Myostatin mRNA Levels Research Articles

Work-induced changes in skeletal muscle IGF-1 and myostatin gene expression in uremia

Resistance to growth hormone (GH)-induced insulin-like growth factor-1 (IGF-1) gene expression contributes to uremic muscle wasting. Since exercise stimulates muscle IGF-1 expression independent of GH, we tested whether work overload (WO) could increase skeletal muscle IGF-1 expression in uremia and thus bypass the defective GH action. Furthermore, to provide insight into the mechanism of uremic wasting and the response to exercise we examined myostatin expression. Unilateral plantaris muscle WO was initiated in uremic and pairfed (PF) normal rats by ablation of a gastrocnemius tendon and adjoining part of this muscle with the contralateral plantaris as a control. Some rats were GH treated for 7 days. WO led to similar gains in plantaris weight in both groups and corrected the uremic muscle atrophy. GH increased plantaris IGF-1 mRNA >twofold in PF rats but the response in uremia was severely attenuated. WO increased the IGF-1 mRNA levels significantly in both uremic and PF groups, albeit less brisk in uremia; however, after 7 days IGF-1 mRNA levels were elevated similarly, >2-fold, in both groups. In the atrophied uremic plantaris muscle basal myostatin mRNA levels were increased significantly and normalized after an increase in WO suggesting a myostatin role in the wasting process. In the hypertrophied uremic left ventricle the basal myostatin mRNA levels were reduced and likely favor the cardiac hypertrophy. Together the findings provide insight into the mechanisms of skeletal muscle wasting in uremia and the hypertrophic response to exercise, and suggest that alterations in the balance between IGF-1 and myostatin play an important role in these processes.

Resistance Exercise-Induced Myostatin mRNA Expression Before and After a 6-month Strength Training Period

Myostatin (MSTN) inhibits skeletal muscle growth and it may also inhibit training-induced muscle growth. MSTN acts by binding to the activin IIb receptor (ActIIb) but its binding can be inhibited by follistatin-related gene (FLRG) protein. Decreased MSTN mRNA expression has been shown in untrained subjects 24h after a single resistance exercise (RE). This molecular response may be important in strength training (ST) mediated hypertrophy together with possible changes in ActIIb and FLRG expression. PURPOSE: To determine MSTN, ActIIb and FLRG responses at mRNA level to a single RE before and after six months of ST. METHODS: Nine (61.0±5.5 yrs) previously untrained healthy men participated in the study. RE was performed one wk before and one wk after ST. The RE included 5 sets of 10RM bilateral leg presses. Muscle biopsies were taken from the vastus lateralis 0.5h before (pre), and 1h (post 1h) as well as 48h (post 48h) after the RE. MSTN, ActIIb and FLRG mRNA expression were determined by Taqman probe-based realtime RT-PCR. RESULTS: The basal (pre) MSTN mRNA levels were significantly increased during the ST (p<0.05) (Fig 1). After ST the MSTN post 48h levels were significantly lower compared to the corresponding pre RE levels (p<0.01). The ActIIb mRNA levels decreased before and after ST due to RE (post 1h p<0.05; post 48h p<0.10). No changes were observed in FLRG mRNA.Fig 1: Resistance exercise-induced myostatin mRNA expression before and after ST.CONCLUSION: The present study showed that ST increased basal MSTN levels, which has been shown previously in some but not all studies. The novel finding was that both MSTN and its receptor ActIIb levels decreased due to RE. However, the decrease in MSTN was observed only in post-training RE. Both of these decreases probably decrease the catabolic action of MSTN. The present study indicates that resistance exercise decreases MSTN and ActIIb expression, at least after training. Both of these anabolic responses may be important in the skeletal muscle hypertrophy process, and they should be examined in the future. Supported by the Finnish Ministry of Education.

