Genome of the rainbow trout (Oncorhynchus mykiss) encodes two distinct muscle regulatory factors with homology to myoD

The roles of two myostatins and immune effects after inhibition in Qi river crucian carp (Carassius auratus)

Myostatin, through type I receptor (kinase 4, 5, ALK4/5), functions to participate in the immune system and negatively regulate muscle growth in mammals. However, the role of myostatin (mstn) in the immune system of teleosts is largely unknown. In a previous study, we cloned the mstn1 cDNA encoding myostatin in Qi river crucian carp (Carassius auratus). In the present study, we have cloned mstn2 cDNA, which was characterized and analyzed together with mstn1. Tissue distribution analysis showed that both mstn genes are expressed in numerous tissues, with mstn1 dominantly expressed in the muscle and brain, whereas mstn2 is mainly expressed in the brain. During embryogenesis, mstn1 and mstn2 exhibit different expression patterns. Both mstn1 and mstn2 expression increased stepwise in the brain at different developmental stages. Furthermore, both genes are differentially regulated during different periods of fasting/re-feeding. Following the exposure of C. auratus to polyI:C, lipopolysaccharide (LPS), and Aeromonas hydrophila, both genes were upregulated in different tissues, which indicated that they might be involved in the immune response against pathogenic invasion. Blocking the Mstn signal pathway with SB-431542 (a chemical inhibitor of ALK4/5) resulted in significantly increased body length and weight. However, the mortality of SB-431542-treated fish was higher after A. hydrophila challenge. Moreover, decreased expression of lysozymes (lyz), complement component 3 (c3), β-defensin 3 (defb3), and interferon γ (ifnγ) were exhibited in treated fish, compared with the controls. Furthermore, the expression of nf-κb1, three pro-inflammatory cytokines (il1β, il6, and tnfα), and inflammatory cytokines (il8 and il10) were significantly increased in both the SB-431542-treated group and the control after A. hydrophila infection, suggesting that the NF-κB pathway was not suppressed in the SB-431542-treated fish. Taken together, our data suggest that both mstn1 and mstn2 play important roles in early body development, muscle growth, and the immune system by acting downstream of the NF-κB signal pathway.

Skeletal myogenesis in the zebrafish and its implications for muscle disease modelling

Current evidence indicates that post-embryonic muscle growth and regeneration in amniotes is mediated almost entirely by stem cells derived from muscle progenitor cells (MPCs), known as satellite cells. Exhaustion and impairment of satellite cell activity is involved in the severe muscle loss associated with degenerative muscle diseases such as Muscular Dystrophies and is the main cause of age-associated muscle wasting. Understanding the molecular and cellular basis of satellite cell function in muscle generation and regeneration (myogenesis) is critical to the broader goal of developing treatments that may ameliorate such conditions. Considerable knowledge exists regarding the embryonic stages of amniote myogenesis. Much less is known about how post-embryonic amniote myogenesis proceeds, how adult myogenesis relates to embryonic myogenesis on a cellular or genetic level. Of the studies focusing on post-embryonic amniote myogenesis, most are post-mortem and in vitro analyses, precluding the understanding of cellular behaviours and genetic mechanisms in an undisturbed in vivo setting. Zebrafish are optically clear throughout much of their post-embryonic development, facilitating their use in live imaging of cellular processes. Zebrafish also possess a compartment of MPCs, which appear similar to satellite cells and persist throughout the post-embryonic development of the fish, permitting their use in examining the contribution of these cells to muscle tissue growth and regeneration.

Dietary carbohydrate level affects transcription factor expression that regulates skeletal muscle myogenesis in rainbow trout

Understanding the effects of dietary carbohydrates on transcription factors that regulate myogenesis provides insight into the role of nutrient sensing by satellite cells towards myocyte differentiation. We evaluated the influence of dietary carbohydrate level (0, 15, 25 or 35%) on the temporal mRNA expression patterns (4, 8 or 12 weeks) of transcription factors that regulate satellite cell myocyte addition (MA) in rainbow trout (Oncorhynchus mykiss), a vertebrate with indeterminate growth. Relative to the 0% carbohydrate (NC) diet, 15 (IC-15) and 25% (IC-25) carbohydrate containing diets significantly up-regulate MyoD and Myf5, but not Pax7, after 12 weeks of feeding. Simultaneously, the Pax7/MyoD mRNA expression ratio declined significantly with both the IC diets. Myogenin mRNA expression also increased in rainbow trout (RBT) fed the IC-15 diet. The high carbohydrate (HC) diet (35%) attenuated the increased mRNA expression of these transcription factors. It is of note that the 4 and 8 week samples lacked the promyogenic expression patterns. The myogenic gene expression in fish fed the IC-15 diet for 12 weeks indicate a transcriptional signature that reflects increased satellite cell myogenesis. Our results suggest a potential role for satellite cells in the nutrient sensing ability of a vertebrate with indeterminate skeletal muscle growth.

