Functional characterization of Labeo rohita muscle cell line for in vitro research

BACKGROUND

Labeo rohita represents the most dominant fish species in Indian aquaculture and the fish cell lines have been used as an excellent in vitro platform for performing various biological research.

METHODS AND RESULTS

The LRM cell culture developed from the muscle tissue of L. rohita was used to study the in vitro applications. The developed muscle cells were maintained in a Leibovitz's-15 (L-15) supplemented with 10% FBS (Fetal Bovine Serum) and 10 ng/ml bFGF at 28 ^oC temperature. The LRM cells showed fibroblastic-like morphology and was authenticated by sequencing mitochondrial gene 16S rRNA. The expression of myogenic regulatory factors (MRFs) was studied in different stages of LRM cells; however, the expression patterns varied at different passages. The MEF2A, Mrf-4, and Myogenin expressions were higher in passage 25, while the expression of MyoD was maximum in passage 15, and the expression of Myf-5 was highest in passage 1. The transfection efficiency of LRM cells revealed 14 % of the GFP expression with a pmaxGFP vector DNA. The LRM cells were susceptible to the extracellular products prepared from Aeromonas hydrophilla and Edwardsiella tarda. The acute cytotoxicity of six heavy metals (Hg, Cd, Zn, Cu, Pb, Ni) was assessed in LRM cells by a dose-dependent manner in comparison to IC₅₀ values obtained from MTT and NR assays. A revival rate of 70-75% was achieved when the LRM cells were cryopreserved at - 196 °C using liquid nitrogen.

CONCLUSION

The developed muscle cells serve as an functional in vitro tool for toxicological and biotechnological studies.

Effect of dietary β-mannanase supplementation on growth performance, digestibility, and gene expression levels of Cyprinus carpio (Linnaeus) fingerlings fed a plant protein-rich diet

The aim of this study was to assess possible beneficial effects of dietary β-mannanase supplementation on the nutrient digestibility, growth performance, digestive and metabolic enzyme activity, and immune response of common carp (Cyprinus carpio) fed plant protein-rich diets. An experiment was conducted in triplicate, and a total of 225 fingerlings of common carp with an average body weight of 13.17 ± 0.12 g were stocked in 15 fiberglass tanks (15 fish/tank). Five dietary treatments (control 35% crude protein, plant-rich basal diet without supplement and four diets supplemented with β-mannanase from two sources (commercially available and locally isolated), each at two dosage levels (500 and 1,000 U/kg diet) were prepared and fed to respective groups of fish, twice a day (8:00 AM and 4:00 PM) at 4 % body weight. During the trial, changes in the level of DO and temperature ranged from 5.5 to 6.1 mg L-1 and 21.5 to 23.5°C, respectively. At the end of the feeding experiment, all fish in each tank were weighed and counted to determine growth parameters, while for the study of other indices, nine samples/treatment group were selected. The results of the study indicated a positive effect of both sources and dosage levels of β-mannanase supplementation on all studied indices, that is, significantly improved (P < 0.05), growth performance (%weight gain, specific growth rate), survival %, hematological indices (RBC, Hb, HCT, and MCHC), immunological indices (lysozyme activity, WBC, respiratory burst activity, and phagocytic activity), improved apparent digestibility of nutrients (crude protein, crude fat, and carbohydrates), and digestible energy. Furthermore, higher activity (P < 0.05) of the digestive enzymes (cellulase, lipase, and protease) and upregulation of MyoD gene in muscle and TNF-α gene in liver, intestine, and muscle were also observed, while the activity of serum AST (serum aspartate aminotransferase) and ALT (alanine transaminase) as compared to control group was significantly decreased (P < 0.05). Based on the results, β-mannanase supplementation (500 U/kg) could be recommended for obtaining better carp production when low-cost plant protein-rich diets are used.

Weighted gene co-expression network indicates that the DYNLL2 is an important regulator of chicken breast muscle development and is regulated by miR-148a-3p

Background

The characteristics of muscle fibers determine the growth and meat quality of poultry. In this study, we performed a weighted gene co-expression network analysis (WGCNA) on the muscle fiber characteristics and transcriptome profile of the breast muscle tissue of Gushi chicken at 6, 14, 22, and 30 weeks.