Expression of myostatin RNA transcript and protein in gastrocnemius muscle of rats after sciatic nerve resection

Myostatin, a member of the transforming growth factor-beta (TGF-beta) superfamily, has been identified as an inhibitor of skeletal muscle mass. To have an insight into the expression pattern of myostatin and its potential role in skeletal muscle atrophy induced by denervation, we used an animal model of peripheral nerve resection to examine the time-dependent changes in myostatin mRNA and protein levels in the denervated gastrocnemius muscle of rats after sciatic neurectomy by the aid of quantitative real-time RT-PCR and Western blotting, respectively. We also conducted morphometric analyses to measure the wet weight ratio of the denervated muscle (the operated side/contralateral non-operated side) and the cross sectional area of muscle fibers and to observe muscle morphology. The experimental results showed that myostatin mRNA and protein levels in rat gastrocnemius muscle persistently elevated after denervation, despite a fluctuation of myostatin mRNA level at day 3 after denervation, reached their respective peaks at day 28 after denervation, and then depressed slightly until day 56 after denervation. Furthermore, a significant negative linear correlation was found between myostatin abundance and muscle atrophy degree, suggesting that myostatin might probably play an important role in denervation-induced muscle atrophy. Our present study perhaps provides a new window into myostatin regulation in association with a specific type of muscle atrophy.

The Effect of 30 Minutes of Passive Stretch of the Rat Soleus Muscle on the Myogenic Differentiation, Myostatin, and Atrogin-1 Gene Expressions

Gomes AR, Soares AG, Peviani S, Nascimento RB, Moriscot AS, Salvini TF. The effect of 30 minutes of passive stretch of the rat soleus muscle on the myogenic differentiation, myostatin, and atrogin-1 gene expressions. Objective To evaluate the effect of passive stretch, applied for 30 minutes to the rat soleus muscle, on the myogenic differentiation (myoD), myostatin, and atrogin-1 gene expressions. Design Case-controlled study. Setting University laboratory. Animals Fifty 12-week-old male Wistar rats. Interventions Six groups of animals were given a single stretch bout and were evaluated immediately and 8, 24, 48, 72, and 168 hours later. Another 3 groups were evaluated immediately after 2, 3, and 7 stretches. An intact control group was also analyzed. Main Outcome Measures The messenger ribonucleic acid (mRNA) levels of myoD, myostatin, and atrogin-1 were assessed by real-time polymerase chain reaction. Results Twenty-four hours after a single session of stretch only, the myoD mRNA levels had increased compared with the control group, whereas an increase in the atrogin-1 expression was observed after 2, 3, and 7 stretches. Conclusions A single session of passive stretch increased the myoD gene expression, a factor related to muscle growth. Interestingly, daily stretches increased the atrogin-1 gene expression, a gene primarily associated with muscle atrophy. The results indicated that gene expression was responsive to the number of stretch sessions.

Aging-sensitive cellular and molecular mechanisms associated with skeletal muscle hypertrophy

Sarcopenia is an age-related loss of muscle mass and strength. The aged can increase various measures of muscle size and strength in response to resistance exercise (RE), but this may not normalize specific tension. In rats, aging reduces the hypertrophy response and impairs regeneration. In this study, we measured cellular and molecular markers, indicative of muscle hypertrophy, that also respond to acute increases in loading. Comparing 6- and 30-mo-old rats, the aims were to 1) determine whether these markers are altered with age and 2) identify age-sensitive responses to acute RE. The muscles of old rats exhibited sarcopenia involving a deficit in contractile proteins and decreased force generation. The RNA-to-protein ratio was higher in the old muscles, suggesting a decrease in translational efficiency. There was evidence of reduced signaling via components downstream from the insulin/insulin-like growth factor (IGF)-I receptors in old muscles. The mRNA levels of myostatin and suppressor of cytokine signaling 2, negative regulators of muscle mass, were lower in old muscles but did not decrease following RE. RE induced increases in the mRNAs for IGF-I, mechano-growth factor, cyclin D1, and suppressor of cytokine signaling 3 were similar in old and young muscles. RE induced phosphorylation of the IGF-I receptor, and Akt increased in young but not old muscles, whereas that of S6K1 was similar for both. The results of this study indicate that a number of components of intracellular signaling pathways are sensitive to age. As a result, key anticatabolic responses appear to be refractory to the stimuli provided by RE.

