Exogenous Expression of an Alternative Splicing Variant of Myostatin Prompts Leg Muscle Fiber Hyperplasia in Japanese Quail

Myostatin gene role in regulating traits of poultry species for potential industrial applications

The myostatin (MSTN) gene is considered a potential genetic marker to improve economically important traits in livestock, since the discovery of its function using the MSTN knockout mice. The anti-myogenic function of the MSTN gene was further demonstrated in farm animal species with natural or induced mutations. In poultry species, myogenesis in cell culture was regulated by modulation of the MSTN gene. Also, different expression levels of the MSTN gene in poultry models with different muscle mass have been reported, indicating the conserved myogenic function of the MSTN gene between mammalian and avian species. Recent advances of CRISPR/Cas9-mediated genome editing techniques have led to development of genome-edited poultry species targeting the MSTN gene to clearly demonstrate its anti-myogenic function and further investigate other potential functions in poultry species. This review summarizes research conducted to understand the function of the MSTN gene in various poultry models from cells to whole organisms. Furthermore, the genome-edited poultry models targeting the MSTN gene are reviewed to integrate diverse effects of the MSTN gene on different traits of poultry species.

A novel splice variant of the human MSTN gene encodes a myostatin-specific myostatin inhibitor

BACKGROUND

Myostatin, encoded by the MSTN gene comprising 3 exons, is a potent negative regulator of skeletal muscle growth. Although a variety of myostatin inhibitors have been invented for increasing muscle mass in muscle wasting diseases, no effective inhibitor is currently available for clinical use. Myostatin isoforms in several animals have been reported to inhibit myostatin, but an isoform has never been identified for the human MSTN gene, a conserved gene among animals. Here, a splice variant of the human MSTN gene was explored.

METHODS

Transcripts and proteins were analysed by reverse transcription-PCR amplification and western blotting, respectively. Proteins were expressed from expression plasmid. Myostatin signalling was assayed by the SMAD-responsive luciferase activity. Cell proliferation was assayed by the Cell Counting Kit-8 (CCK-8) assay and cell counting. Cell cycle was analysed by the FastFUCCI system.

RESULTS

Reverse transcription-PCR amplification of the full-length MSTN transcript in CRL-2061 rhabdomyosarcoma cells revealed two bands consisting of a thick expected-size product and a thin additional small-size product. Sequencing of the small-size product showed a 963-bp deletion in the 5' end of exon 3, creating exon 3s, which contained unusual splice acceptor TG dinucleotides. The novel variant was identified in other human cell lines, although it was not identified in skeletal muscle. The 251-amino acid isoform encoded by the novel variant (myostatin-b) was identified in CRL-2061 rhabdomyosarcoma cells. Transfection of a myostatin-b expression plasmid into CRL-2061 and myoblast cells inhibited endogenous myostatin signalling (44%, P < 0.001 and 63%, P < 0.001, respectively). Furthermore, myostatin-b inhibited myostatin signalling induced by recombinant myostatin (68.8%, P < 0.001). In remarkable contrast, myostatin-b did not inhibit the myostatin signalling induced by recombinant growth differentiation factor 11 (9.2%, P = 0.70), transforming growth factor β (+3.1%, P = 0.83) or activin A (+1.1%, P = 0.96). These results indicate the myostatin-specific inhibitory effect of myostatin-b. Notably, the expression of myostatin-b in myoblasts significantly enhanced cell proliferation higher than the mock-transfected cells by the CCK-8 and direct cell counting assays (60%, P < 0.05 and 39%, P < 0.05, respectively). Myostatin-b increased the percentage of S-phase cells significantly higher than that of the mock-transfected cells (53% vs. 80%, P < 0.05).

CONCLUSIONS

We cloned a novel human MSTN variant produced by unorthodox splicing. The variant encoded a novel myostatin isoform, myostatin-b, that inhibited myostatin signalling by myostatin-specific manner and enhanced myoblast proliferation by shifting cell cycle. Myostatin-b, which has myostatin-specific inhibitory activity, could be developed as a natural myostatin inhibitor.

Comprehensive analysis of pre-mRNA alternative splicing regulated by m6A methylation in pig oxidative and glycolytic skeletal muscles

BACKGROUND

Different types of skeletal myofibers exhibit distinct physiological and metabolic properties that are associated with meat quality traits in livestock. Alternative splicing (AS) of pre-mRNA can generate multiple transcripts from an individual gene by differential selection of splice sites. N6-methyladenosine (m6A) is the most abundant modification in mRNAs, but its regulation for AS in different muscles remains unknown.  RESULTS: We characterized AS events and m6A methylation pattern in pig oxidative and glycolytic muscles. A tota1 of 1294 differential AS events were identified, and differentially spliced genes were significantly enriched in processes related to different phenotypes between oxidative and glycolytic muscles. We constructed the regulatory network between splicing factors and corresponding differential AS events and identified NOVA1 and KHDRBS2 as key splicing factors. AS event was enriched in m6A-modified genes, and the methylation level was positively correlated with the number of AS events in genes. The dynamic change in m6A enrichment was associated with 115 differentially skipping exon (SE-DAS) events within 92 genes involving in various processes, including muscle contraction and myofibril assembly. We obtained 23.4% SE-DAS events (27/115) regulated by METTL3-meditaed m6A and experimentally validated the aberrant splicing of ZNF280D, PHE4DIP, and NEB. The inhibition of m6A methyltransferase METTL3 could induce the conversion of oxidative fiber to glycolytic fiber in PSCs.