Results

A total of 27 coexpressed biological functional modules were identified, of which the midnight blue module had the strongest correlation with muscle fiber and diameter. In addition, 7 hub genes were found from the midnight blue module, including LC8 dynein light chain 2 (DYNLL2). Combined with miRNA transcriptome data, miR-148a-3p was found to be a potential target miRNA of DYNLL2. Experiments on chicken primary myoblasts (CPMs) demonstrated that miR-148a-3p promotes the expression of myosin heavy chain (MYHC) protein by targeting DYNLL2, proving that it can promote differentiation of myoblasts.

Conclusions

This study proved that the hub gene DYNLL2 and its target miR-148-3p are important regulators in chicken myogenesis. These results provide novel insights for understanding the molecular regulation mechanisms related to the development of chicken breast muscle.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08522-8.

Common Carp mef2 Genes: Evolution and Expression

The MEF2 (myocyte enhancer factor 2) family belongs to the MADS-box superfamily of eukaryotic transcription factors. The vertebrate genes compose four distinct subfamilies designated MEF2A, -B, -C, and -D. There are multiple mef2 genes in the common carp (Cyprinus carpio). So far, the embryonic expression patterns of these genes and the evolution of fish mef2 genes have been barely investigated. In this study, we completed the coding information of C. carpio mef2ca2 and mef2d1 genes via gene cloning and presented two mosaic mef2 sequences as evidence for recombination. We also analyzed the phylogenetic relationship and conserved synteny of mef2 genes and proposed a new evolutionary scenario. In our version, MEF2B and the other three vertebrate subfamilies were generated in parallel from the single last ancestor via two rounds of whole genome duplication events that occurred at the dawn of vertebrates. Moreover, we examined the expression patterns of C. carpio mef2 genes during embryogenesis, by using whole-mount in situ hybridization, and found the notochord to be a new expression site for these genes except for mef2ca1&2. Our results thus provide new insights into the evolution and expression of mef2 genes.

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.

Differential expression of a retrotransposable element, Rex6, in Colossoma macropomum fish from different Amazonian environments

Transposable elements (TEs) are DNA sequences that have the ability to move and replicate within the genomes. TEs can be classified according to their intermediates of transposition, RNA (retrotransposons) or DNA. In some aquatic organisms, it has been observed that environmental factors such as pH, temperature and pollution may stimulate differential transcription and mobilization of retrotransposons. In light of this information, the present study sought to evaluate the expression of Rex6 TE transcripts in Colossoma macropomum, which is a very commercially exploited fish in Brazil. In order to establish a comparative analysis using real-time PCR, the samples were collected from Amazonian rivers with different physical and chemical characteristics (distinguished by clear water and black water). Quantitative RT-PCR analyses revealed a differential pattern of expression between tissues collected from different types of water (clear and black waters). When it came to the hepatic and muscle tissues sampled, the levels of Rex6 transcripts were significantly different between the two Amazonian water types. These results suggest that environmental conditions operate differently in the regulation of Rex6 transcription in C. macropomum, results which have implications in the reshaping of the genome against environmental variations.

Molecular cloning, characterization and functional assessment of the myosin light polypeptide chain 2 (mylz2) promoter of farmed carp, Labeo rohita

We cloned the 5'-flanking region (1.2 kb) of a muscle-specific gene, encoding myosin light chain 2 polypeptide (mylz2) of a farmed carp, Labeo rohita (rohu). Sequence analysis using TRANSFAC-database search identified the consensus cis acting regulatory elements of TATA-box and E (CANNTG)-box, including the monocyte enhancer factor 2 motif, implying that it is likely to be a functional promoter. The proximal promoter (~620 bp) was highly homologous with that of Danio rerio (zebrafish) as compared to Channa striatus (snakehead murrel) counterparts and showed less identity with Sparus auratus (gilthead sea bream), Xenopus laevis (African clawed frog) and Rattus norvegicus (Norway rat). Direct muscular (skeletal) injection of the construct containing the mylz2 promoter (0.6 kb) fused to a green fluorescent protein (GFP) reporter gene showed efficient expression in L. rohita, validating its functional activity. Further, the functional activity was confirmed by the observation that this promoter drove GFP expression in the skeletal muscle of transgenic rohu. The promoter may have potential applications for value-addition in ornamental fishes and studying gene regulatory functions.

