenopus laevis development

Masculinization of the larynx in Xenopus laevis frogs is essential for the performance of male courtship song. During postmetamorphic (PM) development, the initially female-like phenotype of laryngeal muscle (slow and fast twitch fibers) is converted to the masculine form (entirely fast twitch) under the influence of androgenic steroids. To explore the molecular basis of androgen-directed masculinization, we have isolated cDNA clones encoding portions of a new Xenopus myosin heavy chain (MHC) gene. We have detected expression of this gene only in laryngeal muscle and specifically in males. All adult male laryngeal muscle fibers express the laryngeal myosin (LM). Adult female laryngeal muscle expresses LM only in some fibers. Expression of LM during PM development was examined using Northern blots and in situ hybridization. Males express higher levels of LM than females throughout PM development and attain adult levels by PM3. In females, LM expression peaks transiently at PM2. Treatment of juvenile female frogs with the androgen dihydrotestosterone masculinizes LM expression. Thus, LM appears to be a male-specific, testosterone-regulated MHC isoform in Xenopus laevis. The LM gene will permit analysis of androgen-directed sexual differentiation in this highly sexually dimorphic tissue.

Organization of non-muscle myosin during early murine embryonic differentiation

A monoclonal antibody (3D10) recognizing myosin heavy chain was isolated following immunization with a synthetic peptide sequence of eight amino acids. The antibody reacted with purified rabbit skeletal myosin and light mero-myosin in enzyme-linked immunosorbent assays and Western immunoblotting. A band of approximately 200 kDa was detected in cell extracts of an embryonal carcinoma (EC) cell line (P19EC) and one of its cloned differentiated derivatives, suggesting reactivity against non-muscle myosin. By indirect immunofluorescence, typical myosin banding patterns were observed in cryostat sections of human skeletal and cardiac muscle tissue. In undifferentiated P19EC cells, speckled immunofluorescent staining was observed in the cytoplasm that became organized in cortical rings where the cells made direct contact with each other. These rings consisted of circular bundles of F-actin decorated by myosin. Undifferentiated embryonic stem (ES) cells derived directly from mouse embryos shared the same features, although the pattern was less pronounced. Human testicular primary germ cell tumours showed cortical staining in the embryonal carcinoma component reminiscent of the staining of EC cells in vitro while cytoplasmic staining was observed in tumour cells with a differentiated morphology. In preimplantation embryos, the immunofluorescent staining was observed at cell apices of blastomeres of morula stage embryos. In blastocysts, staining of inner cell mass cells was not detectable. By contrast, various differentiated derivatives of P19EC contained extensive F-actin microfilament bundles throughout the cytoplasm decorated with myosin. Thick stress fibers in filopodious extensions of cells were particularly highly decorated by myosin. Over the nucleus, linear arrays of myosin containing speckled patterns of immunofluorescence were observed that were not associated with F-actin. The same pattern of staining could be observed in trophectoderm cells of the blastocyst. We conclude that embryonic non-muscle myosin is organized in specific patterns depending on the state of differentiation. As the myosin is primarily associated with F-actin we suspect that it forms part of a contractile apparatus that may have significance during embryonic development.

Tunicamycin-inducible polypeptide synthesis during Xenopus laevis embryogenesis

Tunicamycin treatment of Xenopus laevis embryos enhanced the synthesis of a specific set of polypeptides with molecular masses of 98, 78, 59 and 58 kDa. The 78-kDa polypeptide was tentatively identified as glucose-regulated protein (GRP) 78 on the basis of molecular mass, pl (5.2), and tunicamycin inducibility, which took place upon treating embryos after the midblastula transition (MBT). The synthesis of a polypeptide with this electrophoretic mobility was detected but was not tunicamycin-inducible at stages prior to the MBT. GRP78 mRNA was detectable before the MBT but was not inducible by tunicamycin until the tailbud stage. A comparison of tunicamycin-induced polypeptide synthesis in Xenopus embryos, A6 cell line, and white blood cells by 2D-PAGE and fluorography revealed three spots in the GRP78 region of the gel. One was observed in both embryos and adult cells; another was adult-specific; and the third one was possibly an embryo-specific form. These results suggest that GRP78 synthesis might undergo a switch from an embryonic to an adult pattern during Xenopus development.

Transcription of endogenous and injected cytoskeletal actin genes during early embryonic development in Xenopus laevis

The transcriptional regulation of a cytoskeletal actin gene during Xenopus laevis embryonic development has been investigated. New transcripts of this gene begin to accumulate at approximately the mid-gastrula stage, between 12 and 16 hours after fertilization, replenishing maternal supplies of this transcript. To study the molecular processes which act to determine the timing of transcriptional activation of this gene, a gene-injection assay was devised, utilizing a cloned copy of the gene which has been marked by a small DNA insertion. Accurately-initiated transcripts of the injected gene accumulate in concert with those of the endogenous gene, showing that injected genes can undergo developmental regulation. As little as 485 nucleotides of upstream sequence is sufficient for proper temporal control of activation of an injected gene. The results presented here demonstrate the feasibility of a microinjection assay for the identification of regulatory gene sequences and transacting regulatory factors in amphibian embryos. Such an assay will be useful in achieving an understanding of general transcriptional control mechanisms acting in early development, and should also provide a means to study certain aspects of long-standing developmental problems, such as cytoplasmic localization and embryonic induction.

Functions of maternal mRNA in early development

In this review, the types of mRNAs found in oocytes and eggs of several animal species, particularly Drosophila, marine invertebrates, frogs, and mice, are described. The roles that proteins derived from these mRNAs play in early development are discussed, and connections between maternally inherited information and embryonic pattern are sought. Comparisons between genetically identified maternally expressed genes in Drosophila and maternal mRNAs biochemically characterized in other species are made when possible. Regulation of the meiotic and early embryonic cell cycles is reviewed, and translational control of maternal mRNA following maturation and/or fertilization is discussed with regard to specific mRNAs.