CONCLUSION

Our study suggested that m6A modification could contribute to significant difference in phenotypes between oxidative and glycolytic muscles by mediating the regulation of AS. These findings would provide novel insights into mechanisms underlying muscle fiber conversion.

Differential Expression of MSTN Isoforms in Muscle between Broiler and Layer Chickens

Simple Summary

Increasing the growth rate of food animals with a concomitant increase in muscle yield is a primary goal of animal producers. The identification of factors and a better understanding of the underlying mechanisms of these factors for muscle growth are important if we are to improve meat production in domestic animals. In avian species, MSTN-A has anti-myogenic activities and MSTN-B functions as a pro-myogenic factor. In this study, the expression of the Mstn isoforms and the sizes of myofibers and muscle bundles during development were analyzed in broiler and layer chicken breeds. Although the expression levels of total Mstn were not different between the two breeds, the ratios of Mstn-B to -A were significantly higher in the broilers with muscle hypertrophy and hyperplasia compared to the layers. Further characterizations of the muscle in broilers revealed a greater bundle area but similar fiber number per bundle compared to the layers. Our study demonstrated higher ratios of Mstn-B/-A expression in the muscles of broiler chickens, suggesting that the development of strategies to enhance the expression of Mstn-B will lead to increased muscle growth and poultry production.

Abstract

Myostatin (Mstn)-A, the main isoform among Mstn splicing variants, functions as a negative regulator, whereas Mstn-B functions as a positive regulator in muscle development. Because broiler chickens are a fast-growing breed raised for meat production and layer chickens are a slow-growing breed raised for egg production, differences in the expression of Mstn isoforms between the two distinct breeds were analyzed in this study. There was no difference in the expression levels of total Mstn (Mstn-A and -B forms) during embryonic development and at D33 between the two breeds. Interestingly, the ratios of Mstn-B to -A were significantly higher in the broiler compared to the layer at most ages. In pectoralis major muscle (PM) tissue, the cross-sectional area (CSA) of muscle fiber was significantly greater in the broiler. The broiler also showed greater bundle CSA and a similar fiber number per bundle compared to the layer at D5 and D33. These data suggest that the greater bundle CSA with myofiber hypertrophy in the broilers is associated with greater muscle growth. The relationship between the expression of Mstn isoforms and growth rate can be used as a potential genetic marker for the selection of higher muscle growth in chickens.

Transcriptome-Wide Analyses Identify Dominant as the Predominantly Non-Conservative Alternative Splicing Inheritance Patterns in F1 Chickens

Transcriptome analysis has been used to investigate many economically traits in chickens; however, alternative splicing still lacks a systematic method of study that is able to promote proteome diversity, and fine-tune expression dynamics. Hybridization has been widely utilized in chicken breeding due to the resulting heterosis, but the dynamic changes in alternative splicing during this process are significant yet unclear. In this study, we performed a reciprocal crossing experiment involving the White Leghorn and Cornish Game chicken breeds which exhibit major differences in body size and reproductive traits, and conducted RNA sequencing of the brain, muscle, and liver tissues to identify the inheritance patterns. A total of 40 515 and 42 612 events were respectively detected in the brain and muscle tissues, with 39 843 observed in the liver; 2807, 4242, and 4538 events significantly different between two breeds were identified in the brain, muscle, and liver tissues, respectively. The hierarchical cluster of tissues from different tissues from all crosses, based on the alternative splicing profiles, suggests high tissue and strain specificity. Furthermore, a comparison between parental strains and hybrid crosses indicated that over one third of alternative splicing genes showed conserved patterns in all three tissues, while the second prevalent pattern was non-additive, which included both dominant and transgressive patterns; this meant that the dominant pattern plays a more important role than suppression. Our study provides an overview of the inheritance patterns of alternative splicing in layer and broiler chickens, to better understand post-transcriptional regulation during hybridization.