Myostatin auto‐regulates its expression by feedback loop through Smad7 dependent mechanism

Myostatin, a secreted growth factor, is a member of the TGF-beta superfamily and an inhibitor of myogenesis. Previously, we have shown that myostatin gene expression is regulated at the level of transcription and that myostatin is a downstream target gene of MyoD. Here we show that myostatin gene expression is auto-regulated by a negative feedback mechanism. Northern blot analysis indicated that there are relatively higher levels of myostatin mRNA in the biceps femoris muscle of cattle that express a non- functional myostatin allele (Belgian Blue) as compared to normal cattle. In contrast, addition of exogenous myostatin decreases endogenous myostatin mRNA. Consistent with this result, wild type myostatin protein is able to repress myostatin promoter activity via Activin type IIb receptor (ActRIIB) and ALK5 (P < 0.001). However, non-functional myostatin (Piedmontese) failed to repress the myostatin promoter suggesting that myostatin auto-regulates its promoter by negative feedback inhibition. Auto-regulation by myostatin appears to be signaled through Smad7, since the expression of the inhibitory Smad7 is induced by myostatin and the over-expression of Smad7 in turn inhibits the myostatin promoter activity (P < 0.001). In contrast down regulation of Smad7 by siRNA results in increased myostatin mRNA indicating that Smad7 is a negative regulator of myostatin gene expression. Consistent with these results, a decrease in Smad7 mRNA and concomitant increase in myostatin expression is seen in myotubes that express non functional myostatin. In addition, interference with myostatin signaling prevents the induction of Smad7 promoter activity by myostatin. Based on these results, we propose that myostatin auto-regulates its gene expression through a Smad7 dependent mechanism in myogenic cells.

Proteolytic processing of myostatin is auto-regulated during myogenesis

Myostatin, a potent negative regulator of myogenesis, is proteolytically processed by furin proteases into active mature myostatin before secretion from myoblasts. Here, we show that mature myostatin auto-regulates its processing during myogenesis. In a cell culture model of myogenesis, Northern blot analysis revealed no appreciable change in myostatin mRNA levels between proliferating myoblasts and differentiated myotubes. However, Western blot analysis confirmed a relative reduction in myostatin processing and secretion by differentiated myotubes as compared to proliferating myoblasts. Furthermore, in vivo results demonstrate a lower level of myostatin processing during fetal muscle development when compared to postnatal adult muscle. Consequently, high levels of circulatory mature myostatin were detected in postnatal serum, while fetal circulatory myostatin levels were undetectable. Since Furin proteases are important for proteolytically processing members of the TGF-β superfamily, we therefore investigated the ability of myostatin to control the transcription of furin and auto-regulate the extent of its processing. Transfection experiments indicate that mature myostatin indeed regulates furin protease promoter activity. Based on these results, we propose a mechanism whereby myostatin negatively regulates its proteolytic processing during fetal development, ultimately facilitating the differentiation of myoblasts by controlling both furin protease gene expression and subsequent active concentrations of mature myostatin peptide.

Effect of testosterone on insulin-like growth factor-I, androgen receptor, and myostatin gene expression in splenius and semitendinosus muscles in sheep

Testosterone is known to act differentially on skeletal muscle from different regions. Two genes likely to mediate the testosterone effect are IGF-I, an important growth regulator acting in an autocrine and paracrine way, and androgen receptor (AR), as receptor density could account for differential muscle growth. Another muscle-specific gene that may play a role in differential muscle growth is myostatin, a member of the transforming growth factor-beta superfamily, shown to be a negative regulator of skeletal muscle mass. The objective of this study was to quantify and compare the expression of these three genes in two different skeletal muscles in sheep. East Friesian x Dorset-sired ram lambs from Dorset ewes were used in a 2 x 4 factorial experiment. Eighteen sets of twins were assigned to four age groups corresponding to 77, 105, 133, and 161 d of age, and one individual from each set was castrated at birth. Total RNA was extracted from samples of splenius (SP) and semitendinosus muscles collected at the time of slaughter. Insulin-like growth factor-I mRNA was measured using competitive reverse-transcription PCR. Androgen receptor and myostatin mRNA were measured by ribonuclease protection assay with standard curves. Weight of SP was greater than semitendinosus in rams compared with wethers at 105, 133, and 161 d (P = 0.05, P = 0.04, and P = 0.02, respectively). The difference in IGF-I mRNA levels between the two muscles was greater in rams than in wethers at 133 (P = 0.001) and 161 d (P = 0.014), and the difference in AR mRNA levels was greater in rams than in wethers at 105, 133, and 161 d (P = 0.002, P < 0.001, and P < 0.001, respectively), with greater abundance in the SP. No difference was found in myostatin mRNA level between the two muscles in rams and wethers at any age. These results suggest that locally produced IGF-I and the regulation of AR expression are important for sexually dimorphic muscle growth patterns.

