Discovery of a mammalian splice variant of myostatin that stimulates myogenesis

Discovery of Novel Stimulators of Pax7 and/or MyoD: Enhancing the Efficacy of Cultured Meat Production through Culture Media Enrichment

The principles of myogenesis play crucial roles in the production of cultured meat, and identifying protein stimulators associated with myogenesis holds great potential to enhance the efficiency of this process. In this study, we used surface plasmon resonance (SPR)-based screening of a natural product library to discover ligands for Pax7 and MyoD, key regulators of satellite cells (SCs), and performed cell-based assays on Hanwoo SCs (HWSCs) to identify substances that promote cell proliferation and/or differentiation. Through an SPR analysis, we found that six chemicals, including one Pax7+/MyoD- chemical, four Pax7+/MyoD+ chemicals, and one Pax7-/MyoD+ chemical, bound to Pax7 and/or MyoD proteins. Among four Pax7+/MyoD+ chemicals, parthenolide (0.5 and 1 µM) and rutin (100 and 200 µM) stimulated cell proliferation in the medium with 10% FBS similar to the medium with 20% FBS, without affecting differentiation. Adenosine, a Pax7-/MyoD+ chemical, accelerated differentiation. These chemicals could be potential additives to reduce the reliance of FBS required for HWSC proliferation and differentiation in cultured meat production.

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.

INVITED REVIEW: Inhibitors of myostatin as methods of enhancing muscle growth and development

With the increasing demand for affordable, high-quality meat, livestock and poultry producers must continually find ways to maximize muscle growth in their animals without compromising palatability of the meat products. Muscle mass relies on myoblast proliferation during prenatal or prehatch stages and fiber hypertrophy through protein synthesis and nuclei donation by satellite cells after birth or hatch. Therefore, understanding the cellular and molecular mechanisms of myogenesis and muscle development is of great interest. Myostatin is a well-known negative regulator of muscle growth and development that inhibits proliferation and differentiation in myogenic cells as well as protein synthesis in existing muscle fibers. In this review, various inhibitors of myostatin activity or signaling are examined that may be used in animal agriculture for enhancing muscle growth. Myostatin inhibitors are relevant as potential therapies for muscle-wasting diseases and muscle weakness in humans and animals. Currently, there are no commercial myostatin inhibitors for agriculture or biomedical purposes because the safest and most effective option has yet to be identified. Further investigation of myostatin inhibitors and administration strategies may revolutionize animal production and the medical field.

A novel mechanism of myostatin regulation by its alternative splicing variant during myogenesis in avian species

Myostatin (MSTN) is a key negative regulator of muscle growth and development, and an increase of muscle mass is achieved by inhibiting MSTN signaling. In the current study, five alternative splicing isoforms of MSTN mRNAs in avian species were identified in various tissues. Among these five, three truncated forms of myostatin, MSTN-B, -C, and -E created premature stop codons and produced partial MSTN prodomains encoded from exon 1. MSTN-B is the second dominant isoform following full-length MSTN-A, and their expression was dynamically regulated during muscle development of chicken, turkey, and quail in vivo and in vitro. To clarify the function of MSTN-B, two stable cell lines of quail myoblasts (QM7) were generated to overexpress MSTN-A or MSTN-B. Interestingly, MSTN-B promoted both cell proliferation and differentiation similar to the function of the MSTN prodomain to counteract the negative role of MSTN on myogenesis. The coimmunoprecipitation assay revealed that MSTN-B binds to MSTN-A and reduces the generation of mature MSTN. Furthermore, the current study demonstrated that the partial prodomain encoded from exon 1 is critical for binding of MSTN-B to MSTN-A. Altogether, these data imply that alternative splicing isoforms of MSTN could negatively regulate pro-myostatin processing in muscle cells and prevent MSTN-mediated inhibition of myogenesis in avian species.

Undernutrition regulates the expression of a novel splice variant of myostatin and insulin-like growth factor 1 in ovine skeletal muscle

Undernutrition suppresses the growth of skeletal muscles and alters the expression of insulin-like growth factor 1 (IGF1), a key mitogen, and myostatin, a potent inhibitor of myogenesis. These changes can explain, at least in part, the reduced growth of skeletal muscles in underfed lambs. We have recently identified a myostatin splice variant (MSV) that binds to and antagonizes the canonical signaling of myostatin. In the present study, we hypothesized that the expression of MSV would be reduced in conjunction with myostatin and IGF1 in response to underfeeding in skeletal muscles of sheep. Young growing ewes were fed either ad libitum or an energy-restricted diet (30% of maintenance requirements) for 28 d. This regime of underfeeding resulted in a 24% reduction in body mass (P < 0.001) and a 36% reduction in the mass of the semitendinosus muscles relative to controls (P < 0.001) by day 28. The concentrations of MSV and IGF1 messenger RNA (mRNA) were reduced (both P < 0.001), but myostatin mRNA was not altered in semitendinosus muscles. Unlike the reduced expression of mRNA, the abundance of MSV protein was increased (P < 0.05) and there was no change in the abundance of myostatin protein. Our results suggest that undernutrition for 28 d decreases the signaling of myostatin by increasing the abundance of MSV protein. Although this action may reduce the growth inhibitory activity of myostatin, it cannot prevent the loss of growth of skeletal muscles during undernutrition.

Identification and expression of a novel transcript of the growth and differentiation factor-11 gene

The growth and differentiation factor-11 (GDF-11) gene is thought to code for a single protein that plays a crucial role in regulating the development of multiple tissues. In this study, we aimed to investigate if the GDF-11 gene has another transcript and, if so, to characterise this transcript and determine its tissue-specific and developmental expression. We have identified a novel transcript of GDF-11 in mouse muscle, which contains the 3' region of intron 1, exon 2, exon 3 and 3'UTR, and has two transcription initiation sites and a single termination site. We named the novel transcript GDF-11ΔEx1 because it does not contain exon 1 of canonical GDF-11. The GDF-11ΔEx1 transcript was expressed in the skeletal muscles, heart, brain and kidney, but was undetectable in the liver and gut. The concentration of the GDF-11ΔEx1 transcript was increased in gastrocnemius muscles from three to 6 weeks of age, a period of accelerated muscle growth, steadily declined thereafter and was higher in male than female mice (P < 0.001 for age and sex). GDF-11ΔEx1 cDNA was predicted to code for a putative N-terminal-truncated propeptide and the canonical ligand for GDF-11. However, propeptide-specific antibodies could not identify proteins of the expected size in skeletal muscle. Interestingly, in silico analysis of the GDF-11ΔEx1 RNA predicted a secondary structure with the potential to coordinate multiple protein interactions as a molecular scaffold. Therefore, we postulate that GDF-11ΔEx1 may act as a long non-coding RNA to regulate the transcription of canonical GDF-11 and/or other genes in skeletal muscle and other tissues.