Molecular characterization and expression patterns of myogenin in compensatory growth of Megalobrama amblycephala

Myogenin (myog) is a muscle-specific basic helix-loop-helix (bHLH) transcription factor that plays an essential role in regulating skeletal muscle development and growth. To investigate molecular characterization of myog and the effect of starvation/refeeding on the gene expression, we isolated the myog cDNA sequence and analyzed the expression patterns using quantitative real-time polymerase chain reaction in Megalobrama amblycephala. Sequence analysis indicated that M. amblycephala myog shared an analogous structure with the highly conserved His/Cys-rich, bHLH and C-terminal helix III domains with other vertebrates. Sequence alignment and phylogenetic tree showed that M. amblycephala myog had the highest identity with the homologues of Ctenopharyngodon idella and Cyprinus carpio. Spatio-temporal expression patterns revealed that myog mRNA levels at the segmentation period and 12 h post-hatching (hph) were significantly higher than at other development stages (P<0.05). Furthermore, the highest myog expression level was predominantly observed in white muscle compared with the other types of muscle. Fish body weight continuously decreased during 21-day starvation and then significantly increased after 7days of refeeding and reached the similar level to the control at 21days of refeeding, indicating that the pattern of complete compensatory growth possibly occurred in M. amblycephala; meanwhile, the relative somatic growth rate after refeeding was also dramatically higher than the control group. In addition, the myog expression decreased during 21days of starvation and then exhibited a strong rebound effect after 7days of refeeding and subsequently declined gradually to the control level by 21days of refeeding.

Differential Expression of Myogenic Regulatory Factor Genes in the Skeletal Muscles of Tambaqui Colossoma macropomum (Cuvier 1818) from Amazonian Black and Clear Water

Hypothesizing that the Amazonian water system differences would affect the expression of muscle growth-related genes in juvenile tambaqui Colossoma macropomum (Cuvier 1818), this study aimed to analyze the morphometric data and expression of myogenic regulatory factors (MRFs) in the white and red muscle from tambaqui obtained from clear and black Amazonian water systems. All of the MRF transcript levels (myod, myf5, myogenin, and mrf4) were significantly lower in the red muscle from black water fish in comparison to clear water fish. However, in white muscle, only the myod transcript level was significantly decreased in the black water tambaqui. The changes in MRFs gene expression in muscle fibers of tambaqui from black water system provide relevant information about the environmental influence as that of water systems on gene expression of muscle growth related genes in the C. macropomum. Our results showed that the physical and chemical water characteristics change the expression of genes that promote muscle growth, and these results may be also widely applicable to future projects that aim to enhance muscle growth in fish that are of substantial interest to the aquaculture.

Insulin-like growth factors effects on the expression of myogenic regulatory factors in gilthead sea bream muscle cells

Gilthead sea bream (Sparus aurata) is a widely cultured fish; however, muscle development regulation is poorly known. Myogenesis can be activated by the myogenic regulatory factors (MRFs: MyoD, Myf5, myogenin and MRF4) and by endocrine signals from the growth hormone (GH)/insulin-like growth factors (IGFs) axis. We cultured gilthead sea bream myocytes to better understand the role of IGFs in muscle growth and differentiation through the regulation of MRFs expression. First, we studied the expression pattern during culture development of IGFs and IGF-I splice variants. The expression of igf-II was highest at the beginning of the culture and decreased when the cells started to differentiate, similarly to that observed for total igf-I. Igf-Ib showed a paralleled expression pattern as that of total igf-I, whereas igf-Ic was more stable during culture progression. Next, we analyzed the expression of IGFs and MRFs after incubation of cells at day 4 with GH, IGF-I, IGF-II and combinations of them at 3, 6 and 18 h. IGF-II increased myod2 and myf5 expression, genes involved in early muscle cell proliferation. Moreover, IGF-I caused an increase on mrf4 and myogenin expression, both involved in the later stages of development corresponding to differentiation. Regarding the regulation of IGFs expression, igf-I was stimulated by GH and IGF-II alone and combined, whereas igf-II expression was increased in response to IGF-I, suggesting a nice model of crossed regulation. Overall, the present model could be very useful to understand the different regulatory roles of these endocrine and transcription factors on fish myogenesis.