An update on MyoD evolution in teleosts and a proposed consensus nomenclature to accommodate the tetraploidization of different vertebrate genomes

Background

MyoD is a muscle specific transcription factor that is essential for vertebrate myogenesis. In several teleost species, including representatives of the Salmonidae and Acanthopterygii, but not zebrafish, two or more MyoD paralogues are conserved that are thought to have arisen from distinct, possibly lineage-specific duplication events. Additionally, two MyoD paralogues have been characterised in the allotetraploid frog, Xenopus laevis. This has lead to a confusing nomenclature since MyoD paralogues have been named outside of an appropriate phylogenetic framework.

Methods and Principal Findings

Here we initially show that directly depicting the evolutionary relationships of teleost MyoD orthologues and paralogues is hindered by the asymmetric evolutionary rate of Acanthopterygian MyoD2 relative to other MyoD proteins. Thus our aim was to confidently position the event from which teleost paralogues arose in different lineages by a comparative investigation of genes neighbouring myod across the vertebrates. To this end, we show that genes on the single myod-containing chromosome of mammals and birds are retained in both zebrafish and Acanthopterygian teleosts in a striking pattern of double conserved synteny. Further, phylogenetic reconstruction of these neighbouring genes using Bayesian and maximum likelihood methods supported a common origin for teleost paralogues following the split of the Actinopterygii and Sarcopterygii.

Conclusion

Our results strongly suggest that myod was duplicated during the basal teleost whole genome duplication event, but was subsequently lost in the Ostariophysi (zebrafish) and Protacanthopterygii lineages. We propose a sensible consensus nomenclature for vertebrate myod genes that accommodates polyploidization events in teleost and tetrapod lineages and is justified from a phylogenetic perspective.

Temperature influences the coordinated expression of myogenic regulatory factors during embryonic myogenesis in Atlantic salmon (Salmo salarL.)

Potential molecular mechanisms regulating developmental plasticity to temperature were investigated in Atlantic salmon embryos (Salmo salarL.). Six orthologues of the four myogenic regulatory factors (MRFs:individually: smyf5, smyoD1a/1b/1c, smyoG and sMRF4), the master transcription factors regulating vertebrate myogenesis, were characterised at the mRNA/genomic level. In situ hybridisation was performed with specific cRNA probes to determine the expression patterns of each gene during embryonic myogenesis. To place the MRF data in the context of known muscle fibre differentiation events, the expression of slow myosin light chain-1 and Pax7 were also investigated. Adaxial myoblasts expressed smyoD1a prior to and during somitogenesis followed by smyoD1c (20-somite stage, ss),and sMRF4 (25–30 ss), before spreading laterally across the myotome, followed closely by the adaxial cells. Smyf5 was detected prior to somitogenesis, but not in the adaxial cells in contrast to other teleosts studied. The expression domains of smyf5, smyoD1band smyoG were not confined to the s-smlc1 expression field,indicating a role in fast muscle myogenesis. From the end of segmentation,each MRF was expressed to a greater or lesser extent in zones of new muscle fibre production, the precursor cells for which probably originated from the Pax7 expressing cell layer external to the single layer of s-smlc1+ fibres. SmyoD1a and smyoGshowed similar expression patterns with respect to somite stage at three different temperatures investigated (2°C, 5°C and 8°C) in spite of different rates of somite formation (one somite added each 5 h, 8 h and 15 h at 8°C, 5°C and 2°C, respectively). In contrast, the expression of smyf5, sMRF4 and s-smlc1 was retarded with respect to somite stage at 2°C compared to 8°C, potentially resulting in heterochronies in downstream pathways influencing later muscle phenotype.

