Selection for growth rate or against back fat thickness in pigs is associated with changes in growth hormone axis plasma protein concentration and mRNA level.

Selection for increased growth rate or decreased back fat thickness results in concomitant changes in endocrine and metabolic status. Growth hormone (GH) changes in blood plasma concentration related to selection for growth rate and fat deposition were reported in pigs. The molecular mechanisms regulating selection-induced changes in GH plasma concentration remain largely unknown. We investigated selection-associated changes in GH axis parameters in 2 pig lines selected for increased growth rate (F-line), or decreased back fat thickness (L-line), respectively. First, we investigated selection-associated changes in GH pulse parameters. In both selection lines we found each generation a declining GH peak maximum concentration and area under the GH curve. GH pulse width was not associated with generation number. In both lines generation number was associated with a declined pulse interval, indicating that the number of pulses per day increased on average with 1 pulse per 24 h per generation. Second, plasma concentration of GH axis related Insulin-like growth factor-I (IGF-I) and insulin were investigated. Plasma IGF-I concentration was not associated with generation number in the F-line. Mean plasma insulin concentration declined each generation in both lines. Third, we investigated changes in GH and Pit-1 mRNA levels. In both selection lines GH and Pit-1 mRNA levels increased approximately 50% each generation. The high SD of the GH mRNA levels in both lines may suggest that the GH mRNA levels are pulsatile in vivo. We postulate a molecular mechanism that may explain how selection is associated with increased GH mRNA levels and GH pulse numbers, while lowering GH release per pulse.

Associations of heart and adipocyte fatty acid-binding protein gene expression with intramuscular fat content in pigs.

Intramuscular fat content is a major determinant of meat quality in pigs. Previously, polymorphisms in the adipocyte and heart fatty acid-binding protein genes, A-FABP and H-FABP, have been significantly associated with genetic variation of intramuscular fat content in a Duroc pig population. Further support for the role of H-FABP but not for A-FABP was found in a Meishan crossbred population. However, the effect of closely linked genes could not be excluded in these analyses. To validate the role of A-FABP and H-FABP in intramuscular fat accretion, 153 pigs of a crossbred genotype were evaluated for the A-FABP and H-FABP polymorphisms, mRNA, and protein expression levels of both FABP genes and intramuscular fat content in the longissimus lumborum muscle. For H-FABP, statistical analyses showed significant differences in mRNA but not protein expression levels between H-FABP HaeIII PCR-RFLP genotype classes. Between these genotype classes, significant differences in intramuscular fat content were found within barrows but not in gilts. Moreover, H-FABP mRNA but not protein expression levels were significantly related to intramuscular fat content. For A-FABP genotype classes, no significant differences in mRNA and protein expression levels were found. However, a significant difference in intramuscular fat content was found within barrows but not in gilts. In addition, a significant relationship between A-FABP mRNA but not protein expression levels and intramuscular fat content was found. In conclusion, variation of intramuscular fat content could not be explained by differences in A-FABP and H-FABP mRNA and protein expression levels. However, this may be due to limitations of the assays used and(or) the inappropriateness of the time of sampling. Finally, results suggest that A-FABP and H-FABP expression are translationally rather than transcriptionally regulated.

Glucocorticoid inhibition of C2C12 proliferation rate and differentiation capacity in relation to mRNA levels of the MRF gene family.

The muscle regulatory factors (MRF) gene family regulate muscle fibre development. Several hormones and drugs also affect muscle development. Glucocorticoids are the only drugs reported to have a beneficial effect on muscle degenerative disorders. We investigated the glucocorticoid-related effects on C2C12 myoblast proliferation rate, morphological differentiation, and subsequent mRNA expression patterns of the MRF genes. C2C12 cells were incubated with the glucocorticoids dexamethasone or alpha-methyl-prednisolone. Both glucocorticoids showed comparable effects. Glucocorticoid treatment of C2C12 cells during the proliferative phase reduced the proliferation rate of the cells dose dependently, especially during the third and fourth day of culture, increased MyoD1, myf-5, and MRF4 mRNA levels, and reduced myogenin mRNA level, compared to untreated control cells. Thus, the mRNA level of proliferation-specific MyoD1 and myf-5 expression does not seem to associate with C2C12 myoblast proliferation rate. Glucocorticoid treatment of C2C12 cells during differentiation reduced the differentiation capacity dose dependently, which is accompanied by a dose dependent reduction of myogenin mRNA level, and increased MyoD1, myf-5, and MRF4 mRNA levels compared to untreated control cells. Therefore, we conclude that glucocorticoid treatment reduces differentiation of C2C12 myoblasts probably through reduction of differentiation-specific myogenin mRNA level, while inducing higher mRNA levels of proliferation-associated MRF genes.

Messenger ribonucleic acid expression of the MyoD gene family in muscle tissue at slaughter in relation to selection for porcine growth rate.

