Expression of myostatin is not altered in lines of poultry exhibiting myofiber hyper- and hypoplasia

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.

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

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.

MicroRNA-27b-3p Targets the Myostatin Gene to Regulate Myoblast Proliferation and Is Involved in Myoblast Differentiation

microRNAs play an important role in the growth and development of chicken embryos, including the regulation of skeletal muscle genesis, myoblast proliferation, differentiation, and apoptosis. Our previous RNA-seq studies showed that microRNA-27b-3p (miR-27b-3p) might play an important role in regulating the proliferation and differentiation of chicken primary myoblasts (CPMs). However, the mechanism of miR-27b-3p regulating the proliferation and differentiation of CPMs is still unclear. In this study, the results showed that miR-27b-3p significantly promoted the proliferation of CPMs and inhibited the differentiation of CPMs. Then, myostatin (MSTN) was confirmed to be the target gene of miR-27b-3p by double luciferase reporter assay, RT-qPCR, and Western blot. By overexpressing and interfering with MSTN expression in CPMs, the results showed that overexpression of MSTN significantly inhibited the proliferation and differentiation of CPMs. In contrast, interference of MSTN expression had the opposite effect. This study showed that miR-27b-3p could promote the proliferation of CPMs by targeting MSTN. Interestingly, both miR-27b-3p and MSTN can inhibit the differentiation of CPMs. These results provide a theoretical basis for further understanding the function of miR-27b-3p in chicken and revealing its regulation mechanism on chicken muscle growth.

Distinct cell proliferation, myogenic differentiation, and gene expression in skeletal muscle myoblasts of layer and broiler chickens

Myoblasts play a central role during skeletal muscle formation and growth. Precise understanding of myoblast properties is thus indispensable for meat production. Herein, we report the cellular characteristics and gene expression profiles of primary-cultured myoblasts of layer and broiler chickens. Broiler myoblasts actively proliferated and promptly differentiated into myotubes compared to layer myoblasts, which corresponds well with the muscle phenotype of broilers. Transcriptomes of layer and broiler myoblasts during differentiation were quantified by RNA sequencing. Ontology analyses of the differentially expressed genes (DEGs) provided a series of extracellular proteins as putative markers for characterization of chicken myogenic cells. Another ontology analyses demonstrated that broiler myogenic cells are rich in cell cycle factors and muscle components. Independent of these semantic studies, principal component analysis (PCA) statistically defined two gene sets: one governing myogenic differentiation and the other segregating layers and broilers. Thirteen candidate genes were identified with a combined study of the DEGs and PCA that potentially contribute to proliferation or differentiation of chicken myoblasts. We experimentally proved that one of the candidates, enkephalin, an opioid peptide, suppresses myoblast growth. Our results present a new perspective that the opioids present in feeds may influence muscle development of domestic animals.

Myostatin Increases Smad2 Phosphorylation and Atrogin-1 Expression in Chick Embryonic Myotubes

Skeletal muscle mass is an important trait in poultry meat production. In mammals, myostatin, a negative regulator of skeletal muscle growth, activates Smad transcription factors and induces the expression of atrogin-1 by regulating the Akt/FOXO pathway. Although the amino acid sequence of chicken myostatin is known to be completely identical to its mammalian counterpart, previous studies in chicken skeletal muscles have implied that the physiological roles of chicken myostatin are different from those of mammals. Furthermore, it remains to be elucidated whether myostatin affects cellular signaling factors and atrogin-1 expression. In this study, using chick embryonic myotubes, we found that myostatin significantly increased the phosphorylation rate of Smad2 and mRNA levels of atrogin-1. No significant change was observed in the phosphorylation of Akt and FOXO1. These in vitro results suggest that the molecular mechanisms underlying myostatin-induced expression of atrogin-1 might be different between chickens and mammals.