Three embryonic myosin heavy chain genes encoding different motor domain structures from common carp show distinct expression patterns in cranial muscles

Three embryonic myosin heavy chain (MYH) genes >> (MYHs) including MYH(emb1), MYH(emb2) and MYH(emb3) and encoding a C-terminal part of MYH were previously cloned and demonstrated to be expressed transiently in this order during development of common carp Cyprinus carpio embryos. The present study determined the full-length cDNA nucleotide sequences encoding the motor domain of the three MYHs, suggesting the implication of loop 1 and loop 2 sequences for the differences in the motor functions. Phylogenetic analysis based on the full-length amino acid sequences showed that MYH(emb1) and MYH(emb2) both belong to the fast types, though clearly differ from fast-type MYHs expressed in adult fast muscle previously reported. In contrast, MYH(emb3) was in a clade containing slow/cardiac type. Whole-mount immunostaining and in situ hybridization showed that the transcripts of the three embryonic MYHs are localized in the same or different cranial muscles of common carp larvae, suggesting that the three MYHs function cooperatively or individually in various cranial muscles.

Molecular structure of the largemouth bass (Micropterus salmoides) Myf5 gene and its effect on skeletal muscle growth

Myogenic Regulatory Factors (MRFs), a family of basic helix-loop-helix (bHLH) transcription factors, play important roles in regulating skeletal muscle development and growth. Myf5, the primary factor of MRFs, initiates myogenesis. Its expression pattern during somitomyogenesis in some fish has been revealed. To further study its effect on fish muscle during postembryonic growth, characterization and function analysis of myf5 cDNA were carried out in largemouth bass. The 1,093 bp cDNA sequence was identified by RT-PCR and 3'RACE, then the ORF of Myf5 cDNA was cloned into the expression vector pcDNA3.1(-)/mycHisB. The recombinant plasmid pcDNA3.1(-)/mycHisB-Myf5 was injected into the dorsal muscle of tilapias. RT-PCR and histochemical results showed that the exogenous gene was transcribed and translated in vivo. Its effect on muscle growth focused on myofiber hypertrophy in white muscle 60 days post injection. This indicated that overexpression of Myf5 can promote myogenesis during the fish muscle postembryonic growth period.

Expression of myogenic regulatory factors in the muscle-derived electric organ of Sternopygus macrurus

In most groups of electric fish, the current-producing cells of electric organs (EOs) derive from striated muscle fibers but retain some phenotypic characteristics of their precursor muscle cells. Given the role of the MyoD family of myogenic regulatory factors (MRFs) in the transcriptional activation of the muscle program in vertebrates, we examined their expression in the electrocytes of the gymnotiform Sternopygus macrurus. We estimated the number of MRF genes in the S. macrurus genome and our Southern blot analyses revealed a single MyoD, myogenin, myf5 and MRF4 gene. Quantitative RT-PCR showed that muscle and EO transcribe all MRF genes. With the exception of MyoD, the endogenous levels of myogenin, myf5 and MRF4 transcripts in electrocytes were greater than those detected in muscle fibers. These data indicate that MRF expression levels are not sufficient to predict the level to which the muscle program is manifested. Qualitative expression analysis of MRF co-regulators MEF2C, Id1 and Id2 also revealed these genes not to be unique to either muscle or EO, and detected similar expression patterns in the two tissues. Therefore, the partial muscle program of the EO is not associated with a partial expression of MRFs or with apparent distinct levels of some MRF co-factors. In addition, electrical inactivation by spinal cord transection (ST) resulted in the up-regulation of some muscle proteins in electrocytes without an accompanying increase in MRF transcript levels or notable changes in the co-factors MEF2C, Id1 and Id2. These findings suggest that the neural regulation of the skeletal muscle program via MRFs in S. macrurus might differ from that of their mammalian counterparts. Together, these data further our understanding of the molecular processes involved in the plasticity of the vertebrate skeletal muscle program that brings about the muscle-like phenotype of the non-contractile electrogenic cells in S. macrurus.