Role of insulin, insulin-like growth factors, and muscle regulatory factors in the compensatory growth of the trout (Oncorhynchus mykiss)

To examine the various mechanisms involved in compensatory growth in Oncorhynchus mykiss, an experimental protocol involving 1, 2 or 4 weeks of fasting followed by a single ad libitum re-feeding period of 4 weeks was designed for alevins. Morphological parameters including body weight, specific growth rates (SGR), and coefficient factor decreased significantly during fasting. Re-feeding accelerated growth and restored final body weight in groups previously fasted. Plasma insulin and glucose decreased in fasting, while normal levels were restored in all re-fed groups. The expression profile of insulin-like growth factors (IGFs) in liver and of the main muscle growth regulators in white muscle was examined using real-time quantitative RT-PCR. Fasting decreased the expression of IGF-I mRNA in both tissues, while re-feeding restored expression to control values. In contrast, IGF-II expression was not affected by any treatment in either tissue. Insulin- and IGF-I-binding assays in partial semi-purifications (of soluble proteins) in white skeletal muscle showed that insulin binding was not affected by either fasting or re-feeding, whereas fasting up-regulated IGF-I binding. The expression of IGFRIb mRNA in white skeletal muscle also increased with fasting, while IGFRIa increased with re-feeding, indicating that the two receptor isoforms are differentially regulated. The mRNA expression of myogenic regulator factors and fibroblast growth factors (FGFs) was not affected throughout the experiment, except for myogenin, which first decreased and then showed a rebound effect after 4 weeks of fasting. Myostatin mRNA expression did not change during fasting, although re-feeding caused a significant decrease. In conclusion, re-feeding of previously fasted trout induced compensatory growth. The differential regulation in muscle expression of IGF-I, IGF-I receptors, and myostatin indicates their contribution to this compensatory mechanism.

Characterisation and expression of the paired box protein 7 (Pax7) gene in polymorphic Arctic charr (Salvelinus alpinus)

Arctic charr (Salvelinus alpinus L.) from Lake Thingvallavatn, Iceland occur as four distinct morphs: large benthivorous (LB), dwarf benthivorous (DB), piscivorous (PI) and planktonivorous (PL). The morphs differ with respect to body size, head morphology, growth rate, and life history. The aim of this study was to investigate the paired box protein 7 (Pax7) gene as a candidate for such polymorphisms due to its importance in cranio-facial, skeletal muscle, and central nervous system development. No variation in coding and intronic sequences was found between morphs. We identified 10 alternate Pax7 isoforms with insertions/deletions: a four-residue (GNRT) deletion, a GEASS insertion truncated by the first serine residue (GEAS), and a thirteen-residue insertion (GQYA/TGPEYVYCGT). The latter insertion with a threonine (T) contains a putative casein kinase II (CK-2) phosphorylation site. Pax7 spatial expression patterns were identical in embryos of DB-, LB-, and PL-morphs, and were similar to those described for zebrafish Pax7c, but a difference in temporal expression for segmentation was observed between DB and LB morphs. At the end of segmentation, novel expression was observed in the mandibular region as two bilateral domains. The potential role of multiple alternative splicing of the Pax7 gene for the generation of different Arctic charr morphs is briefly discussed.

Somite formation and expression ofMyoD,myogeninandmyosinin Atlantic halibut (Hippoglossus hippoglossusL.)embryos incubated at different temperatures: transient asymmetric expression ofMyoD

Genes encoding the myogenic regulating factors MyoD and myogenin and the structural muscle proteins myosin light chain 2 (MyLC2) and myosin heavy chain(MyHC) were isolated from juvenile Atlantic halibut (Hippoglossus hippoglossus L.). The impact of temperature on their temporal and spatial expression during somitogenesis were examined by incubating halibut embryos at 4, 6 and 8°C, and regularly sampling for whole-mount in situhybridisation and reverse transcription (RT)–PCR.

There were no significant effects of temperature on the onset of somitogenesis or number of somites at hatching. The rate of somite formation increased with increasing temperature, and the expression of MyoD, myogenin and MyHC followed the cranial-to-caudal somite formation. Hence, no significant effect of temperature on the spatial and temporal expression of the genes studied was found in relation to somite stage. MyoD, which has subsequently been shown to encode the MyoD2 isoform, displayed a novel bilaterally asymmetric expression pattern only in white muscle precursor cells during early halibut somitogenesis. The expression of myogenin resembled that previously described for other fish species, and preceded the MyHC expression by approximately five somites. Two MyLC2 cDNA sequences were for the first time described for a flatfish, probably representing embryonic (MyLC2a) and larval/juvenile(MyLC2b) isoforms.

