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Departments of 1 Anatomy and
2 Medical Microbiology and
Immunology, Myogenic
regulatory factors (MRFs) are a family of skeletal muscle-specific
transcription factors that regulate the expression of several muscle
genes. This study was designed to determine whether MRF transcripts
were increased in hypertrophy-stimulated muscle of adult quails and
whether equivalent increases occurred in muscles of older quails.
Slow-tonic anterior latissimus dorsi and fast-twitch patagialis muscles
of adult, middle-aged, aged, and senescent quails were stretch
overloaded for 6, 24, or 72 h, with contralateral muscles serving as
controls. RNase protection assays showed that MRF4 and MyoD transcript
levels were increased and myogenin and Myf5 transcripts were induced
in stretch-overloaded muscles. However, MRF4 and MyoD increases were
significantly attenuated in patagialis muscles of older quails. RT-PCR
analyses of three MRF-regulated genes showed that increases in the
transcription of these genes occurred with stretch overload, but the
increases were less in muscles of older quails. In summary, attenuated
MRF responses in muscles from aged animals may partially explain why muscles from older animals do not hypertrophy to the same extent as
muscles from younger animals.
aging; stretch overload; skeletal muscle hypertrophy; MyoD; MRF4
HYPERTROPHY OF SKELETAL MUSCLE involves an
increased rate of synthesis and accumulation of proteins, and therefore
increased transcription of muscle genes is necessary. The mechanisms by which nuclei increase transcription of specific skeletal muscle mRNA in
response to a hypertrophy stimulus are not known. We suspect that a
family of skeletal muscle-specific transcription factors called
myogenic regulatory factors (MRFs) is involved. This family is composed
of four members that were identified on the basis of their ability to
convert nonmuscle cells to myoblasts: MRF4/Myf6/herculin (33), MyoD
(7), myogenin (39), and Myf5 (4). These factors belong to a larger
basic helix-loop-helix (bHLH) class of transcription factors. The basic
region binds DNA, whereas the HLH region is involved in homo- or
heterodimerization with other HLH proteins. Heterodimerization with a
ubiquitous E protein is most common, and this dimerization increases
the efficiency of binding to target E boxes that are present in the
promoter region of several skeletal muscle genes (30). E box-containing
genes, which MRFs have been shown to bind and regulate, include
MRFs play pivotal roles in establishing the myogenic lineage and in
controlling terminal differentiation (23), and thus most studies of the
MRFs have been conducted from a developmental perspective. In general,
Myf5 and MyoD are required for the determination of myoblasts,
myogenin is critical for differentiation, and the function of MRF4
appears to be in fully differentiated fibers. The roles of MRFs in
regenerating muscle and in maintaining muscle phenotype have also been
investigated. Muscle regeneration involves activation of satellite
cells, the only myogenic cells in adult skeletal muscle. Proliferation
and differentiation of these cells are similar to events that occur
during embryogenesis, including increased expression of MRFs (34). MyoD
and myogenin have also been implicated in regulating muscle fiber type,
as myogenin accumulates in slow-twitch fibers and MyoD accumulates in
fast-twitch fibers (13, 35). Few studies have investigated MRFs in
fully differentiated, hypertrophic muscle (14, 22). The first objective
of this study was to determine whether skeletal muscle stimulated to
hypertrophy by stretch overload had an increased expression of MRFs and
whether increases in the transcription of genes regulated by MRFs
occurred.
Protein synthesis and accumulation are attenuated in skeletal muscle of
older individuals (24). Consequently, muscles of aged animals have a
reduced ability to hypertrophy. For example, 30 days of overload
resulted in a 44% increase of muscle mass in adult quails but only a
26% increase in aged quails (19). It is not known which step(s) in the
chain of processes that lead to an increase in muscle protein content
is responsible for the attenuation. It is possible that specific genes
are not activated in hypertrophy-stimulated muscle in aged animals.
This would occur if transcription factors, such as the MRFs, are lower
or have reduced activity in these muscles. Two studies have shown MRF expression in muscle from aged animals (27, 29). In those studies it
was reported that basal levels of MRF transcripts were higher in
hindlimb muscles from aged mice (29) and rats (27) than in muscles from
younger mice and rats, respectively. However, it remains to be
determined whether MRF expression in muscle from older animals can be
elevated further with a hypertrophy stimulus. Thus our second objective
of this study was to determine whether hypertrophy-stimulated muscle in
aged and senescent animals showed increases in MRF expression and
transcription of genes regulated by MRFs similar to that in younger
animals. The final objective was to define whether MRFs were regulated
differently in fast- and slow-twitch muscles during hypertrophy.
Animals and experimental protocol.
Japanese quails (Couturnix couturnix
Japonica) of four different ages were studied. Quails were obtained
from a breeder (Willow Games Farms) at 16-18 mo of age and used as
the oldest animals in this study when they reached 28 mo of age
(senescent). Eggs produced by the quails were incubated and hatched in
an incubator (Humidaire Incubator, New Madison, WI). Chicks were housed
in a chicken brooder for 2 wk and then housed five to seven quails per
cage, along with the older quails, at 20-23°C with a 12:12-h light-dark cycle. Chicks were studied when they reached 4 (adult), 8 (middle aged), and 18 mo (aged). The life span of Japanese quails is
~2.5 yr, with a mortality rate of ~60% by 28 mo (38). Quails in
our colony have lived as long as 32 mo. Japanese quails are physically
and sexually mature at 6 wk and show no maturational changes in carcass
composition or body weight beyond 2 mo after hatching (25, 26, 38).