Cloning and expression analysis of myogenin from flounder (Paralichthys olivaceus) and promoter analysis of muscle-specific expression

Myogenin is a bHLH transcription factor of the MyoD family. It plays a crucial role in myoblast differentiation and maturation. We report here the isolation of flounder myogenin gene and the characterization of its expression patterns. Sequence analysis indicated that flounder myogenin shared a similar structure and the conserved bHLH domain with other vertebrate myogenin genes. Flounder myogenin gene contains 3 exons and 2 introns. Sequence alignment and phylogenetic showed that flounder myogenin was more homologous with halibut (Hippoglossus hippoglossus) myogenin and striped bass (Morone saxatilis) myogenin. Whole-mount embryo in situ hybridization revealed that flounder myogenin was first detected in the medial region of somites that give rise to slow muscles, and expanded later to the lateral region of the somite that become fast muscles. The levels of myogenin transcripts dropped significantly in matured somites at the trunk region. Its expression could only be detected in the caudal somites, which was consistent with the timing of somite maturation. Transient expression analysis showed that the 546 bp flounder myogenin promoter was sufficient to direct muscle-specific GFP expression in zebrafish embryos.

Molecular cloning and characterization of the Myf5 gene in sea perch (Lateolabrax japonicus)

The cDNA of myogenic factor (Myf5) was isolated from sea perch (Lateolabrax japonicus) using Reverse-transcription Polymerase Chain Reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). The 5' flanking sequence of the cDNA contains a TATA box, GC box, CAAT box, several E box sites and muscle-specific regulatory elements determined by genome walking. The Myf5 gene consists of 3 exons and 2 introns. The open reading frame was found to code a protein with 238 amino-acid residues, containing the conserved basic helix-loop-helix domain (bHLH). RT-PCR indicated the Myf5 was highly expressed in muscle, and weakly expressed in brain, eyes, spleen, gill, liver, kidney, intestine and heart. In early embryonic stages, Myf5 mRNA transcripts are highly detectable in the early gastrula stage while decreasing up to a low level at the late gastrula stage, subsequently greatly increased up to the highest level in the somites stage, then gradually decreases from the tail-bud stage to 15 d larvae after hatching, but they are still detectable. Further, Myf5 mRNA was expressed in several sea perch cell lines such as LJES1, LJHK, LJH-1, LJH-2, LJS, LJL, although its expression level varied greatly among different cell lines.

Molecular structure and expression patterns of flounder (Paralichthys olivaceus) Myf-5, a myogenic regulatory factor

Myf-5, a member of the myogenic regulatory factors (MRF), has been shown to be expressed in muscle precursors in early stage zebrafish embryos. The MRFs, including MyoD, Myf-5, Myogenin and MRF4, belong to the basic Helix-Loop-Helix transcription factors that contain a conserved basic Helix-Loop-Helix (bHLH) domain. To better understand the role of Myf-5 in the development of fish muscles, we have isolated the Myf-5 genomic sequence and cDNA from Flounder (Paralichthys olivaceus), and analyzed its structures and patterns of expression. Promoter analysis identified several putative transcription factor binding sites such as an E-box, NF-Y sites that might confer muscle-specific expression. Myf-5 transcripts were first detected in the paraxial mesoderm that gives rise to slow muscles. During somitogenesis, Myf-5 expression was found in developing somites. Myf-5 expression decreased gradually in somites in the anterior region, but remained strong in the newly formed somites. In the hatching stage, the expression was also detected in other muscle cells such as head muscle and fin muscle. In the growing fish, RT-PCR results showed that Myf-5 was expressed in the skeletal muscle and intestine.

Molecular cloning and mRNA expression analysis of carp embryonic, slow and cardiac myosin heavy chain isoforms