Factors regulating muscle determination, differentiation and development have so far mostly been studied in vertebrates with external bilateral symmetry. The findings of the present study suggest that more such investigations of flatfish species could provide valuable information on how muscle-regulating mechanisms work in species with different anatomical,physiological and ecological traits.

Insight into the complex genetic network of tetraploid Atlantic salmon (Salmo salar L.): description of multiple novel Pax-7 splice variants

Paired box transcription factor 7 (Pax-7) cDNA was isolated from the skeletal muscle and brain of alevin and adult stages of Atlantic salmon, identifying 10 variants categorised as novel or established insertions (ins) or deletions (del). Two putative Pax-7 paralogs were identified (denoted Pax-7alpha and Pax-7beta) on the basis of the length and sequences of intron 3 (218 and 248 bp) and versions of ins1 and ins2. Pax-7beta contained a threonine variant of ins1 (GQY[T]GPEYVYCGT), and a shortened variant of ins2 (GEAS). Pattern identification revealed the threonine variant of ins1 includes a potential phosphorylation site (casein kinase II). Thus, the tetraploid Atlantic salmon genome appears to contain at least two putative copies and multiple splice variants of Pax-7. In situ hybridisation localised Pax-7 to mononuclear cells in the fast muscle of adult Atlantic salmon, while quantitative real-time PCR showed Pax-7alpha to be more highly expressed in brain than in skeletal muscle.

Alterations in expression of genes associated with muscle metabolism and growth during nutritional restriction and refeeding in rainbow trout

Rainbow trout, as well as many other species of fish, demonstrate the ability to survive starvation for long periods of time. During starvation, growth rate is decreased and muscle exhibits signs of wasting. However, upon resumption of feeding, accelerated growth is often observed. Alterations in muscle metabolism occur during feed restriction and refeeding, although the ways in which these alterations affect the molecular pathways that control muscle growth have not been fully determined. To analyze changes in muscle metabolism and growth during starvation and refeeding, real-time PCR was used to test the expression of six metabolic-related genes and eight muscle-specific genes in rainbow trout white muscle prior to and after 30 days of starvation, and after 4 and 14 days of refeeding. The six metabolic-related genes chosen are indicative of specific metabolic pathways: glycolysis, glycogenesis, gluconeogenesis, the pentose phosphate pathway, and fatty acid formation. The eight muscle specific genes chosen are key components in muscle growth and structural integrity, i.e., MRFs, MEFs, myostatins, and myosin. Alterations in expression of the tested metabolic-related genes and muscle-specific genes suggest that during both starvation and refeeding, changes in specific metabolic pathways initiate shifts in muscle that result mainly in the modification of myotube hypertrophy. The expression levels of many of the metabolic-related genes were altered during the refeeding period compared to those observed before the starvation period began. However, the accelerated growth often observed during refeeding is likely driven by changes in normal muscle metabolism, and the altered expression observed here may be a demonstration of those changes.

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.

Cloning and characterization of myogenic regulatory genes in three Ictalurid species

We report sequence, tissue expression and map-position data for myogenin, MYOD1, myostatin and follistatin in three Ictalurid catfish species: channel catfish (Ictalurus punctatus), blue catfish (I. furcatus) and white catfish (Ameiurus catus). These genes are involved in muscle growth and development in mammals and may play similar roles in catfish. Amino acid sequences were highly conserved among the three Ictalurid species (>95% identity), moderately conserved among catfish and zebrafish (approximately 80% identity), and less conserved among catfish and humans (approximately 40-60% identity) for all four genes. Gene structure (number of exons and introns and exon-intron boundaries) was conserved between catfish and other species for all genes. Myogenin and MYOD1 expression was limited to skeletal muscle in juvenile channel catfish, similar to expression patterns for these genes in other fish and mammalian species. Myostatin was expressed in a variety of tissues in juvenile channel catfish, a pattern common in other fish species but contrasting with data from mammals where myostatin is primarily expressed in skeletal muscle. Follistatin was expressed in juvenile catfish heart, testes and spleen. All four genes contained polymorphic microsatellite repeats in non-coding regions and linkage analysis based on inheritance of these microsatellite loci was used to place the genes on the channel catfish linkage map. Information provided in this study will be useful in further studies to determine the role these genes play in muscle growth and development in catfish.

