Transitory expression of alpha cardiac myosin heavy chain in a subpopulation of secondary generation muscle fibers in the pig

Integrated Analysis of Proteomic and Transcriptomic Data Highlights Late Fetal Muscle Maturation Process

In pigs, the perinatal period is the most critical time for survival. Piglet maturation, which occurs at the end of gestation, is an important determinant of early survival. Skeletal muscle plays a key role in adaptation to extra-uterine life, e.g. motor function and thermoregulation. Progeny from two breeds with extreme neonatal mortality rates were analyzed at 90 and 110 days of gestation (dg). The Large White breed is a highly selected breed for lean growth and exhibits a high rate of neonatal mortality, whereas the Meishan breed is fatter and more robust and has a low neonatal mortality. Our aim was to identify molecular signatures underlying late fetal longissimus muscle development. First, integrated analysis was used to explore relationships between co-expression network models built from a proteomic data set (bi-dimensional electrophoresis) and biological phenotypes. Second, correlations with a transcriptomic data set (microarrays) were investigated to combine different layers of expression with a focus on transcriptional regulation. Muscle glycogen content and myosin heavy chain polymorphism were good descriptors of muscle maturity and were used for further data integration analysis. Using 89 identified unique proteins, network inference, correlation with biological phenotypes and functional enrichment revealed that mitochondrial oxidative metabolism was a key determinant of neonatal muscle maturity. Some proteins, including ATP5A1 and CKMT2, were important nodes in the network related to muscle metabolism. Transcriptomic data suggest that overexpression of mitochondrial PCK2 was involved in the greater glycogen content of Meishan fetuses at 110 dg. GPD1, an enzyme involved in the mitochondrial oxidation of cytosolic NADH, was overexpressed in Meishan. Thirty-one proteins exhibited a positive correlation between mRNA and protein levels in both extreme fetal genotypes, suggesting transcriptional regulation. Gene ontology enrichment and Ingenuity analyses identified PPARGC1A and ESR1 as possible transcriptional factors positively involved in late fetal muscle maturation.

Expression of Unique and Developmental Myosin Heavy Chain Isoforms in Adult Human Digastric Muscle

Digastric muscle (DGM) is a powerful jaw-opening muscle that participates in chewing, swallowing, breathing, and speech. For better understanding of its contractile properties, five pairs of adult human DGMs were obtained from autopsies and processed with immunocytochemistry and/or immunoblotting. Monoclonal antibodies against α-cardiac, slow tonic, neonatal, and embryonic myosin heavy chain (MHC) isoforms were employed to determine whether the DGM fibers contain these MHC isoforms, which have previously been demonstrated in restricted specialized craniocervical skeletal muscles but have not been reported in normal adult human trunk and limb muscles. The results showed expression of all these MHC isoforms in adult human DGMs. About half of the fibers reacted positively to the antibody specific for the α-cardiac MHC isoform in DGMs, and the number of these fibers decreased with age. Slow tonic MHC isoform containing fibers accounted for 19% of the total fiber population. Both the α-cardiac and slow tonic MHC isoforms were found to coexist mainly with the slow twitch MHC isoform in a fiber. A few DGM fibers expressed the embryonic or neonatal MHC isoform. The findings suggest that human DGM fibers may be specialized to facilitate performance of complex motor behaviors in the upper airway and digestive tract. (J Histochem Cytochem 52:851–859, 2004)