Short‐term endurance training results in a muscle‐specific decrease of myostatin mRNA content in the rat

Myostatin has been characterized as a negative regulator of skeletal muscle growth. To examine a probable function of myostatin during the adaptation of skeletal muscle in response to training, we analysed the effect of short-term endurance training on myostatin and insulin-like growth factor-I (IGF-I) mRNA contents in rat skeletal muscles. To assess the impact of the training stimulus, mRNA levels of metabolic genes were analysed simultaneously. Male Wistar rats were trained for 5 days by swimming, while another group remained untrained. Myostatin, IGF-I, glucose transporter 4 (GLUT4), hexokinase II (HK II) and hydroxyacyl-CoA dehydrogenase (HAD) mRNA levels were determined by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) in gastrocnemius, vastus lateralis and soleus muscles. A time course experiment was conducted, in order to examine transient changes of myostatin mRNA contents in gastrocnemius 7 and 24 h after one-swimming session as well as 24 h after a 3-day swimming training. No significant changes in IGF-I and GLUT4 mRNA levels were found in any of the muscles analysed. mRNA contents of myostatin were significantly reduced in gastrocnemius and vastus lateralis but not in soleus. In agreement to this pattern, we found significantly higher mRNA levels of HK II and HAD in the trained group. The time course experiment revealed significantly reduced myostatin mRNA contents in gastrocnemius 7 but not 24 h post-exercise. The 3-day swimming training resulted also in significantly lower myostatin mRNA levels in the trained group. This study demonstrated that short-term endurance training may modulate myostatin mRNA levels, implying a probable role of myostatin in remodelling of skeletal muscle in response to training.

Changes in muscle myostatin expression in obese subjects after weight loss.

Myostatin is a member of transforming growth factor-beta superfamily that plays an important inhibitory role during muscle development; in fact mutations of myostatin gene result in a hypermuscular phenotype. Moreover myostatin-deficient mice have a significant reduction in fat depots and a depression of adipogenesis. Little is known about myostatin function in muscle growth regulation in humans and in particular during caloric restriction. In the present work we quantified by real-time RT-PCR myostatin expression in muscle biopsies of a group of morbidly obese patients before and after weight loss obtained by biliopancreatic diversion (BPD). The patients reduced body weight by 38.9%, mostly due to fat-mass loss, showing also a significant reduction in the 24-hour EE as assessed by the respiratory chamber. Myostatin mRNA levels result clearly decreased after weight loss, suggesting a role in counteracting the progressive decline of muscle mass after BPD. Myostatin may provide therefore another mechanistic explanation for the control of energy partitioning between protein and fat, working against muscle wasting. Our data suggest that myostatin might represent an important regulator of skeletal muscle size also in conditions of food restriction in obese subjects.

Follistatin alters myostatin gene expression in C2C12 muscle cells.

The objective of the present study was to determine the effects of follistatin addition on myostatin and follistatin gene expression patterns in C2C12 muscle cells. C2C12 cells were administered with 100 ng/ml recombinant human (rh) follistatin in Dulbecco's modified Eagle medium (DMEM) containing 10% fetal bovine serum (FBS), 4 mM glutamine and antibiotics daily for three days. Rh follistatin was not added in the control wells. Follistatin and myostatin gene cDNAs were synthesised by reverse transcriptase polymerase chain reaction (RT-PCR). The time course of follistatin gene expression pattern was similar in both the control and the follistatin-treated group. Myostatin mRNA level significantly increased in the follistatin-treated group after 24 h of culture (Fig. 3, P < 0.01). Amounts then sharply decreased (Fig. 3, P < 0.01) at 48 h of culture, whereas there was no significant difference between the control and the follistatin-treated group at 72 h of culture. Our results demonstrated that myostatin and follistatin mRNA were expressed in C2C12 cells and rh follistatin changed the myostatin expression pattern.

