Molecular cloning of the Atlantic salmon activin receptor IIB cDNA - Localization of the receptor and myostatin in vivo and in vitro in muscle cells

Calanus finmarchicus hydrolysate improves growth performance in feeding trial with European sea bass juveniles and increases skeletal muscle growth in cell studies

The world will be dependent on the development of novel feed ingredients from renewable sources to ensure sustainable growth of the aquaculture industry. Zooplankton like Calanus finmarchicus are viable new raw material candidates, as they have optimal nutrient profiles for aquatic animals and may be sustainably harvested in large volumes. In this study, the aim was to investigate if a protein hydrolysate of C. finmarchicus was able to influence the growth performance of fish. The effect of dietary inclusion of hydrolysates was tested in a feeding trial with European sea bass (Dicentrarchus labrax) juveniles, benchmarking calanus hydrolysate (CH) against commercially available hydrolysates. The diet with CH inclusion yielded increased growth, with significantly higher body weight than hydrolysates of sardine and tuna fish at the end of the trial. The observed growth-promoting effects were further examined using an in vitro model with skeletal muscle cells from Atlantic salmon. Through bioactivity experiments with muscle cells grown in media containing CH, low-molecular fractions were found to have the greatest positive effect on proliferation, viability, and expression of muscle-specific genes. Characterization of the most potent fraction revealed an abundance of small peptides, along with amino acids and marine metabolites associated with increased muscle growth.

Activin receptor type IIB in rohu (Labeo rohita): molecular characterization, tissue distribution and immunohistochemical localization during different stages of gonadal maturation

Activin receptor type IIB (ActRIIB) is a transmembrane serine/threonine kinase receptor which plays a pivotal role in regulating the reproduction in vertebrates including teleost. Earlier studies have documented its importance in governing gonadal maturation in higher vertebrates. However, reports on the regulation of fish reproductive system by ActRIIB gene are still limited. Here, we report the identification and characterization of ActRIIB cDNA of Labeo rohita, a commercially important fish species of the Indian subcontinent. The full-length gene encoding rohu ActRIIB was cloned and found to be of 1674 bp in length. Functional similarities were evident from evolutionary analysis across vertebrates. Real-time PCR to measure the expression of ActRIIB transcript in rohu revealed significant mRNA levels in gonads followed by non-reproductive tissues, including the brain, pituitary and muscle. With respect to different gonadal maturation stages, predominant expression of ActRIIB mRNA was observed during the pre-spawning phase of both sexes. To further delineate its role in rohu reproduction, a recombinant protein of the extracellular domain of ActRIIB (rECD-ActRIIB) was produced, and polyclonal antibody is raised against the protein for its immuno-localization studies during different gonadal maturation stages. Strong immunoreactivity was noticed in the pre-vitellogenic oocytes which decreased dramatically in the fully mature oocytes. Similarly, the strong and intense immunoreactivity was found in the spermatids and spermatocytes of the immature testis, and eventually the intensity reduced with the progression of the maturation stage. These results provide the first evidence of the presence of ActRIIB in rohu gonadal tissues. Taken together, our observations lay the groundwork for further understanding and investigating on the potential role of ActRIIB in fish reproduction system in the event of gonadal maturation.

The effects of exogenous cortisol on myostatin transcription in rainbow trout, Oncorhynchus mykiss

Glucocorticoids (GCs) strongly regulate myostatin expression in mammals via glucocorticoid response elements (GREs), and bioinformatics methods suggest that this regulatory mechanism is conserved among many vertebrates. However, the multiple myostatin genes found in some fishes may be an exception. In silico promoter analyses of the three putative rainbow trout (Oncorhynchus mykiss) myostatin promoters have failed to identify putative GREs, suggesting a divergence in myostatin function. Therefore, we hypothesized that myostatin mRNA expression is not regulated by glucocorticoids in rainbow trout. In this study, both juvenile rainbow trout and primary trout myoblasts were treated with cortisol to examine the effects on myostatin mRNA expression. Results suggest that exogenous cortisol does not regulate myostatin-1a and -1b expression in vivo, as myostatin mRNA levels were not significantly affected by cortisol treatment in either red or white muscle tissue. In red muscle, myostatin-2a levels were significantly elevated in the cortisol treatment group relative to the control, but not the vehicle control, at both 12 h and 24 h post-injection. As such, it is unclear if cortisol was acting alone or in combination with the vehicle. Cortisol increased myostatin-1b expression in a dose-dependent manner in vitro. Further work is needed to determine if this response is the direct result of cortisol acting on the myostatin-1b promoter or through an alternative mechanism. These results suggest that regulation of myostatin by cortisol may not be as highly conserved as previously thought and support previous work that describes potential functional divergence of the multiple myostatin genes in fishes.

Revisiting the paradigm of myostatin in vertebrates: insights from fishes

In the last decade, myostatin (MSTN), a member of the TGFβ superfamily, has emerged as a strong inhibitor of muscle growth in mammals. In fish many studies reveal a strong conservation of mstn gene organization, sequence, and protein structures. Because of ancient genome duplication, teleostei may have retained two copies of mstn genes and even up to four copies in salmonids due to additional genome duplication event. In sharp contrast to mammals, the different fish mstn orthologs are widely expressed with a tissue-specific expression pattern. Quantification of mstn mRNA in fish under different physiological conditions, demonstrates that endogenous expression of mstn paralogs is rarely related to fish muscle growth rate. In addition, attempts to inhibit MSTN activity did not consistently enhance muscle growth as in mammals. In vitro, MSTN stimulates myotube atrophy and inhibits proliferation but not differentiation of myogenic cells as in mammals. In conclusion, given the strong mstn expression non-muscle tissues of fish, we propose a new hypothesis stating that fish MSTN functions as a general inhibitors of cell proliferation and cell growth to control tissue mass but is not specialized into a strong muscle regulator.