Characterization and Comparative Transcriptomic Analysis of Skeletal Muscle in Pekin Duck at Different Growth Stages Using RNA-Seq

Simple Summary

Skeletal muscle is an important tissue and its development is strictly regulated by genes. In this study, in order to understand the muscle-related gene expression in Pekin duck, RNA-seq was performed to analyze and compare skeletal muscle at different growth stages. Alternative splicing, single nucleotide polymorphisms and insertion–deletions were detected, and 299 novel genes were discovered. MYL4, IGF2BP1, CSRP3, SPP1, KLHL31, LAMB2, LAMA2, ITGB1 and OPN played crucial roles in skeletal muscle development. Oxidative phosphorylation, ECM-receptor interaction, focal adhesion, carbon metabolism, and biosynthesis of amino acids participated in the regulation of skeletal muscle development in Pekin duck. This study provides an important reference for revealing the developmental mechanisms of pectoral and leg muscles in duck.

Abstract

Skeletal muscle, accounting for approximately 50% of body weight, is the largest and most important tissue. In this study, the gene expression profiles and pathways in skeletal muscle of Pekin duck were investigated and compared at embryonic day 17, 21, and 27 and postnatally at 6 months of age. An average of 49,555,936 reads in each sample was obtained from the transcriptome libraries. Over 70.0% of alternative splicing (AS) in each sample was mainly alternative 5′ first exon (transcription start site)—the first exon splicing (TSS) and alternative 3′ last exon (transcription terminal site)—the last exon splicing (TTS), indicating that TSS and TTS were the most common AS event in Pekin ducks, and these AS events were closely related to the regulation of muscle development at different growth stages. The results provided a valuable genomic resource for selective breeding and functional studies of genes. A total of 299 novel genes with ≥2 exons were obtained. There were 294 to 2806 differentially expressed genes (DEGs) in each pairwise comparison of Pekin duck. Notably, 90 DEGs in breast muscle and 9 DEGs in leg muscle were co-expressed at all developmental points. DEGs were validated by qPCR analysis, which confirmed the tendency of the expression. DEGs related to muscle development were involved in biological processes such as “endodermal cell differentiation”, “muscle cell cellular homeostasis”, “skeletal muscle tissue growth” and “skeletal muscle cell differentiation”, and were involved in pathways such as oxidative phosphorylation, ECM-receptor (extracellular matrix receptor) interaction, focal adhesion, carbon metabolism, and biosynthesis of amino acids. Some DEGs, including MYL4, IGF2BP1, CSRP3, SPP1 and KLHL31, as well as LAMB2, LAMA2, ITGB1 and OPN, played crucial roles in muscle growth and development. This study provides valuable information about the expression profile of mRNAs and pathways from duck skeletal muscle at different growth stages, and further functional study of these mRNAs and pathways could provide new ideas for studying the molecular networks of growth and development in duck skeletal muscle.

The Roles of CircRNAs in Regulating Muscle Development of Livestock Animals

The muscle growth and development of livestock animals is a complex, multistage process, which is regulated by many factors, especially the genes related to muscle development. In recent years, it has been reported frequently that circular RNAs (circRNAs) are involved widely in cell proliferation, cell differentiation, and body development (including muscle development). However, the research on circRNAs in muscle growth and development of livestock animals is still in its infancy. In this paper, we briefly introduce the discovery, classification, biogenesis, biological function, and degradation of circRNAs and focus on the molecular mechanism and mode of action of circRNAs as competitive endogenous RNAs in the muscle development of livestock and poultry. In addition, we also discuss the regulatory mechanism of circRNAs on muscle development in livestock in terms of transcription, translation, and mRNAs. The purpose of this article is to discuss the multiple regulatory roles of circRNAs in the process of muscle development in livestock, to provide new ideas for the development of a new co-expression regulation network, and to lay a foundation for enriching livestock breeding and improving livestock economic traits.

Muscle Hyperplasia in Japanese Quail by Single Amino Acid Deletion in MSTN Propeptide

Mutation in myostatin (MSTN), a negative regulator of muscle growth in skeletal muscle, resulted in increased muscle mass in mammals and fishes. However, MSTN mutation in avian species has not been reported. The objective of this study was to generate MSTN mutation in quail and investigate the effect of MSTN mutation in avian muscle growth. Recently, a new targeted gene knockout approach for the avian species has been developed using an adenoviral CRISPR/Cas9 system. By injecting the recombinant adenovirus containing CRISPR/Cas9 into the quail blastoderm, potential germline chimeras were generated and offspring with three base-pair deletion in the targeted region of the MSTN gene was identified. This non-frameshift mutation in MSTN resulted in deletion of cysteine 42 in the MSTN propeptide region and homozygous mutant quail showed significantly increased body weight and muscle mass with muscle hyperplasia compared to heterozygous mutant and wild-type quail. In addition, decreased fat pad weight and increased heart weight were observed in MSTN mutant quail in an age- and sex-dependent manner, respectively. Taken together, these data indicate anti-myogenic function of MSTN in the avian species and the importance of cysteine 42 in regulating MSTN function.