The influence of undernutrition during gestation on skeletal muscle cellularity and on the expression of genes that control muscle growth.

We examined the effects of two levels of gestational undernutrition (50 % and 40 % of ad libitum) on postnatal growth rate, skeletal muscle cellularity and the expression of genes that control muscle growth, in the offspring at weaning. The results showed that the rat pups born to mothers fed the 50 % diet during gestation and a control diet during lactation had an increased postnatal growth rate compared with the pups fed the more restricted diet (40 % of ad libitum). Surprisingly, the growth rate of the control group (ad libitum) was intermediate between the 50 % and 40 % groups. The restricted diets did not alter the number of muscle fibres in the semitendinosus muscle of the offspring but the number of muscle nuclei was reduced by 16 % in the 40 % group compared with the control group. In the 50 % group, the lightest pups at birth (L) had elevated muscle insulin-like growth factor (IGF)-1, IGF binding protein (BP)-5 and proliferating cell nuclear antigen (PCNA) mRNA compared with the L pups from both the control and 40 % groups. The heaviest pups at birth (H) in the 50 % group had increased levels of IGFBP-4, PCNA and M-cadherin mRNA compared with both the control and 40 % groups. Levels of IGF-1 receptor, myostatin and MyoD mRNA did not correlate with postnatal growth. Both H and L pups from the 40 % group had reduced muscle IGF-1 mRNA but all other transcripts examined were similar to control levels. The results suggest that the increased postnatal growth rate, which accompanied milder fetal undernutrition (50 %), may be due to a more active local muscle IGF system and increased muscle-cell proliferation.

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 24 h 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.

Time course of changes in growth factor mRNA levels in muscle of steroid-implanted and nonimplanted steers.

We used a muscle biopsy technique in conjunction with real-time PCR analysis to examine the time course of changes in muscle IGF-I, IGFBP-3, myostatin, and hepatocyte growth factor (HGF) mRNA in the longissimus muscles of Revalor-S-implanted and nonimplanted steers on d 0, 7, 12, and 26 after implantation (nine steers/treatment group). Administration of a Revalor-S implant increased (P < 0.01) ADG and improved (P < 0.05) feed efficiency, 36 and 34%, respectively, compared with steers that received no implant during the 26-d trial. Daily dry matter intake did not differ (P > 0.15) between nonimplanted and implanted steers. Steers receiving the Revalor-S implant had increased (P < 0.001) circulating IGF-I concentrations compared with nonimplanted steers. The longissimus muscles of steers receiving the Revalor-S implant contained increased (P < 0.001) IGF-I mRNA levels compared with longissimus muscles of nonimplanted steers over the 26-d duration of the study. Longissimus muscle IGF-I mRNA levels in implanted steers were increased (P < 0.003) relative to d-0 concentrations on d 7 and 12 (101% and 128%, respectively), and byd 26, longissimus muscle mRNA levels were more than three times (P < 0.0001) those in the longissimus muscles of the same steers on d 0. There was no treatment effect on the level of IGFBP-3, myostatin, or HGF mRNA in the longissimus muscle at any time point; however, levels of IGFBP-3, myostatin, and HGF mRNA increased with time on feed. Based on current and previous studies, we hypothesize that the increased IGF-I level in muscle of implanted steers by d 7 of implantation stimulates satellite cell proliferation and maintains a high number of proliferating satellite cells at a point in the growth curve where satellite cell numbers and activity are normally dropping off. This would prolong the period of rapid muscle growth, resulting in the observed increased rate and efficiency of muscle deposition in implanted steers.

Myostatin gene expression is reduced in humans with heavy-resistance strength training: a brief communication.