Gene expression profiling of gastrocnemius of "minimuscle" mice

Few studies have investigated heterogeneity of selection response in replicate lines subjected to equivalent selection. We developed four replicate lines of mice based on high levels of voluntary wheel running (high runner or HR lines) while also maintaining four nonselected control lines. This led to the unexpected discovery of the HR minimuscle (HRmini) phenotype, recognized by a 50% reduction in hindlimb muscle mass, which became fixed in 1 of the four HR selected lines. Here, we report genome-wide expression profiling describing transcriptome differences between HRnormal and HRmini medial gastrocnemius. Consistent with the known reduction of type IIB fibers in HRmini, Myh4 gene expression was -8.82-fold less (P = 0.0001) in HRmini, which was closely associated with differences in the "calcium signaling" canonical pathway, including structural genes (e.g., Mef2c, twofold greater in HRmini, P = 0.0003) and myogenic factors (e.g., Myog, 3.8-fold greater in HRmini, P = 0.0026) associated with slow-type myofibers. The gene that determines the HRmini phenotype is known to reside in a 2.6335-Mb interval on mouse chromosome 11 and 7 genes (Myh10, Chrnb1, Acadvl, Senp3, Gabarap, Eif5a, and Clec10a) from this region were differentially expressed. Verification by real-time PCR confirmed 1.5-fold greater (P < 0.05) expression of very long chain acyl-CoA dehydrogenase (Acadvl) in HRmini. Ten other genes associated with fatty acid metabolism were also upregulated in HRmini, suggesting differences in the ability to metabolize fatty acids in HRnormal and HRmini muscles. This work provides a resource for understanding differences in muscle phenotypes in populations exhibiting high running capacity.

Expression of a cardiac myosin gene in non-heart tissues of developing frogs

Direct developing frogs, like Eleutherodactylus coqui, provide opportunities to investigate limb early development in anuran amphibians that are less available in species with tadpoles. We have found that myosin heavy chain 6 (myh6), a myosin gene usually considered heart-specific in Xenopus and other animals, is expressed in limbs of E. coqui embryos. The gene for microRNA(miR)-208 is contained in an intron of the E. coqui myh6 gene as in mammals, and miR -208 was detected as a microRNA, more highly expressed in a microarray of E. coqui limb buds, compared to Xenopus laevis limb buds. Myh6 is also expressed in several muscles of tadpoles and froglets of Xenopus tropicalis. These connections raise the possibility of an involvement of myh6 and miR-208 in the thyroid dependent metamorphosis of anurans.

Intrauterine crowding impairs formation and growth of secondary myofibers in pigs

There are indications that intrauterine crowding may cause intrauterine growth retardation with the possibility of an impaired myofiber hyperplasia. The aim of the study was to confirm this by generating large differences in uterine space using sows that were unilaterally hysterectomized-ovariectomized (HO; crowded) or unilaterally oviduct ligated (OL; non-crowded). In the study, seven HO and seven OL Swiss Large White third parity sows were used. At farrowing, litter size and litter birth weight were determined. Subsequently, within each litter two male and two female progenies each with the respectively lowest (L) and highest (H) birth weight were sacrificed. Internal organs and brain were weighed, and longissimus (LM) and semitendinosus muscle (SM) samples were collected. Histological analyses were performed in both muscles using mATPase staining after preincubation at pH 4.3 and 10.2. Myosin heavy chain (MyHC) polymorphism was determined in the LM by means of SDS-PAGE. The number of piglets born alive was similar in both sow groups, but litter size expressed per uterine horn was lower (P < 0.05) in OL than HO sows. Consequently, OL progeny were markedly heavier (P < 0.01). Regardless of gender, the organs, the brain and the SM were heavier (P < 0.001) in OL and H compared with HO and L offspring, respectively. Compared with HO pigs, the SM of OL offspring tended (P < 0.1) to have more myofibers, which were of larger (P < 0.05) size. However, myofiber density appeared to be lower (P < 0.1) in the SM of OL than HO pigs. The impact of birth weight on myofiber characteristics was limited to the lower (P < 0.05) myofiber density in the SM and the larger (P < 0.01) myofiber size in the light portion of the SM of H than L offspring, whereas myofiber hyperplasia did not differ between birth weight categories. The SM, but not the LM, of male offspring had a greater (P < 0.05) myofiber density. This did not affect total SM myofiber number. The relative abundance of fetal and type I MyHC in the LM was lower (P < 0.05) and that of type II MyHC was greater (P < 0.001) in OL than HO pigs. The current data suggest that regardless of birth weight and gender, in the LM and SM of individuals born from a crowded environment, not only hyperplasia but also hypertrophy of myofibers is impaired and their maturity seems delayed.