Three embryonic class II myosin heavy chains (MYHs) were cloned from the common carp (Cyprinus carpio L.), MYHemb1, MYHemb2 and MYHemb3. MYH DNA clones were also isolated from the slow muscle of adult carp acclimated to 10 degrees C (MYHS10) and 30 degrees C (MYHS30). Phylogenetic analysis demonstrated that MYHemb1 and MYHemb2 belonged to the fast skeletal muscle MYH clade. By contrast, the sequence of MYHemb3 was similar to the adult slow muscle isoforms, MYHS10 and MYHS30. MYHemb1 and MYHemb2 transcripts were first detected by northern blot analysis in embryos 61 h post-fertilization (h.p.f.) at the heartbeat stage, with peak expression occurring in 1-month-old juveniles. MYHemb1 continued to be expressed at low levels in 7-month-old juveniles when MYHemb2 was not detectable. MYHemb3 transcripts appeared at almost the same stage as MYHemb1 transcripts did (61 h.p.f.), and these genes showed a similar pattern of expression. Whole mount in situ hybridization analysis revealed that the transcripts of MYHemb1 and MYHemb2 were expressed in the inner part of myotome, whereas MYHemb3 was expressed in the superficial compartment. MYHS10 and MYHS30 mRNAs were first detected at hatching. In adult stages, the expression of slow muscle MYH mRNAs was dependent on acclimation temperature. MYHS10 mRNA was expressed at an acclimation temperature of 10 and 20 degrees C, but not at 30 degrees C. In contrast, MYHS30 mRNA was strongly expressed at all acclimation temperatures. The predominant MYH transcripts found in adult slow muscle and in embryos at hatching were expressed in adult fast muscle at some acclimation temperatures but not others. A MYH DNA clone was isolated from the cardiac muscle of 10 degrees C-acclimated adult fish (MYHcard). MYHcard mRNA was first detected at 61 h.p.f., but strong signals were only observed in the adult myocardium. The present study has therefore revealed a complex pattern of expression of MYH genes in relation to developmental stage, muscle type and acclimation temperature. None of the skeletal muscle MYHs identified so far was strongly expressed during the late juvenile stage, indicating further developmentally regulated members of the MYH II gene family remain to be discovered.

Generality of vertebrate developmental patterns: evidence for a dermomyotome in fish

The somitic compartment that gives rise to trunk muscle and dermis in amniotes is an epithelial sheet on the external surface of the somite, and is known as the dermomyotome. However, despite its central role in the development of the trunk and limbs, the evolutionary history of the dermomyotome and its role in nonamniotes is poorly understood. We have tested whether a tissue with the morphological and molecular characteristics of a dermomyotome exists in nonamniotes. We show that representatives of the agnathans and of all major clades of gnathostomes each have a layer of cells on the surface of the somite, external to the embryonic myotome. These external cells do not show any signs of terminal myogenic or dermogenic differentiation. Moreover, in the embryos of bony fishes as diverse as sturgeons (Chondrostei) and zebrafish (Teleostei) this layer of cells expresses the pax3 and pax7 genes that mark myogenic precursors. Some of the pax7-expressing cells also express the differentiation-promoting myogenic regulatory factor Myogenin and appear to enter into the myotome. We therefore suggest that the dermomyotome is an ancient and conserved structure that evolved prior to the last common ancestor of all vertebrates. The identification of a dermomyotome in fish makes it possible to apply the powerful cellular and genetic approaches available in zebrafish to the understanding of this key developmental structure.

Characterization of muscle-regulatory gene, MyoD, from flounder (Paralichthys olivaceus) and analysis of its expression patterns during embryogenesis

Specification and differentiation of skeletal muscle cells are driven by the activity of genes encoding members of the myogenic regulatory factors (MRFs). In vertebrates, the MRF family includes MyoD, Myf5, myogenin, and MRF4. The MRFs are capable of converting a variety of nonmuscle cells into myoblasts and myotubes. To better understand their roles in fish muscle development, we isolated the MyoD gene from flounder (Paralichthys olivaceus) and analyzed its structure and patterns of expression. Sequence analysis showed that flounder MyoD shared a structure similar to that of vertebrate MRFs with three exons and two introns, and its protein contained a highly conserved basic helix-loop-helix domain (bHLH). Comparison of sequences revealed that flounder MyoD was highly conserved with other fish MyoD genes. Sequence alignment and phylogenetic analysis indicated that flounder MyoD, seabream (Sparus aurata) MyoD1, takifugu (Takifugu rubripes) MyoD, and tilapia (Oreochromis aureus) MyoD were more likely to be homologous genes. Flounder MyoD expression was first detected as two rows of presomitic cells in the segmental plate. From somitogenesis, MyoD transcripts were present in the adaxial cells that give rise to slow muscles and the lateral somitic cells that give rise to fast muscles. After 30 somites formed, MyoD expression decreased in the somites except the caudal somites, coincident with somite maturation. In the hatching stage, MyoD was expressed in other muscle cells and caudal somites. It was detected only in muscle in the growing fish.