This study examined changes in myostatin gene expression in response to strength training (ST). Fifteen young and older men (n = 7) and women (n = 8) completed a 9-week heavy-resistance unilateral knee extension ST program. Muscle biopsies were obtained from the dominant vastus lateralis before and after ST. In addition to myostatin mRNA levels, muscle volume and strength were measured. Total RNA was reverse transcribed into cDNA, and myostatin mRNA was quantified using quantitative PCR by standard fluorescent chemistries and was normalized to 18S rRNA levels. A 37% decrease in myostatin expression was observed in response to ST in all subjects combined (2.70 +/- 0.36 vs 1.69 +/- 0.18 U, arbitrary units; P < 0.05). Though the decline in myostatin expression was similar regardless of age or gender, the small number of subjects in these subgroups suggests that this observation needs to be confirmed. No significant correlations were observed between myostatin expression and any muscle strength or volume measure. Although further work is necessary to clarify the findings, these data demonstrate that myostatin mRNA levels are reduced in response to heavy-resistance ST in humans.

Effect of refeeding on IGFI, IGFII, IGF receptors, FGF2, FGF6, and myostatin mRNA expression in rainbow trout myotomal muscle

Fish endure long periods of fasting and demonstrate an extensive capacity for rapid and complete recovery after refeeding. The underlying mechanisms through which nutrient intake activates an increase in somatic growth and especially in muscle growth is poorly understood. In this study we examined the expression profile of major muscle growth regulators in trout white muscle 4, 12, and 34 days after refeeding, using real-time quantitative RT-PCR. Mean insulin-like growth factor I (IGFI) mRNA level in muscle increased dramatically 8- and 15-fold, 4 and 12 days, respectively, after refeeding compared to fasted trout. This declined thereafter. Conversely, only a weak but gradual increase in mean insulin-like growth factor II (IGFII) mRNA level was observed during refeeding. Inversely to IGFI, mean IGF receptor Ia (IGFRIa) mRNA level declined after ingestion of food. In contrast, IGF receptor Ib (IGFRIb) mRNA level was not affected by refeeding. Mean fibroblast growth factor 2 (FGF2) mRNA level increased by 2.5-fold both 4 and 12 days after refeeding, whereas fibroblast growth factor 6 (FGF6) and myostatin mRNA levels were unchanged. Subsequent to IGFI and FGF2 gene activation, an increase in myogenin mRNA accumulation was observed at 12 days post-refeeding suggesting that an active differentiation of myogenic cells succeeds their proliferation. In conclusion, among the potential growth factors we examined in this study, IGFI and FGF2 were identified as candidate genes whose expression may contribute to muscle compensatory growth induced by refeeding.

Growth factor messenger RNA levels in muscle and liver of steroid-implanted and nonimplanted steers.

Ribonuclease protection assays were used to measure steady-state semimembranosus muscle and/or hepatic levels of IGF-I, IGFBP-3, IGFBP-5, hepatocyte growth factor (HGF), and myostatin messenger RNA (mRNA) in steers implanted from 32 to 38 d with Revalor-S, a combined trenbolone acetate and estradiol implant. Insulin-like growth factor-ImRNA levels were 69% higher (P < 0.01, n = 7) in the livers of implanted steers than in the livers of nonimplanted steers. Similarly, IGF-I mRNA levels were 50% higher (P < 0.05, n = 7) in the semimembranosus muscles of implanted steers than in the same muscles from nonimplanted steers. Hepatic IGFBP-3 mRNA levels were 24% higher (P < 0.07, n = 7) in implanted steers than in nonimplanted steers. Hepatic HGF and IGFBP-5 mRNA levels did not differ between implanted and nonimplanted steers. Similarly, muscle IGFBP-3, IGFBP-5, HGF, and myostatin mRNA levels were not affected by treatment. Previous data from these same steers have shown that circulating IGF-I and IGFBP-3 concentrations were 30 to 40% higher (P < 0.01, n = 7) in implanted steers than in nonimplanted, control steers. Additionally, the number of actively proliferating satellite cells that could be isolated from the semimembranosus muscle was 45% higher (P < 0.01, n = 7) for implanted steers than for nonimplanted steers. Viewed together, these data suggest that increased muscle IGF-I levels stimulate increased satellite cell proliferation, resulting in the increased muscle growth observed in Revalor-S implanted steers.