In vitro characterization of proliferation and differentiation of pig satellite cells

Skeletal muscle contains various muscle fiber types exhibiting different contractile properties based on the myosin heavy chain (MyHC) isoform profile. Muscle fiber type composition is highly variable and influences growth performance and meat quality, but underlying mechanisms regulating fiber type composition remain poorly understood. The aim of the present work was to develop a model based on muscle satellite cell culture to further investigate the regulation of adult MyHC isoforms expression in pig skeletal muscle. Satellite cells were harvested from the mostly fast-twitch glycolytic longissimus (LM) and predominantly slow-twitch oxidative rhomboideus (RM) muscles of 6-week-old piglets. Satellite cells were allowed to proliferate up to 80% confluence, reached after 7 day of proliferation (D7), and then induced to differentiate. Kinetics of proliferation and differentiation were similar between muscles and more than 95% of the cells were myogenic (desmin positive) at D7 with a fusion index reaching 65 ± 9% after 4 day of differentiation. One-dimensional SDS polyacrylamide gel electrophoresis revealed that satellite cells from both muscles only expressed the embryonic and fetal MyHC isoforms in culture, without any of the adult MyHC isoforms that were expressed in vivo. Interestingly, triiodothyronine (T3) induced de novo expression of adult fast and α-cardiac MyHC in vitro making our culture system a valuable tool to study de novo expression of adult MyHC isoforms and its regulation by intrinsic and/or extrinsic factors.

Limited and excess protein intake of pregnant gilts differently affects body composition and cellularity of skeletal muscle and subcutaneous adipose tissue of newborn and weanling piglets

AIM

This study investigated whether dietary protein intake less (50%) or greater (250%) than requirements throughout gestation differently affects offspring body composition and cellular properties of skeletal muscle and subcutaneous adipose tissue (SCAT).

METHODS

Primiparous gilts were fed iso-energetic diets containing adequate (22 AP), high (21 HP), or low (19 LP) protein contents. Newborn (n = 166) and weanling piglets cross-fostered to sows fed a standard diet (day 28; n = 83) were examined by morphological, biochemical, histological, and molecular analyses of the body, SCAT, and semitendinosus, longissimus, biceps femoris muscles.

RESULTS

Lowered birth weight (BW) in response to the HP and LP diets (p < 0.01) resulted from decreases in all body constituents in LP, and mainly from reduced body fat in HP piglets (p < 0.05). In the light BW class within litters, HP piglets exhibited a greater percentage of muscle tissue (p < 0.05) than LP piglets. Less SCAT mass in HP and LP piglets resulted from reduced (p < 0.05) number, but not the size of adipocytes. The LP diet adversely affected myogenesis and muscular differentiation derived from less (p < 0.01) primary and secondary myofibers, lower creatine kinase activity (p < 0.05), less IGF2 mRNA (p < 0.10), and greater expression of the embryonic myosin heavy chain isoform (p < 0.01). Catch-up growth of LP but not HP pigs until day 28 increased body fat (p = 0.01). Despite compensated muscle growth in LP piglets, the deficit in myofiber number remained.

CONCLUSION

Poor intrauterine environment by limited and excess protein supply retards fetal growth, but only limited protein supply impairs myogenesis, persistently restricts muscle growth potential, and favors obesity at infancy.

Myosin heavy chain fibre type composition in foals: analyses at the mRNA and protein level

REASONS FOR PERFORMING STUDY

An optimal developed musculoskeletal system is vital for the performance of the horse. Previously, we showed that in the m. gluteus medius from adult untrained horses, identical mRNA and protein expression patterns were found in the majority of fibres. However, co-expression of IIa and IId/x myosin heavy chain (MyHC) was substantially more common at the protein than at the mRNA level, suggesting a transcriptionally controlled fine-tuning of these 2 genes.

OBJECTIVE

To analyse the MyHC transcripts and proteins (including the cardiac alpha isoform) in the same muscle during post natal development when the muscle is adapting to movement and load.

METHODS

Biopsies were taken from the m. gluteus medius of 2 Dutch Warmblood foals at 0, 2, 4, 22 and 48 weeks of age. mRNA was compared to protein expression on a fibre-to-fibre basis using in situ hybridisation and immunofluorescence. The MyHC slow (I), alpha, IIa and IId/x isoforms were analysed.

RESULTS

At all ages the expression of the mRNA and protein MyHC isoforms was almost identical. Surprisingly, coexpression of the IIad isoform was also detected at the mRNA level especially early in life. The transcript of the alpha isoform was only detectable at young age, indicating silencing of the gene around birth.