Myogenin in model pufferfish species: Comparative genomic analysis and thermal plasticity of expression during early development

Myogenin (Myog) is a muscle-specific basic helix-loop-helix transcription factor that plays an essential role in the specification and differentiation of myoblasts. The myogenin genes from the tiger pufferfish, Takifugu rubripes, and green-spotted pufferfish, Tetraodon nigroviridis, were cloned and a comparative genomic analysis performed. The gene encoding myogenin is composed of three exons and has a relatively similar genomic structure in T. rubripes, T. nigroviridis and human. Introns 1 and 2 were approximately 2-fold and 8-fold longer respectively in human than pufferfish. Myogenin is located in a 100 kb region of conserved synteny between these organisms, corresponding to chromosome 1 in human, chromosome 11 in T. nigroviridis and scaffold 208 in T. rubripes. Pufferfish myogenin contained a serine-rich region at the carboxyl terminus that is highly conserved amongst teleosts. During embryonic development of T. rubripes, myogenin was expressed in a rostral-caudal gradient in the developing somites and subsequently during the pharyngula period in the pectoral fin bud primordia, jaw muscles and extraocular muscle precursors. In T. rubripes, the time required to form a somite pair during the linear phase of somitogenesis ( identical withsomite-interval) was 122 min, 97 min and 50 min in embryos incubated at 15, 18 and 21 degrees C, respectively. Myogenin mRNA transcripts were quantified using qPCR and normalised to the highest level of expression. Peak myogenin expression occurred later with respect to developmental stage (standardised using somite-intervals) and was over 2-fold higher at 21 degrees C than at either 18 or 15 degrees C. Changes in the relative timing and intensity of myogenin expression are a potential mechanism for explaining thermal plasticity of muscle phenotype in larvae via effects on the differentiation programme.

Quantitative expression analysis of genes affecting muscle growth during development of rainbow trout(Oncorhynchus mykiss)

The molecular characterization of the hyperplasia and hypertrophy that characterize postembryonic muscle development in rainbow trout is of great interest to aquaculturists because of the commercial value of the species. Determination of temporal expression levels of the genes that control muscle development is an important step in molecular analysis. Real-time quantitative reverse transcriptase polymerase chain reaction was used to characterize expression in the muscle of 3 MRF, 2 MEF, and 2 myostatin genes during 9 stages of trout development. Expression of genes that promote muscle growth (MRF and MEF) peaked in swim-up fry, and in some cases again in 25-g, 140-g, and spawning fish. Myostatin genes, which restrict muscle growth, were expressed at very low levels early in development, but their expression levels were elevated in 140-g and spawning fish. Expression levels and the known function of each tested gene were used to infer the extent of hyperplasia, hypertrophy, and restriction of muscle growth during each stage. Both hyperplasia and hypertrophy appeared to peak in swim-up fry and spawning females, and hyperplasia also appeared to peak in 25-g fish. These results should provide valuable information for developmental biologists and those interested in understanding muscle growth in fish.

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.

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.

Muscle-specific expression of myogenin in zebrafish embryos is controlled by multiple regulatory elements in the promoter

Myogenin is a member of the basic Helix-Loop-Helix transcription factor family that play key roles in myoblast specification and differentiation. Myogenin is specifically expressed in developing somite and skeletal muscles in zebrafish embryos. To determine the regulation of myogenin expression, we reported here the characterization of zebrafish myogenin gene and analysis of its promoter activity in zebrafish embryos. Our data showed that a 0.8-kb myogenin promoter was sufficient to direct correct temporal and spatial muscle-specific green fluorescence protein expression in zebrafish embryos. Sequence analysis identified two putative E box sites in the myogenin gene promoter. In addition, a MEF2 recognition site and a MEF3 binding site were also found in the promoter. Mutation of the E boxes, MEF2 or MEF3 binding site individually had little effect on the muscle-specificity and activity of the myogenin promoter. However, mutating these sites in various combinations, e.g. E boxes and MEF2 binding site, or MEF2 and MEF3 sites significantly reduced the activity of the promoter. Moreover, mutating the E boxes, MEF2 and MEF3 sites together almost abolished the activity of the promoter. These data indicate that muscle-specific expression of myogenin in zebrafish embryos is controlled by multiple regulatory elements in the promoter. In addition, because these regulatory elements control myogenin expression in mouse and human embryos, these data suggest that the regulatory mechanism controlling myogenin expression might be conserved during evolution.