Evolution of duplicated growth hormone genes in autotetraploid salmonid fishes

A defining character of the piscine family Salmonidae is autotetraploidy resulting from a genome-doubling event some 25-100 million years ago. Initially, duplicated genes may have undergone concerted evolution and tetrasomic inheritance. Homeologous chromosomes eventually diverged and the resulting reduction in recombination and gene conversion between paralogous genes allowed the re-establishment of disomic inheritance. Among extant salmonine fishes (e.g. salmon, trout, char) the growth hormone (GH) gene is generally represented by two functional paralogs, GH1 and GH2. Sequence analyses of salmonid GH genes from species of subfamilies Coregoninae (whitefish, ciscos) and Salmoninae were used to examine the evolutionary history of the duplicated GH genes. Two divergent GH gene paralogs were also identified in Coregoninae, but they were not assignable to the GH1 and GH2 categories. The average sequence divergence between the coregonine GH genes was more than twofold lower than the corresponding divergence between the salmonine GH1 and GH2. Phylogenetic analysis of the coregonine GH paralogs did not resolve their relationship to the salmonine paralogs. These findings suggest that disomic inheritance of two GH genes was established by different mechanisms in these two subfamilies.

Sequence, conservation, and quantitative expression of rainbow trout Myf5

The success of rainbow trout as an aquaculture species is dependent on the ability to produce fish with large amounts of high-quality lean muscle. It is therefore important to understand not only the best conditions under which to raise the fish but also the molecular control of muscle growth. Vertebrate muscle growth is initiated by the specification of myogenic precursor cells into myoblasts. The myoblasts proliferate and fuse to form multinucleated myotubes, which mature into myofibers. A family of basic helix-loop-helix (bHLH) transcription factors, the Myogenic Regulatory Factors (MRFs), controls these events. In trout, two MRF-encoding genes, TMyoD (of which there are two) and Tmyogenin, have been identified. However, the primary MRF-encoding Myf5 is not yet sequenced. Here, using degenerate PCR and 5' and 3' RACE, the cDNA sequence of trout Myf5 (TMyf5) is identified. Translation of the cDNA reveals that TMyf5 is a bHLH protein with homology to Myf5 and MRFs in other organisms. It is expressed mainly in red and white muscle, suggesting that it shares functional homology to Myf5 in other species. The molecular control of muscle growth has been well-characterized in mammals, but there are differences in the growth of fish muscle, highlighting the need for characterization of MRFs in fish species, particularly those in which understanding muscle growth will have a positive impact on the economic potential of the species.

Genomic characterization of a novel pair of ID genes in the rainbow trout (Oncorhynchus mykiss)

The ID (inhibitors of DNA binding/differentiation) proteins represent a family of dominant negative regulators of the basic helix-loop-helix transcription factors whose activities result in delayed cell differentiation and prolonged proliferation. A pair of expressed sequence tag clones with homologies to the ID proteins were identified and used to screen a rainbow trout bacterial artificial chromosome (BAC) library to identify clones containing homologues sequences. Phylogenetic analysis of the predicted amino acid sequences revealed close similarities to the rainbow trout ID1 protein, the genes were therefore classified as rainbow trout ID1B and ID1C. Genome characterization based on BAC sequencing showed each gene to have two exons separated by a small intron. The genes are 83% similar in their transcribed regions, yet they are only 64 and 65% similar in the upstream and downstream sequences, respectively. Using reverse transcription polymerase chain reaction, we found both genes to be expressed in a variety of tissues in the adult rainbow trout.

Transcription of MyoD and myogenin in the non-contractile electrogenic cells of the weakly electric fish, Sternopygus macrurus