CONCLUSION

During the first year of life, MyHCs are continuously adapting at the mRNA and protein level. Additionally, the regulation of hybrid fibres is different from that in adult fibres.

POTENTIAL RELEVANCE

We postulate that interfering in this process by e.g. early training will be levelled out by the maturation of the muscle.

Effects of gradual coronary artery occlusion and exercise training on gene expression in swine heart

Gradual occlusion (O) of the swine left circumflex coronary artery (LCX) with an ameroid occluder results in complete O within 3 weeks, collateral vessel development, and compensatory hypertrophy. The purpose of this investigation was to determine the independent and combined effects of O and exercise training (E) on gene expression in the swine heart. Adult Yucatan miniature swine were assigned to one of the following groups (n=6-9/group): sedentary control (S), exercise-trained (E), sedentary swine subjected to LCX occlusion (SO), and exercise-trained swine with LCX occlusion (EO). Exercise consisted of progressive treadmill running conducted 5 d/wk for 16 weeks. Gene expression was studied in myocardium isolated from the collateral-dependent left ventricle free wall (LV) and the collateral-independent septum (SEP) by RNA blotting. E and O each stimulated cardiac hypertrophy independently (p<0.001) with no interaction. O but not E increased atrial natriuretic factor expression in the LV, but not in the SEP. E decreased the expression of beta-myosin heavy chain in the LV, but not in the SEP. E retarded the expression of collagen III mRNA in SEP; but not in the LV. Exercise training and coronary artery occlusion each stimulate cardiac hypertrophy independently and induce different patterns of gene expression.

Number of conceptuses in utero affects porcine fetal muscle development

Unmodified, third parity, control sows (CTR; n = 30) or sows subjected to unilateral oviduct ligation before breeding (LIG; n = 30), were slaughtered at either day 30 or day 90 of gestation and used to determine the effects of numbers of conceptuses in utero on prenatal, and particularly muscle fibre, development. Ovulation rate, number of conceptuses in utero, placental and fetal size, and (day 90 sows) fetal organ and semitendinosus muscle development were recorded. Tubal ligation reduced (P < 0.05) the number of viable embryos at day 30 and fetuses at day 90. Placental weight at day 30 and day 90, and fetal weight at day 90, were lower (P < 0.05) in CTR sows. All body organs except the brain were lighter, and the brain:liver weight ratio was higher in CTR fetuses (P < 0.05), indicative of brain sparing and intrauterine growth restriction in fetuses from CTR sows. Muscle weight, muscle cross-sectional area and the total number of secondary fibres were also lower (P < 0.05) in CTR fetuses. The number of primary fibres, the secondary:primary muscle fibre ratio, and the distribution of myosin heavy chain-Iβ, -IIa, fetal and embryonic isoforms did not differ between groups. Thus, even the relatively modest uterine crowding occurring naturally in CTR sows negatively affected placental and fetal development and the number of secondary muscle fibres. Consequences of more extreme crowding in utero on fetal and postnatal development, resulting from changing patterns of early embryonic survival, merit further investigation.

Slow myosins in muscle development

Myogenesis has been a system central to investigations on mechanisms of diversification within groups of differentiating cells. Diversity among cell types has been well described in striated muscle tissue at the protein and enzymatic-function levels for decades, but it is only in recent years that some understanding of the molecular mechanisms responsible for this diversity has begun to emerge. Study of the expression of the slow isoforms of the myosin heavy chain has contributed to our understanding of how cell diversity arises within skeletal and cardiac muscle. Slow MyHc isoforms are developmentally responsive to a number of cues provided by the nervous systems, the endocrine system and, later in development, to functional demands on these developing tissues. Perhaps most informative have been studies on the mechanism for regulation of slow MyHc expression in mammals and birds where studies on the calcineurin-NF-AT pathways and nuclear hormone action have been shown to control MyHC gene expression in skeletal muscle and in the developing heart. The mechanisms involved in cell diversification in myogenesis are undoubtedly more varied and complex than those currently offered to explain cell diversification, but these recent studies have broadened our understanding of the interplay between the nervous system, the endocrine system and cell lineages in controlling cell diversification. Greater focus on the first fibers and cardiomyocytes to form in the embryo are likely to bring additional insights into the mechanism crucial for establishing the patterns of diversity required for successful formation of embryonic tissues.