Gene expression profile of IGF1 and MSTN mRNA and their correlation with carcass traits in different breeds of geese at 70 d of age

1. The expression of insulin-like growth factor-1 (IGF1) and myostatin (MSTN) mRNA in breast and leg muscle was quantified in 70-d-old Taihu and Wanxi geese by using a Multiplex Competitive Fluorescent-PCR method and the correlations between mRNA levels and carcass traits were analysed. 2. IGF1 mRNA expression in breast muscle in Taihu geese was significantly higher than that in Wanxi geese and the MSTN mRNA level in leg muscle in Taihu geese was significantly higher than that in Wanxi geese. 3. There was no significant difference in breast muscle MSTN or leg muscle IGF1 mRNA expression between the two breeds. 4. Within the same breed, the IGF1 mRNA expression in leg muscle of male geese was significantly higher than that in female geese, and MSTN mRNA expression in leg muscle was significantly higher than that in breast muscle. 5. There was no difference in the IGF1 mRNA expression between tissues. 6. There was a positive correlation between IGF1 mRNA and MSTN mRNA and a negative correlation between IGF1 mRNA expression of breast muscle and leg muscle ratio. 7. In Wanxi geese, MSTN mRNA expression in leg muscle was negatively associated with body weight and leg muscle weight.

Dietary Lysine Affect Carcass Characteristics and Myostatin Gene Exon 1 Region Methylation in Muscle Tissue of Broilers

Amino acids emerged in various biochemical cycles in vivo, affecting the performance and carcass characteristics of broilers. In this study, the contents of dietary lysine and their effects on carcass performance of broiler were investigated. Experimental results showed that deficient dietary lysine significantly reduced live weight, carcass weight, eviscerated yield with gihlet (EYG) and eviscerated yield (EY) of broilers. Whereas, the percentage of slaughter yield, the percentage of eviscerated yield (PEY) and the percentage of eviscerated yield with giblet (PEG) were slightly affected. Many CpG sites were found at the myostatin exon 1 region (nucleotides 2360-2980 bp). At the myostatin exon 1 region where CpG sites enriched (nucleotides 2360-2980 bp), we found significantly lower methylation rate of 18% in low-lysine group of 55-day-old broilers, compared with methylation rate of 87% in control group, which suggested the deficient dietary lysine may lead to demethylation of their original methylated sites. In muscle tissues, the exon1 hypermethylation status of myostatin gene was detected, which was negative correlated with the expression of this gene. These results suggested that methylation of this gene was a dynamic process, which plays a dominant role in regulation of gene expression for development of individuals.

Seasonal variation of myostatin gene expression in pectoralis muscle of house sparrows (Passer domesticus) is consistent with a role in regulating thermogenic capacity and cold tolerance

Winter acclimatization in small birds overwintering in cold climates, including house sparrows (Passer domesticus), is associated with improved cold tolerance, elevated summit metabolic rates (M(sum) = maximum cold-induced metabolic rate), and increased pectoralis muscle mass compared to summer birds. Myostatin is a potent autocrine/paracrine inhibitor of skeletal muscle growth in mammals and birds and is a potential candidate for regulation of seasonal phenotypic flexibility in birds. As a first step toward examining such a role for myostatin in small birds, we measured summer and winter gene expression of myostatin and its potential metalloproteinase activators TLL-1 and TLL-2 in house sparrows from southeastern South Dakota. Gene expression of myostatin decreased significantly in winter, with summer values exceeding winter values by 1.52-fold. Moreover, gene expression of TLL-1 was also significantly reduced in winter, with summer values exceeding winter values by 1.55-fold. These data are consistent with the hypothesis that the winter increases in pectoralis muscle mass, M(sum), and cold tolerance in house sparrows are mediated by reduced levels of myostatin and its activator TLL-1, and they suggest the possibility that myostatin may be a common mediator of phenotypic flexibility of muscle mass in birds.