The MyoD family of basic helix-loop-helix (bHLH) myogenic regulatory factors (MRFs) are transcriptional activators of skeletal muscle gene expression and are pivotal in inducing the full myogenic program. The expression of these factors after muscle differentiation is complete and the mechanism by which they modulate (or maintain) the muscle phenotype is less well understood. The myogenically derived electric organ (EO) of the electric fish Sternopygus macrurus is an excellent model to address this question. The electrocytes, i.e., the electrogenic cells of the EO, are not contractile but they do retain some muscle proteins. In order to examine the molecular regulatory pathways that control the muscle-to-electrocyte cell conversion, we have cloned the MyoD and myogenin cDNAs from S. macrurus. Clustal-based alignments showed that the functional domains observed in mammalian MyoD and myogenin are highly conserved in these MRF homologs. Expression analyses revealed that mature electrocytes, which retain the muscle proteins dystrophin, desmin, acetylcholine receptors (AChRs), alpha-actin, and alpha-actinin, also transcribe the MyoD and myogenin genes. RT-PCR studies confirmed that expression of these MRFs is confined to the myogenic lineage. Surprisingly, the levels of MyoD and myogenin transcripts in skeletal muscle and EO could not be used to predict the level to which a cell manifests the muscle program. We conclude that expression of multiple MRFs is not sufficient to induce non-contractile cells to fully express the skeletal muscle program. These data also suggest that the MRF transcriptional program in S. macrurus may be distinct from MRF-dependent myogenesis in other vertebrate systems.

Temperature and the expression of seven muscle-specific protein genes during embryogenesis in the Atlantic cod Gadus morhua L

Seven cDNA clones coding for different muscle-specific proteins (MSPs) were isolated from the fast muscle tissue of Atlantic cod Gadus morhua L. In situ hybridization using cRNA probes was used to characterize the temporal and spatial patterns of gene expression with respect to somite stage in embryos incubated at 4 degrees C, 7 degrees C and 10 degrees C. MyoD transcripts were first observed in the presomitic mesoderm prior to somite formation, and in the lateral compartment of the forming somites. MyoD expression was not observed in the adaxial cells that give rise to the slow muscle layer, and expression was undetectable by in situ hybridization in the lateral somitic mesoderm after the 35-somite stage, during development of the final approximately 15 somites. RT-PCR analysis, however, confirmed the presence of low levels of the transcript during these later stages. A phylogenetic comparison of the deduced aminoacid sequences of the full-length MyoD cDNA clone and those from other teleosts, and inference from the in situ expression pattern suggested homology with a second paralogue (MyoD2) recently isolated from the gilthead seabream Sparus aurata. Following MyoD expression, alpha-actin was the first structural gene to be switched on at the 16-somite stage, followed by myosin heavy chain, troponin T, troponin I and muscle creatine kinase. The final mRNA in the series to be expressed was troponin C. All genes were switched on prior to myofibril assembly. The troponin C sequence was unusual in that it showed the greatest sequence identity with the rainbow trout Oncorhynchus mykiss cardiac/slow form, but was expressed in the fast myotomal muscle and not in the heart. In addition, the third TnC calcium binding site showed a lower level of sequence conservation than the rest of the sequence. No differences were seen in the timing of appearance or rate of posterior progression (relative to somite stage) of any MSP transcripts between embryos raised at the different temperatures. It was concluded that myofibrillar genes are activated asynchronously in a distinct temporal order prior to myofibrillar assembly and that this process was highly canalized over the temperature range studied.

Expression, purification and DNA-binding activity of tilapia muscle-specific transcription factor, MyoD, produced in Escherichia coli

MyoD is one of several helix-loop-helix proteins regulating muscle-specific gene expression. Using a reverse transcription-polymerase chain reaction, 5'-rapid cDNA end amplification, and plaque hybridization, MyoD cDNA was cloned from the mRNA of tilapia dorsal skeletal muscle. The 1015 bp MyoD cDNA product contained an 846 bp open reading frame with flanking regions of 115 and 64 bp at the 5'- and 3'-ends, respectively. Results showed that the tilapia MyoD sequence, which includes one polypeptide of 281 amino acids, shared sequence identities of 64.3, 64.1, 62.6 and 62.4% with those of zebrafish, carp, and two rainbow trout, respectively. Results from a molecular phylogenic tree assay showed that the tilapia MyoD was more closely related to those of other fishes than of higher vertebrates. Using Escherichia coli, a pET expression system, and an Ni(2+)-NTA column, we purified approximately 35 kDa recombinant tilapia MyoD. Results from an electrophoretic mobility shift assay demonstrated that the purified E. coli-produced tilapia MyoD was capable of binding to the DNA fragment sequence CA(C/T)(C/A)TG.

Embryonic temperature and the relative timing of muscle-specific genes during development in herring (Clupea harengus L.)