Early postnatal food intake alters myofiber maturation in pig skeletal muscle

We examined the effects of undernutrition on muscle development during the first postnatal week in pigs. Eighteen piglets were subjected to three nutritional levels (300, 200 or 100 g/(kg body. d) of colostrum then milk) between birth and slaughter at 7 d of age. Longissimus lumborum (LL), a fast-twitch glycolytic muscle, and rhomboideus (RH), a mixed slow- and fast-twitch oxido-glycolytic muscle, were taken for myofiber typing and biochemical analyses. Enzyme activities of lactate dehydrogenase (LDH), citrate synthase (CS) and beta-hydroxy-acyl-CoA-dehydrogenase (HAD) were used as markers of glycolytic, oxidative and lipid beta-oxidation capacities, respectively. Undernutrition selectively decreased (P < 0.001) hypertrophy of the future fast-twitch glycolytic fibers in LL. Contractile and metabolic maturation was delayed in the later maturing LL, as reflected by a decrease in muscle protein concentration (P < 0.01), an increase (P < 0.05) in the percentage of myofibers still expressing the fetal myosin heavy chain (MyHC), a lower postnatal increase in LDH activity (P < 0.001) and a delayed decrease in the percentage of IIa MyHC positive fibers (P < 0.001). Otherwise, restriction tended (P < 0.10) to increase the percentage of slow type I MyHC containing fibers in both muscles and of alpha-cardiac MyHC positive fibers in RH (P < 0.05). The LDH/CS ratio decreased dramatically (P < 0.001) after restriction, to a greater extent in LL than in RH. These changes denoted a more oxidative metabolism using fewer carbohydrates and more lipids in restricted pigs, as suggested by the increased activity of HAD (P < 0.001) and decreased respiratory quotient (P < 0.001).

Muscle-derived CD45-SCA-1+c-kit- progenitor cells give rise to skeletal muscle myotubes in vitro

A stem cell population isolated from murine skeletal muscle has recently been shown to differentiate into hematopoietic cells after transplantation in vivo. In the present study, we tested the hypothesis that this cell population would also, under appropriate culture conditions, differentiate into skeletal muscle cells in vitro. Lower-extremity skeletal muscle tissue isolated from 3- to 4-wk-old mice was dissected free from bone and vessels, enzymatically digested, and flow cytometrically sorted to yield CD45(-)Sca-1(+)c-Kit(-) (S+) cells. These cells were further sorted into CD34(+) and CD34(-) fractions and examined for skeletal, cardiac, and hematopoietic lineage-specific messenger RNA (mRNA) transcripts immediately after isolation and after a 10- to 14-d culture period. Freshly isolated S(+)CD34(+) cells lacked expression of skeletal-, cardiac-, or hematopoietic-specific mRNA transcripts, whereas S(+)CD34(-) cells expressed c-met, a marker for skeletal muscle satellite cells. During 10-14 d in culture, both S(+)CD34(+) and S(+)CD34(-) cell populations underwent a period of attachment followed by elongation and, ultimately, fusion to create large multinucleated contractile myotubes expressing skeletal muscle lineage mRNA transcripts but not hematopoietic or cardiac lineage transcripts. We conclude that murine skeletal muscle possesses two populations of progenitor cells that can be directly isolated. One population expressing the phenotype S(+)CD34(-) may contain satellite cells, whereas the S(+)CD34(+) population is devoid of satellite cell markers. Both populations possess the ability to differentiate into skeletal muscle cells in vitro.