In vitro indeterminate teleost myogenesis appears to be dependent on Pax3

The zebrafish (Danio rerio) has been used extensively as a model system for developmental studies but, unlike most teleost fish, it grows in a determinate-like manner. A close relative, the giant danio (Devario cf. aequipinnatus), grows indeterminately, displaying both hyperplasia and hypertrophy of skeletal myofibers as an adult. To better understand adult muscle hyperplasia, a postlarval/postnatal process that closely resembles secondary myogenesis during development, we characterized the expression of Pax3/7, c-Met, syndecan-4, Myf5, MyoD1, myogenin, and myostatin during in vitro myogenesis, a technique that allows for the complete progression of myogenic precursor cells to myotubes. Pax7 appears to be expressed only in newly activated MPCs while Pax3 is expressed through most of the myogenic program, as are c-Met and syndecan-4. MyoD1 appears important in all stages of myogenesis, while Myf5 is likely expressed at low to background levels, and myogenin expression is enriched in myotubes. Myostatin, like MyoD1, appears to be ubiquitous at all stages. This is the first comprehensive report of key myogenic factor expression patterns in an indeterminate teleost, one that strongly suggests that Pax3 and/or Myf5 may be involved in the regulation of this paradigm. Further, it validates this species as a model organism for studying adult myogenesis in vitro, especially mechanisms underlying nascent myofiber recruitment.

Immunolocalization of myostatin (GDF-8) following musculoskeletal injury and the effects of exogenous myostatin on muscle and bone healing

The time course and cellular localization of myostatin expression following musculoskeletal injury are not well understood; therefore, the authors evaluated the temporal and spatial localization of myostatin during muscle and bone repair following deep penetrant injury in a mouse model. They then used hydrogel delivery of exogenous myostatin in the same injury model to determine the effects of myostatin exposure on muscle and bone healing. Results showed that a "pool" of intense myostatin staining was observed among injured skeletal muscle fibers 12-24 hr postsurgery and that myostatin was also expressed in the soft callus chondrocytes 4 days following osteotomy. Hydrogel delivery of 10 or 100 µg/ml recombinant myostatin decreased fracture callus cartilage area relative to total callus area in a dose-dependent manner by 41% and 80% (p<0.05), respectively, compared to vehicle treatment. Myostatin treatment also decreased fracture callus total bone volume by 30.6% and 38.8% (p<0.05), with the higher dose of recombinant myostatin yielding the greatest decrease in callus bone volume. Finally, exogenous myostatin treatment caused a significant dose-dependent increase in fibrous tissue formation in skeletal muscle. Together, these findings suggest that early pharmacological inhibition of myostatin is likely to improve the regenerative potential of both muscle and bone following deep penetrant musculoskeletal injury.

Myostatin (MSTN) gene duplications in Atlantic salmon (Salmo salar): evidence for different selective pressure on teleost MSTN-1 and -2

Whereas the negative muscle regulator myostatin (MSTN) in mammals is almost exclusively expressed in the muscle by a single encoding gene, teleost fish possess at least two MSTN genes which are differentially expressed in both muscular and non-muscular tissues. Duplicated MSTN-1 genes have previously been identified in the tetraploid salmonid genome. From Atlantic salmon we succeeded in isolating the paralogous genes of MSTN-2, which shared about 70% identity with MSTN-1a and -1b. The salmon MSTN-2a cDNA encoded a predicted protein of 363 residues and included the conserved C-terminal bioactive domain. MSTN-2a seemed to be primarily expressed in the brain, and a functional role of teleost MSTN-2 in the neurogenesis similar to the inhibitory action of the closely related GDF-11 in the mammalian brain was proposed. In contrast, a frame-shift mutation in exon 1 of salmon MSTN-2b would lead to the synthesis of a putatively non-functional truncated protein. The absence of processed MSTN-2b mRNA in the examined tissues indicated that this gene has become a non-functional pseudogene. The differential, but partially overlapping, expression patterns of salmon MSTN-2a, -1a and -1b in muscular and non-muscular tissues are probably due to the different arrangement of the potential cis-acting regulatory elements identified in their putative promoter regions. Single and paired E-boxes in the MSTN-1b promoter were shown to bind both homo-and hetero-dimers of the myogenic regulatory factor MyoD and E47 in vitro of importance for initiating the myogenic program. Analyses of nucleotide substitution patterns indicated that the teleost MSTNs essentially have evolved under purifying selection, but a subset of amino acid sites under positive selective pressure were identified within the MSTN1 branch. The results may reflect the evolutionary forces related to adoption of the different functional roles proposed for the teleost MSTN isoforms. The phylogenetic analysis of multiple vertebrate MSTNs suggested at least two separate gene duplication events in the fish lineage. Linkage analysis of polymorphic microsatellites within intron 2 of salmon MSTN-1a and -1b mapped the two genes to different linkage groups in agreement with the tetraploid origin of the duplicated salmonid MSTN-1 and MSTN-2 genes.