Muscle development, insulin-like growth factor-I and myostatin mRNA levels in chickens selected for increased breast muscle yield.

Insulin-like growth factor-I (IGF-I) and myostatin (MSTN) are paracrine regulators of muscle growth. The present study was conducted to relate their expression with muscle fibre development in chickens selected for high breast meat yield and their controls. Both mRNA levels were measured by real-time RT-PCR in the Pectoralis major (PM) muscle between 14 days in ovo and 6 weeks post-hatch and in the Sartorius (SART) muscle between 2 and 6 weeks. The data show that PM growth was slow during in ovo development and rapid in the early post-hatch period. Chickens from the selected genotype exhibited significantly higher breast muscle yields from 2 to 6 weeks of age, and muscle fibre hypertrophy. In the PM, IGF-I and MSTN mRNA levels decreased markedly around hatch, while the IGF-I/MSTN ratio increased, suggesting that it could contribute to the explosive growth observed in the early post-hatch period. Between 4 and 6 weeks of age in selected chickens, IGF-I mRNA levels were significantly higher (p=0.04) with a similar trend in MSTN mRNA levels (p=0.07) in the PM muscle but not in the SART muscle. Our results support the hypothesis that the relative levels of IGF-I and MSTN mRNA may participate to set muscle growth rate along development, while other factors are required to explain differences between genotypes.

Myostatin expression during development and chronic stress in zebrafish (Danio rerio).

Myostatin, a member of the transforming growth factor-beta superfamily, is a negative regulator of skeletal muscle mass in mammals. We have studied myostatin expression during embryonic and post-hatching development in zebrafish by semiquantitative RT-PCR. The transcript is present in just-fertilized eggs and declines at 8 h post-fertilization (hpf), suggesting a maternal origin. A secondary rise occurs at 16 hpf, indicating the onset of embryonic transcription at the time of muscle cell differentiation. The level of myostatin mRNA decreases slightly at 24 hpf, when somitogenesis is almost concluded, and rises again at and after hatching, during the period of limited muscle hyperplastic growth that is typical of slow-growing, small fish. In the adult muscle, we found the highest expression of myostatin mRNA and protein, which were detectable by Northern and Western blot analyses respectively. Although only the precursor protein form was revealed in the adult lateral muscle, we demonstrated that zebrafish myostatin is proteolytically processed and secreted in cultured cells, as is its mammalian counterpart. These results suggest that myostatin may play an important regulatory role during myogenesis and muscle growth in fish, as it does in mammals. In chronically stressed fish, grown from 16 days post-fertilization to adulthood in an overcrowded environment, we observed both depression of body growth and a diminished level of myostatin mRNA in the adult muscle, as compared with controls. We propose that chronic stunting in fish brings about a general depression of muscle protein synthesis which does not spare myostatin.

Prolonged fasting and cortisol reduce myostatin mRNA levels in tilapia larvae; short-term fasting elevates.

Myostatin negatively regulates muscle growth and development and has recently been characterized in several fishes. We measured fasting myostatin mRNA levels in adult tilapia skeletal muscle and in whole larvae. Although fasting reduced some growth indexes in adults, skeletal muscle myostatin mRNA levels were unaffected. By contrast, larval myostatin mRNA levels were sometimes elevated after a short-term fast and were consistently reduced with prolonged fasting. These effects were specific for myostatin, as mRNA levels of glyceraldehyde-3-phosphate dehydrogenase and glucose-6-phosphatase were unchanged. Cortisol levels were elevated in fasted larvae with reduced myostatin mRNA, whereas in addition immersion of larvae in 1 ppm (2.8 microM) cortisol reduced myostatin mRNA in a time-dependent fashion. These results suggest that larval myostatin mRNA levels may initially rise but ultimately fall during a prolonged fast. The reduction is likely mediated by fasting-induced hypercortisolemia, indicating divergent evolutionary mechanisms of glucocorticoid regulation of myostatin mRNA, since these steroids upregulate myostatin gene expression in mammals.