Temperature influences many aspects of muscle development in herring (Clupea harengus). In Clyde herring, myofibril synthesis occurred later with respect to somite stage in embryos reared at 5°C compared with 12°C. The aim of the present study was to test the hypothesis that the relative timing of expression of myogenic regulatory factors (MRFs) and myosin heavy chain (MyHC) transcripts changes with developmental temperature. Reverse transcriptase/polymerase chain reaction (RT-PCR) was used to clone partial coding regions of MyoD, myogenin and MyHC from juvenile Clyde herring. Embryos were reared at 5, 8 and 12°C, and the spatial and temporal expression patterns of transcripts were investigated using cRNA probes and in situ hybridisation. Antisense probes revealed a rostral–caudal progression of all three transcripts. MyoD transcription initially took place in the adaxial cells of the unsegmented, presomitic mesoderm, whereas myogenin transcription first occurred in newly formed somites. The MyHC gene transcript was not detected until approximately nine somites had formed. Since the somite stage at which the MRFs and MyHC were first expressed was independent of temperature, the hypothesis was rejected. We suggest that the effects of temperature on myofibril synthesis must occur downstream from MyHC transcription either at the level of translation or at the assembly stage.

Two myostatin genes are differentially expressed in myotomal muscles of the trout (Oncorhynchus mykiss)

Myostatin (GDF8) has been shown to be a major genetic determinant of skeletal muscle growth in mammals. In this study, we report the cloning of two trout cDNAs that encode two distinct myostatin-related proteins. The presence in this fish species of two myostatin genes (Tmyostatin 1 and Tmyostatin 2) probably results from the recent tetraploïdisation of the salmonid genome. A comparative reverse-transcriptase-linked polymerase chain reaction assay revealed that Tmyostatin 1 mRNA was present ubiquitously in trout tissues, while Tmyostatin 2 mRNA expression was restricted to muscle and brain. In developing muscle, Tmyostatin 1 expression was observed in eyed-stage embryos well before hatching, whereas Tmyostatin 2 was expressed only in free-swimming larvae. In myotomal muscle from adult animals, Tmyostatin 1 mRNA accumulation was similar in both slow- and fast-twitch fibres, and its concentration did not change during the muscle wasting associated with sexual maturation. In contrast, Tmyostatin 2 mRNA accumulated predominantly in slow-twitch fibres, and its concentration decreased dramatically in wasting muscles from maturing animals. This work shows that two distinct myostatin genes are present in the trout genome. Furthermore, it indicates that these two trout myostatin genes (i) exhibit a distinct expression pattern in muscle and non-muscle tissues and (ii) are not upregulated during the muscle wasting that accompanies sexual maturation.

Lower environmental temperature delays and prolongs myogenic regulatory factor expression and muscle differentiation in rainbow trout (Onchrhynchus mykiss) embryos

The effect of different temperatures (4 degrees C and 12 degrees C) on myogenic regulatory factors (MyoD and myogenin) and myosin heavy chain (MyHC) expression was investigated in rainbow trout (Onchrhynchus mykiss) during early development. MyoD is first switched on at stage 14 [about 5 somites are formed (1/2 epiboly)] while myogenin mRNA is expressed at stage 15 [around 15 somites are visible (2/3 epiboly)] at both temperatures. Subsequently (up to at least stage 20), the most caudal somites exhibit less myogenin mRNA at 4 degrees C compared to 12 degrees C. At the eyed stage (stage 23-24), both myogenin mRNA and protein are present in greater amounts throughout all myotomes at the lower temperature, with mRNA levels in warmer (12 degrees C) embryos at 83% for MyoD and 72% for myogenin of the levels seen in 4 degrees C embryos. Conversely, however, at this same stage, fast-MyHC mRNA and protein are more abundant in 12 degrees C than in 4 degrees C embryos. This indicates relatively advanced muscle differentiation at the warmer temperature. At hatching, myogenin-positive cells are concentrated within the myosepta at both temperatures and they are also sparsely distributed in the myotome at 4 degrees C, but not at 12 degrees C. MyoD, myogenin, and MyHC levels provide an indication of differentiation of muscle cells. These findings suggest that myogenic regulatory factor expression is delayed but prolonged by the lowering of temperature.