Livestock meat production capacity is related to muscle fiber numbers and growth. Muscle fibers develop during early embryonic development from proliferating and differentiating myoblasts. Post-natal muscle growth requires satellite cell proliferation and differentiation. Myoblast and satellite cell proliferation and differentiation is regulated by the genes of the MyoD gene family (myogenin, myf-5, myf-6, and MyoD1). Our aim was to study the mRNA expression of these genes in postnatal muscle tissue in relation to porcine selection for growth rate or leanness. Five boars from a line selected for fast growth (F-line) and five boars from a line selected against backfat thickness (L-line) were slaughtered, and biopsies were taken from 12 muscles. Between-line effects, within-line effects in relation to the performance of the pigs, and muscle-specific effects were studied. Comparing the F-line with the L-line revealed significantly greater myogenin, myf-5, and MyoD1 mRNA expression in some muscles of the F-line. The expression of myf-6 showed a tendency for the opposite effect in some muscles. Muscles were ordered by their muscle-specific growth rate (b-value). Within-line evaluation of the data revealed a systematic muscle effect for the myf-6 expression level in the F-line because higher b-values correlated with increased myf-6 expression level. Backfat thickness was negatively related to myogenin expression in the F-line. A relationship was found between myogenin:MyoD1 mRNA expression ratio and meat color/muscle fiber type composition in the L-line. Furthermore, the myogenin:MyoD1 ratio was greater in muscles from F-line boars than in muscles from L-line boars, which relates to the difference between the lines in muscle fiber type. We conclude that the mRNA levels of the MyoD genes in porcine muscle tissue at slaughter showed different relationships to selection for growth rate when evaluated between selection lines and within selection lines.

Influences of myogenin genotypes on birth weight, growth rate, carcass weight, backfat thickness, and lean weight of pigs.

Lean weight is related to muscle fiber number. Muscle fiber formation (myogenesis) occurs only during embryonic development when it is under the control of the MyoD gene family consisting of myogenin, MyoD1, myf-5, and myf-6. Myogenin has a central position within the MyoD gene family because myogenin expression abrogates myoblast proliferation potential and regulates the differentiation of single nucleated myoblasts into multinucleated myofibers. Thus, myogenin genotype could be related to variation in the number of muscle fibers formed, leading to variation in muscle mass and, thus, lean weight. A polymorphism at the porcine myogenin locus was associated with birth weight, growth rate, lean weight at 200 d, and backfat thickness. Yorkshire pigs from two commercial lines were genotyped, and crosses between heterozygous pigs and heterozygous and homozygous pigs were made. Resulting litters were genotyped, and phenotypic data were collected. Significant differences were found between the two homozygous myogenin genotypes for birth weight, growth rate, and lean weight, but not for backfat thickness. Variation at the myogenin locus explained 4% of the total phenotypic variation in birth weight, growth rate, and carcass weight, and 5.8% of the total variation in lean weight. We conclude that myogenin genotype influences porcine growth rate and muscle mass.

Effect of genetic variants of the heart fatty acid-binding protein gene on intramuscular fat and performance traits in pigs.

In order to find genetic markers to improve the meat quality of pigs by breeding we studied the relationship between variation in the heart fatty acid-binding protein (H-FABP) gene (FABP3) and intramuscular fat (IMF) content. To estimate the effect of H-FABP, pigs from two Duroc populations were selectively mated in such a way that at least two genotypes were present in each litter. In total, data from 983 pigs and pedigree information from three preceding generations were analyzed. Offspring were tested for IMF content as well as backfat thickness (BFT), BW, and drip loss of the meat (DRIP). All pigs were assigned to H-FABP RFLP genotype classes either by the assessed genotype (75%) or based on a probability score determined according to genotypic information of their relatives (25%). Contrasts were detected between homozygous H-FABP RFLP genotype classes for IMF content (.4%, P < .05), BFT (.6 mm, P < .01), and BW (2.4 kg, P < .10). No significant contrasts were detected for DRIP. Results for IMF content, BFT, and BW were confirmed when only genotyped animals were analyzed. Variation in BFT partially explained the effect on IMF content. Although other closely linked genes on porcine chromosome 6 might be responsible for the observed effect, interference of the halothane gene was excluded because all parental animals were noncarriers. In conclusion, H-FABP RFLP can be used as markers to select for increased IMF content and growth in breeding programs.

Gender related and dexamethasone induced differences in the mRNA levels of the MRF genes in rat anterior tibial skeletal muscle.

Muscle formation and postnatal growth is under the control of the muscle regulatory factors (MRF) gene family, consisting of four genes: MyoD1, myogenin, myf-5, and myf-6. Muscle mass is also known to be affected by specific drugs, like glucocorticoids. Glucocorticoids have also been characterized as muscle atrophying agents. However, glucocorticoids are also the only drugs reported to have a beneficial effect on the treatment of muscle degenerative disorders. Since muscle mass relates to gender, this may be partially caused by gender. The aim of this study is to investigate gender-related basal and dexamethasone-induced expression of the MRF genes. Gender-specific MRF mRNA levels were investigated in anterior tibial muscles of the rat. Myogenin, myf-5, and myf-6 mRNA level was significantly higher in female rats than in male rats. Since muscle mass is usually higher in males, we conclude that the development of gender-related differences in muscle mass is not primarily under the control of the mRNA levels of the MRF genes. Male rats treated with dexamethasone for 14 days (1 mg per kg body weight) showed increased levels of MyoD1, myogenin and myf-5 compared to control male rats. Female rats treated with dexamethasone showed decreased expression of myf-6 compared to control female rats. These results suggest that dexamethasone increase satellite cell-specific MRF activity in male muscle tissue, which is suggested to be associated with muscle hypertrophy, while maintenance of muscle tissue is affected in female muscle tissue. Therefore, we conclude that both basal and dexamethasone-induced MRF gene mRNA levels are regulated gender-specific.