Post-translational phosphorylation of the slow/beta myosin heavy chain isoform in adult rabbit masseter muscle

Four different phenotypes of slow muscle fibers, characterized by differential epitope expression in the slow/beta myosin heavy chain (MyHC) isoform, have been identified in adult rabbit masseter muscle. We investigated the role of post-translational phosphorylation in the expression of these four phenotypes. Serial cryostat sections were treated either with alkaline phosphatase to dephosphorylate proteins in the tissue, or with a brain kinase solution and ATP to phosphorylate them, and then stained, using four antibodies that bind specifically to the slow/beta MyHC isoform. In sections pre-treated with phosphatase, immunoreactivity to antibody A4.840 was abolished, but it could be restored by subsequent kinase/ATP treatment or ATP alone, indicating that the expression of its epitope requires phosphorylation. Phosphatase treatment resulted in an exposure of the epitope for antibody A4.951 in cells that normally bind this antibody only weakly or not at all, but since heat treatment alone produced similar effects, the role of phosphorylation in this enhancement is less certain. Immunoreactivity to antibodies S58 and BA-D5 were not influenced by phosphatase pre-treatment. Kinase/ATP treatment was only effective in changing antibody binding when tissues already had been phosphatase treated. We interpret these results to mean that sites of potential phosphorylation may already be occupied by O-linked glycosylation.

Influence of early postnatal cold exposure on myofiber maturation in pig skeletal muscle

Early after birth, piglets rely almost exclusively on muscular shivering thermogenesis to produce heat in the cold and this can possibly modulate skeletal muscle development. An experiment involving 10 individually housed piglets was conducted to determine the influence of cold (24-15 degrees C, D5C group) vs. thermoneutrality (34-30 degrees C, D5TN group) between birth and 5 days on myosin heavy chain (MyHC) polymorphism and metabolic characteristics of longissimus lumborum (LL) and rhomboideus (RH) muscles. Five additional piglets were sacrificed at birth. Piglets exposed to cold received 43% more artificial milk on a liveweight basis in order to achieve similar growth rates. D5C piglets produced 93% more heat and exhibited intense shivering during the whole experiment. Contractile and metabolic characteristics of muscles were determined by immunocytochemistry, electrophoresis and enzyme activities. At least eight MyHC isoforms were detected, including atypical expressions of the alpha-cardiac and extraocular isoforms. Dramatic changes in MyHC composition, myofiber cross-sectional area (CSA) and energy metabolism occurred between birth and 5 days. Cold exposure did not affect either the total number of fibers or the CSA, but it did influence muscle maturation. In particular, it increased the expression of alpha-cardiac and type I MyHC, and decreased that of fetal MyHC, confirming an acceleration in the rate of postnatal maturation. An increase in oxidative enzyme activities was observed in both muscles in the cold, whereas the activity of a glycolytic enzyme, lactate dehydrogenase, remained unchanged. Cold exposure also induced an increase in T3 plasma levels. The extent to which these changes are the result of sustained shivering or are due to the action of hormonal factors, such as thyroid hormones, are discussed.

Jaw-closing muscles of kangaroos express alpha-cardiac myosin heavy chain

The masseter muscle of eutherian grazing mammals typically express beta or slow myosin heavy chain (MyHC). Myosins in the masseter of 4 species of kangaroos and a slow limb muscle of one of them were compared with their cardiac myosin by pyrophosphate and sodium dodecyl sulphate (SDS) gel electrophoresis, immunoblotting and immunohistochemistry. It was found that ventricular muscle contains three isoforms homologous to V1 (alpha-MyHC homodimer), V2 (heterodimer) and V3 (beta-MyHC homodimer) of eutherian cardiac muscle, and that the masseter contained V1, with traces of V2 and V3, in great contrast to eutherian ruminants, which express only V3. A polyclonal antibody (anti-KJM) was raised in rabbits against red kangaroo masseter myosin. After cross-absorption against limb muscle myofibrils, anti-KJM specifically reacted in Westerns with MyHCs from masseter but not limb muscles, and immunohistochemically with masseter, but not limb muscle fibers. In pyrophosphate Western blots, anti-KJM reacted with V1 but not with V3. However, a monoclonal antibody specific for eutherian slow myosin stained all kangaroo slow muscle fibers but only weakly stained scattered fibers in the masseter. The SDS-PAGE revealed that light chain composition of masseter and ventricular myosins is identical, but isoforms of both light chains of kangaroo limb slow myosin were observed. These results confirm that kangaroo jaw muscle express alpha-MyHC rather than beta-MyHC. The difference in MyHC gene expression between marsupial and eutherian grazers may be related to the fact that kangaroos are not ruminants, and have only a single chance to comminute food into fine particles, hence the need for the greater speed and power of muscle contraction associated with V1 containing muscle fibers.