Ubiquitous expression of myostatin in chicken embryonic tissues: Its high expression in testis and ovary

The skeletal muscle of mammals is known to express myostatin (GDF-8) that acts as a potent negative regulator of skeletal muscle growth. However, the function of GDF-8 is not limited to skeletal muscle, because of its ubiquitous expression in fish. Here we investigated whether GDF-8 is expressed in various tissues including gonads during chicken embryogenesis. As revealed by RT-PCR and Western blotting, the transcript and protein for GDF-8 were detected in brain, eye, gizzard, muscle, heart, small gut, large gut, mesonephroi, testis and ovary of chicken embryos at E12, but not in liver. GDF-8 was constitutively expressed in testis and ovary as well as muscle at E6-E21, as demonstrated by in situ hybridization on section and whole-mount. Some cell population in testis, but not identified, highly expressed GDF-8. On the other hand, the medulla and germinal epithelium of ovary highly expressed it. Collectively, these results indicate that GDF-8 is ubiquitously expressed in various tissues of chicken embryos including testis and ovary through the stage of embryogenesis.

Genotypic and nutritional regulation of gene expression in two sheep hindlimb muscles with distinct myofibre and metabolic characteristics

This study investigated whether the expression profile of GDF8 (myostatin), myogenic regulatory factors (MRFs: MYF5, MYOD1, MYOG (myogenin), and MYF6), and IGF-system (IGF1, IGF2, IGF1R) genes are correlated with anatomical muscle, nutrition level, and estimated breeding values (EBVs) for muscling, growth, and/or fatness. Real-time PCR was employed to quantitatively measure the mRNA levels of these genes in the semimembranosus (SM) and semitendinosus (ST) muscles of growing lambs. The lambs were sired by Poll Dorset rams with differing EBVs for growth, muscling, and fatness, and were fed either high or low quality and availability pasture from birth to ~8 months of age. With the exception of MYOD1, the mRNA levels of all genes examined in this study showed varying degrees of nutritional regulation. All the MRF mRNA levels were higher in the SM muscle than the ST muscle, whereas myostatin mRNA was higher in the ST muscle than the SM muscle. Interactions between muscle type and nutrition were detected for IGF2, MYF6, and myogenin, while positive correlations between IGF2 and IGF1R and between MYOD1 and myogenin mRNA levels were apparent in both muscles. At the genotypic level, subtle differences in mRNA levels suggested interactions between nutrition and sire EBV. The findings of this study confirm that the MRFs, IGFs, and myostatin genes are differentially affected by a variety of factors that include nutrition, muscle type, and sire EBVs. Together, these data suggest that this suite of genes has important roles during postnatal muscle growth, even at quite late stages of growth and development.

Comparison of chicken genotypes: myofiber number in pectoralis muscle and myostatin ontogeny

This study was performed to evaluate breast muscle development in chicken genotypes divergently selected for muscularity. In the first experiment, 2 commercial broiler lines (a high breast yield, HBY, and a normal breast yield broiler strain-cross, NBY) and a Leghorn line were grown up to 35 d to evaluate BW, breast weight, and breast yield. At 7 and 21 d of age, pectoralis muscle was used to estimate myofiber density (MFD, number of myofibers per mm2) and total apparent myofiber number (MFN). In the second experiment, the ontogeny of myostatin was determined from broiler- and Leghorn-type chick embryos, at embryonic days 1 to 20 (E1 to E20), using reverse transcription (RT)-PCR. As expected, the Leghorn line had lower BW, breast weight, and breast yield than broiler lines. The HBY line showed higher breast yield at all ages evaluated, but lower BW at 21 and 35 d than the NBY line. The Leghorn line had 45% higher MFD than broilers, which indicates an increased cross-sectional area of the myofibers in broiler lines. No MFD difference was observed between the broiler strains (P > 0.05). The myofiber number of broilers was more than twice that of Leghorns and HBY had 10% higher MFN than the NBY line. Myofiber number was correlated to BW (r = 0.58), breast weight (r = 0.58), and breast yield (r = 0.69). Conversely, MFD showed negative correlation with BW, breast weight, and breast yield (r = -0.85, -0.83, and -0.88, respectively). No effect of genotype or interaction between genotype and embryonic age was observed for myostatin expression. This study showed that broilers have higher MFN in the breast muscles than Leghorn-type chickens, and that high breast yield of broiler strains may be due to increased MFN. Higher muscularity of broilers, as compared with Leghorns, was not attributed to lower expression of myostatin during embryonic development.