TEMPERATURE AND MYOGENIC FACTOR TRANSCRIPT LEVELS DURING EARLY DEVELOPMENT DETERMINES MUSCLE GROWTH POTENTIAL IN RAINBOW TROUT(ONCORHYNCHUS MYKISS) AND SEA BASS (DICENTRARCHUS LABRAX)

The influence of changes in environmental temperature on the mRNA levels of myogenic regulatory factors (MRFs), i.e. MyoD and myogenin, as well as myosin heavy chain (MyHC) were studied during early larval development in rainbow trout and sea bass. Phosphoimager analysis of northern blots indicated that there is an optimum temperature for the RNA transcript levels of MRF and MyHC RNA in trout and in sea bass larvae. In the trout strain studied, the highest concentration for MRF and MyHC transcripts was found at 8°C rather than 4°C or 20°C. In European sea bass, the highest concentrations of MRF and MyHC mRNA were observed at 15-20°C rather than 13°C. Raising sea bass larvae at 15°C was associated with higher MyHC gene expression as well as a trend towards an increase in total muscle fibre number and higher growth rates after transfer at ambient temperature. Results suggest that mRNA levels of MRF and MyHC can be used to optimise early development. An experiment in which the temperature was changed illustrates the consequence of precise temporal expression of MRF genes in specifying muscle fibre number at critical stages during early development.

Regulation and functions of myogenic regulatory factors in lower vertebrates

The transcription factors of the MyoD family have essential functions in myogenic lineage determination and muscle differentiation. These myogenic regulatory factors (MRFs) activate muscle-specific transcription through binding to a DNA consensus sequence known as the E-box present in the promoter of numerous muscle genes. Four members, MyoD, myogenin, myf5 and MRF4/herculin/myf6, have been identified in higher vertebrates and have been shown to exhibit distinct but overlapping functions. Homologues of these four MRFs have also been isolated in a variety of lower vertebrates, including amphibians and fish. Differences have been observed, however, in both the expression patterns of MRFs during muscle development and the function of individual MRFs between lower and higher vertebrates. These differences reflect the variety of body muscle formation patterns among vertebrates. Furthermore, as a result of an additional polyploidy that occurred during the evolution of some amphibians and fish, MyoD, myogenin, myf5 and MRF4 may exist in lower vertebrates in two distinct copies that have evolved separately, acquiring specific roles and resulting in increased complexity of the myogenic regulatory network. Evidence is now accumulating that many of the co-factors (E12, Id, MEF2 and CRP proteins) that regulate MRF activity in mammals are also present in lower vertebrates. The inductive signals controlling the initial expression of MRFs within the developing somite of lower vertebrate proteins are currently being elucidated.

Identification in a fish species of two Id (inhibitor of DNA binding/differentiation)-related helix-loop-helix factors expressed in the slow oxidative muscle fibers

Helix-loop-helix (HLH) proteins related to the inhibitor of DNA binding/differentiation (Id) serve as general antagonists of cell differentiation. They lack a basic DNA-binding domain and are thought to function in a dominant negative manner by sequestering basic HLH (bHLH) transcription factors that are involved in cell determination and differentiation. Four Id-encoding genes have been shown in mammals, they have a distinct pattern of expression suggesting different functions for each member in different cell lineage. In this study we describe the identification and cloning of two trout cDNAs which encode helix-loop-helix proteins showing a high degree of similarity with mammalian Id family members. One cDNA encodes a trout putative Id1 protein (TId1) that is 63% identical to the human Id1 protein over the entire length and 78% identical within the HLH region. The other cDNA encodes a trout putative Id2 protein (TId2) that shows 82% identity to the human Id2 protein and only one change that is conservative over the HLH region. In the 3' untranslated region, TId2 mRNA exhibits 16 nucleotides upstream from the AATAAA site, a palindromic sequence similar to the cytoplasmic polyadenylation element (CPE) which is also present in Id2 and Id3 mRNAs from mammals and in XIdx/XIdI mRNA from Xenopus. In the fish, TId1 and TId2 are expressed in a tissue-specific manner, with slightly different patterns. During myogenesis, TId1 and TId2 are highly expressed in the myotomal musculature of fish embryos and of early alevins but are down-regulated in that of late alevins. In muscle from juveniles and adults, TId1 and TId2 transcripts are abundant in the slow oxidative fibers while they are absent in the fast glycolytic fibers. This expression pattern suggests that Id genes play a role in the regulation of muscle fiber phenotype in addition to controlling early myogenesis. On the whole, the identification of two HLH-Id encoding genes in a major taxonomic group like teleosts, suggests an early divergence of Id genes in vertebrate evolution. The observation that Id transcripts are present selectively in the slow muscle reveals that their expression is more complicated than previously appreciated.