Postnatal myosin heavy chain isoform expression in normal mice and mice null for IIb or IId myosin heavy chains

The patterns of myosin heavy chain (MyHC) isoform expression in the embryo and in the adult mouse are reasonably well characterized and quite distinct. However, little is known about the transition between these two states, which involves major decreases and increases in the expression of several MyHC genes. In the present study, the expression of seven sarcomeric MyHCs was analyzed in the hindlimb muscles of wild-type mice and in mice null for the MyHC IIb or IId/x genes at several time points from 1 day of postnatal life (dpn) to 20 dpn. In early postnatal life, the developmental isoforms (embryonic and perinatal) comprise >90% of the total MyHC expression, while three adult fast isoforms (IIa, IIb, and IId) comprise <1% of the total MyHC protein. However, between 5 and 20 dpn their expression increases to comprise >90% of the total MyHC. Expression of each of the three adult fast isoforms occurs in a spatially and temporally distinct manner. We also show that alpha MyHC, which is almost exclusively expressed in the heart, is expressed in scattered fibers in all hindlimb muscles during postnatal development. Surprisingly, the timing and localization of expression of the MyHC isoforms is unchanged in IIb and IId/x null mice, although the magnitude of expression is altered for some isoforms. Together these data provide a comprehensive overview of the postnatal expression pattern of the sarcomeric MyHC isoforms in the mouse hindlimb.

Accelerated contractile function and improved fatigue resistance of calf muscles in newborn piglets with IUGR

Asymmetrical intrauterine growth restriction is denoted by disproportional reduction of muscle mass compared with body weight reduction. However, effects on contractile function or tissue development of skeletal muscles were not studied until now. Therefore, isometric force output of serial-stimulated hindlimb plantar flexors was measured in thiopental-anesthetized normal weight (NW) and intrauterine growth-restricted (IUGR) 1-day-old piglets under conditions of normal, reduced (aortic cross clamping), and reestablished (clamp release) blood supply (measured by colored microspheres technique). Furthermore, muscle fiber type distribution was determined after histochemical staining, specific muscle force of the plantar flexors [quotient from absolute force divided by muscle mass (N/g)] was calculated, and glycogen content and morphometric data of the investigated muscles were estimated. Regional blood flow of hindlimb muscles was similar in NW (6 +/- 2 ml. min(-1). 100 g(-1)) and IUGR piglets (8 +/- 1 ml. min(-1). 100 g(-1)). Isometric muscle contractions induced a marked increase in regional blood flow of 4.1-fold in NW and 5-fold in stimulated hindlimb muscles of IUGR piglets (baseline blood flow). Specific force of NW piglet muscles (5.2 +/- 0.2 N/g) was significantly lower than IUGR piglet muscles (6.1 +/- 0.6 N/g; P < 0.05). Isometric muscle contractions (NW: 32.7 +/- 4.7 N; IUGR: 21.7 +/- 4.0 N) resulted in a higher rate of force decrease in the calf muscles of NW animals compared with IUGR piglets (8 +/- 2 vs. 3 +/- 1%; P < 0. 01). Functional restoration of contractile performance after hindlimb recirculation was nearly complete in IUGR piglets (98 +/- 1%), whereas in NW piglets a deficit of 9 +/- 3% was found (P < 0. 01). Muscle fiber type estimation revealed an increased proportion of type I fibers in flexor digitalis superficialis and gastrocnemius medialis in IUGR piglets (P < 0.05). These data clearly indicate that contractile function is accelerated in newborn IUGR piglets.

Transient expression of myosin heavy chain MHCI alpha in rabbit muscle during fast-to-slow transition