cDNA array analysis of Japanese quail lines divergently selected for four-week body weight

Decades of divergent selection for 4-wk BW has produced 3 lines of growth-selected Japanese quail. P line quail have been selected for >110 generations for 4-wk posthatch BW and are nearly 3-fold larger than the randomly bred control C line. The H line has been selected for high 4-wk BW for 52 generations and the L line has been selected for low 4-wk BW for 54 generations. To identify differentially expressed genes that may play a role in defining the differences in these lines, a DNA array containing 4,704 random anonymous cDNA clones from 8-d C line embryos was screened using isotopically-labeled cDNA from the different quail lines. Array analysis yielded 3 differentially expressed cDNA clones that were confirmed by Northern blot analysis. The 35-kD quail EB1 protein, previously unidentified, was shown to have elevated transcripts in the L line and decreased transcripts in the H and P lines, compared with the C line. It was also widely expressed in embryonic and adult tissues by BLASTN analysis of chicken expressed sequence tag (EST) libraries. Two other cDNA clones are novel sequences expressed at higher levels in the L line and at lower levels in the H and P lines, one of which was more selectively expressed in embryos and adult tissues by BLASTN analysis. These limited findings suggest that anonymous cDNA array analysis is a productive means to identify differentially expressed genes in growth-selected poultry.

Nutrient supply enhances both IGF-I and MSTN mRNA levels in chicken skeletal muscle

Nutrient supply may control muscle growth directly and indirectly through its influence on regulatory factors. The present study focuses on its effects on muscle insulin-like growth factors (IGF-I and -II) and myostatin (MSTN). Their mRNA levels were quantified by real time RT-PCR in pectoralis major (PM) and sartorius (SART) muscles from broiler chickens submitted to different feeding regimens (fed or fasted for 48 h) between hatch and 2 days of age and at 4 weeks of age. In the PM of 4 weeks old broilers, mRNA levels were also evaluated after a 16 h-fast and a refeeding period (refed 24 or 48 h after a 48 h-fast). In the PM muscle, both IGF-I and MSTN mRNA levels increased between 0 and 2 days of age in the fed group, while they remained low in the unfed one. A comparable trend was observed in the SART, but with lesser amplitude. In both muscles of 4 weeks old chickens, a 48 h-fast induced a significant reduction in MSTN mRNA levels (20% of fed state). In the PM, this effect required more than 16 h of fasting to occur and was fully reversed by only 24h of refeeding. IGF-I mRNA levels also varied with nutritional state. They decreased significantly with fasting in the SART muscle. By contrast, IGF-II mRNA levels did not vary significantly. Our data shows for the first time that two major paracrine regulators of muscle growth, IGF-I and MSTN, are sensitive to nutrient supply in hatching chicks, and also that fasting reduced IGF-I and MSTN mRNA levels in muscles of older chickens.

The embryonated egg: a practical target for genetic based advances to improve poultry production

The embryonated avian egg is an attractive target for applying technology-based advances to improve poultry production. There are a number of reasons for this. First, the egg is immobile and can be easily accessed by high-speed automated equipment such as the commercial egg injection system used for vaccination of broilers worldwide. Second, due to successful breeding techniques, the embryonic period now composes 30 to 40% of a broiler's total lifespan, underscoring the importance of this window in the bird production life cycle. Third, the period of incubation involves rapid development from a ball of 40,000 to 60,000 undifferentiated blastodermal cells to a fully formed chick and associated extra-embryonic compartments in 21 d, allowing development of novel approaches for improving poultry production. Some of these novel approaches will be discussed in this paper and include gender discrimination of embryos and the possibility of changing the breeding paradigm through introduction of undifferentiated cells such as avian blastodermal or embryonic stem cells.