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.

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.

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.

Hedgehog signalling is required for maintenance of myf5 and myoD expression and timely terminal differentiation in zebrafish adaxial myogenesis

Hedgehog proteins have been implicated in the control of myogenesis in the medial vertebrate somite. In the mouse, normal epaxial expression of the myogenic transcription factor gene myf5 is dependent on Sonic hedgehog. Here we examine in zebrafish the interaction between Hedgehog signals, the expression of myoD family genes, including the newly cloned zebrafish myf5, and slow myogenesis. We show that Sonic hedgehog is necessary for normal expression of both myf5 and myoD in adaxial slow muscle precursors, but not in lateral paraxial mesoderm. Expression of both genes is initiated normally in rostral presomitic mesoderm in sonic you mutants, which lack all Sonic hedgehog. Similar initiation continues during tailbud outgrowth when the cells forming caudal somites are generated. However, adaxial cells in sonic you embryos are delayed in terminal differentiation and caudal adaxial cells fail to maintain myogenic regulatory factor expression. Despite these defects, other signals are able to maintain, or reinitiate, some slow muscle development in sonic you mutants. In the cyclops mutant, the absence of floorplate-derived Tiggywinkle hedgehog and Sonic hedgehog has no discernible effect on slow adaxial myogenesis. Similarly, the absence of notochord-derived Sonic hedgehog and Echidna hedgehog in mutants lacking notochord delays, but does not prevent, adaxial slow muscle development. In contrast, removal of both Sonic hedgehog and a floorplate signal, probably Tiggywinkle hedgehog, from the embryonic midline in cyclops;sonic you double mutants essentially abolishes slow myogenesis. We conclude that several midline signals, likely to be various Hedgehogs, collaborate to maintain adaxial slow myogenesis in the zebrafish embryo. Moreover, the data demonstrate that, in the absence of this required Hedgehog signalling, expression of myf5 and myoD is insufficient to commit cells to adaxial myogenesis.

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.

Molecular structure, dynamic expression, and promoter analysis of zebrafish (Danio rerio) myf-5 gene

We isolated a 1,438 bp cDNA fragment that encoded Myf-5 myogenic factor of zebrafish. The deduced amino acid contained 237 residues, including the basic helix-loop-helix domain that is conserved in all known Myf-5. The zebrafish myf-5 transcripts were first detectable at 7.5 hpf, increased substantially until 16 hpf, and then declined gradually to an undetectable level by 26 hpf. During somitogenesis, zebrafish myf-5 transcripts were distributed mainly in the somites and segmental plates. Prominent signals occurred transiently in adaxial cells in two parallel rows but did not extend beyond the positive-signal somites. Various lengths of upstream region of zebrafish myf-5 fused with EGFP gene were used to carry out transgenic analysis. Results showed that a small, 82 bp (nucleotide positions from -82 to -1), regulatory cassette is sufficient to control the somite- and stage-specific expression of zebrafish myf-5 during early development.

Muscle regulatory factor gene: zebrafish (Danio rerio) myogenin cDNA

Myogenin is one of the basic helix-loop-helix proteins that regulate muscle-specific gene expression. Using reverse transciption-polymerase chain reaction (RT-PCR), 5'- and 3'-rapid amplification of cDNA ends (RACE), zebrafish myogenin cDNA was cloned from mRNA of embryos at 10-96 h post-fertilization. The cDNA, at 1384 base pairs (bp), contained a 771-bp open reading frame with 113- and 500-bp flanking regions at the 5'- and 3'-ends, respectively. The deduced amino acid sequences of zebrafish myogenin encoded a 256-amino-acid polypeptide. In a comparison with myogenin of carp, trout, Xenopus, chicken and human, zebrafish myogenin shared 90.9, 77.6, 70.3, 62.9 and 51.5% amino acid identity, respectively. The basic helix-loop-helix domains in myogenin are all conserved. The molecular phylogenic tree demonstrated that myogenin of zebrafish is more closely related to that of fish than to the myogenin of other vertebrates.