The expression of an alpha-cardiac-like myosin heavy chain, MHCI alpha, was investigated at both the mRNA and protein levels in rabbit tibialis anterior muscle undergoing fast-to-slow transition by continuous chronic low-frequency stimulation (CLFS). According to sequence analyses of the PCR product, the MHCI alpha isoform was found to be identical to the alpha-cardiac MHC expressed in rabbit atrium. In muscles at different degrees of transformation, the upregulation of MHCI alpha mRNA preceded that of the MHCI beta mRNA. At more advanced stages of the transformation, MHCI alpha mRNA decayed while MHCI beta mRNA persisted at high levels. The expression of MHCI alpha, therefore, was transitory. Studies at the protein level were based on immunoblotting using a monoclonal antibody (F88 12F8,1), characterized to be specific to MHCI alpha in rabbit muscle. These studies revealed a similar relationship between initial increase and successive decline of the MHCI alpha protein as seen at the mRNA level. Immunohistochemistry of 30-day stimulated muscle revealed that up to 65% of the fibres expressed the MHCI alpha isoform in combination with other adult MHC isoforms. The most frequent patterns of coexistence were MHCIIa + MHCI alpha + MHCI beta (28%), MHCI alpha + MHCI beta (18%), and MHCIIa + MHCI alpha (11%). According to these combinations, the upregulation of MHCI alpha may be assigned as an intermediate step in the transformation of existing fibres during the MHCIIa-->MHCI beta transition. A small fraction of fibres contained, in addition to the MHCI alpha + MHCI beta and MHCIIa + MHCI alpha combinations, developmental myosin, suggesting that MHCI alpha was also expressed in regenerating fibres originating from satellite cell-derived myotubes.

Alpha-cardiac-like myosin heavy chain MHCI alpha is not upregulated in transforming rat muscle

The expression of MHCI alpha, an alpha-cardiac-like myosin heavy chain isoform, was studied in extensor digitorum longus (EDL) and tibialis anterior (TA) rat muscles undergoing fast-to-slow transition by chronic low-frequency stimulation (CLFS), a condition inducing a transient upregulation of MHCI alpha in rabbit muscle. In order to enhance the transformation process, CLFS was applied to hypothyroid rats. mRNA analyses were performed by RT-PCR, and studies at the protein level by immunoblotting and immunohistochemistry, using the F88 antibody (F88 12F8,1) demonstrated in the accompanying paper to be specific for MHCI alpha. In total RNA preparations from slow- and fast-twitch muscles, MHCI alpha mRNA was present at minute levels, at least three orders of magnitude lower than in cardiac atrium. As verified immunohistochemically, MHCI alpha is present only in intrafusal fibres of rat muscle. Moreover, MHCI alpha is not expressed in extrafusal fibres and, contrary to the rabbit, was not upregulated at both the mRNA and protein levels by CLFS. These results support our notion of species-specific responses to CLFS. Another antibody reported to be specific to MHCI alpha, BA-G5, was also investigated by immunoblot and immunohistochemical analyses. Its specificity could not be validated for skeletal muscles of the rat. BG-A5 was shown to cross-react with MHCIIb and MHCI beta. These results question an upregulation of MHCI alpha in transforming rat muscles as reported in studies based on the use of this antibody.

Developmental expression and 5' end cDNA cloning of the porcine 2x and 2b myosin heavy chain genes

Muscle research on farm animals suffers from a lack of basic molecular probes and immunochemical reagents. To complete the cloning of a comprehensive range of isoform-specific postnatal myosin heavy chain (MyHC) probes from the pig, we report here on the 5 '-end cDNA isolation, identification, and in vivo characterization of the porcine equivalent of the 2x (fast oxidative-glycolytic) and 2b (fast glycolytic) MyHC isoforms. Comparative analyses of primary nucleotide and deduced amino acid sequences of the porcine slow, 2a, 2x, and 2b cDNAs showed that the 2x and 2b isoforms were most homologous with each other and the slow isoform was the least similar. At or just after birth, at least 3 postnatal muscle fiber types (slow/beta, 2a, and 2x) were present. MyHC 2x expressing fibers were arranged distinctively as large outer rings, each surrounding an inner ring of 2a fibers, which in turn circumscribed one or more central slow fibers. Subsequently, different muscles expressed at different times the MyHC 2b isoform such that by 10 weeks, 2b fibers became the most dominant fiber type, in size and number in nearly all muscles examined except for its notable absence in the soleus muscle. Co-expression of two or more MyHC isoforms within the same fiber was not a feature of porcine muscles unlike that observed in rat and human muscles. For the first time, porcine muscle fibers can now be classified into 4 adult phenotypes (slow/beta 2a, 2x, and 2b) according to the expression of specific MyHC isoforms.