Body weight did not differ among adult, middle-aged, aged, and
senescent quails in this study: 173 ± 28, 176 ± 22, 170 ± 23, and 174 ± 20 (SD) g, respectively (P = 0.92, n = 9 for middle-aged group and 21 for
adult, aged, and senescent groups). It was important that the quails
were at a stable body weight during these experiments so that
hypertrophy-stimulated growth was not confounded by normal growth of
the animals. Male and female quails were used in these studies, because
hypertrophic responses are not different between the sexes in these
animals (S. E. Alway, unpublished observations).
ABSTRACT
Top
Abstract
Introduction
Methods
Results
Discussion
References
INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References
-actin (28), desmin (20), nicotinic ACh receptor (AChR) (31), muscle
creatine kinase (17), troponin I (21), and myosin light chain (36).
Because increases in structural and contractile proteins (e.g., desmin
and troponin I), as well as muscle-specific enzymes (e.g., muscle
creatine kinase), are necessary for muscle to hypertrophy and remain
functional, we hypothesize that MRFs are upregulated in response to a
hypertrophy stimulus. Additionally, hyperplasia often accompanies
hypertrophy (1, 2, 5, 19), and this would require increases in AChR
proteins to enable neural connections to the new fibers.
METHODS
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Abstract
Introduction
Methods
Results
Discussion
References
80°C.
Riboprobes.
Quail cDNA clones for MyoD
(qmf1-cc509) (6), myogenin
(qmf2-cc527) (32), and Myf5
(qmf3-cc528) (32), provided by Dr. Charles Emerson (Fox Chase Cancer Center, Philadelphia, PA), were subcloned to make riboprobes that could be used simultaneously in RNase
protection assays (RPAs). MyoD riboprobes were made from a
Dde
I-EcoR I 267-bp fragment of
qmf1, myogenin probes from a Hind III-BamH I
203-bp fragment of qmf2, and Myf5
probes from a Sac
II-EcoR I 408-bp fragment of
qmf3. Restriction fragments were
cloned into pBluescript KS vectors (Stratagene, La Jolla, CA). For the
quail MRF4 riboprobe, total RNA from quail muscle was reverse
transcribed (Superscript II RNase
H
RT, Life Technologies)
using oligo(dT) primers (Promega, Madison, WI) and then amplified by
PCR. Oligonucleotides used for PCR were derived from the chicken MRF4
mRNA sequence (9) and produced a 330-bp PCR product
(5'-GGCTGGATCAGCAGGACAAA and 3'-AGGGCCGTTCGCCGGGGGGA; annealing temperature 62°C). The resulting cDNA was cloned using the pCR-Script Amp SK(+) Cloning Kit (Stratagene), and the insert was
verified by DNA sequencing; the quail 330-bp cDNA was 98% homologous
to chicken MRF4 mRNA. A 150-bp segment of quail
glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was also amplified by
RT-PCR. The lower primer was synthesized, including a T3 phage promoter
element (Table 1). The resulting PCR
product was then transcribed directly. Riboprobes were transcribed
using biotin-14-CTP and T7 or T3 RNA polymerase (BrightStar
BIOTINscript Kit, Ambion) and then gel purified. In the MyoD, myogenin,
and Myf5 riboprobes, 100% of the CTP was biotinylated;
only 50 and 10% of the CTP was biotinylated in the MRF4 and GAPDH
riboprobes, respectively. Each probe was verified for specificity by
Northern blot analyses using 25 µg of total RNA from a control Pat
muscle; each probe detected a single band at ~1.2 kb.
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RNA analyses.
Total RNA was isolated from ALD and Pat muscles with use of TriReagent
(Molecular Research Center, Cincinnati, OH), which is based on the
guanidine thiocyanate method. Frozen muscles were mechanically
homogenized in 1 ml of TriReagent. Extracted RNA was solubilized in
RNase-free H2O and quantitated in
duplicate by absorbance at 260 nm. Twenty-five micrograms of RNA from
an ALD muscle or 40 µg of RNA from a Pat muscle were used for each RPA. Any remaining RNA was stored at 80°C. RPAs were done
according to the directions from the manufacturer (HybSpeed RPA,
Ambion). Positive and negative control samples contained all five
riboprobes and either yeast mRNA or mouse liver mRNA. Protected
fragments were not observed when control samples were digested with
RNase (negative controls), and full-length riboprobes (
10% larger
than protected fragments) were observed when RNase digestion was
omitted (positive controls). Century Marker Template (Ambion) was
transcribed using biotin-14-CTP and used as size standards in the RPAs.
Protected fragments, controls, and standards were electrophoresed on
5% acrylamide-8 M urea gels, electroblotted onto positive-charged nylon membranes, and immobilized by ultraviolet cross-linking. Chemiluminescence detection was used (BrightStar BioDetect Nonisotopic Detection Kit, Ambion), and exposure to film was 2-4 h. Signals were quantitated on a densitometer (model 620, Bio-Rad, Hercules, CA),
and each MRF signal was normalized by the GAPDH signal in that lane.
This normalization accounted for variability in initial amounts of RNA
used in the assays, losses during the assays, or blotting
inefficiencies. Preliminary experiments were done to ensure that
1) riboprobes were used in excess of
the message they were to detect and
2) the resulting signals were within
the linear range of the detection system and the film (i.e., each probe
was biotinylated such that all 5 resulting signals could be analyzed simultaneously).
RT) in a total volume of
10 µl. Control RT reactions that contained no RT were done. Thirty
cycles of PCR were carried out using 1 µl of cDNA generated from the
RT, 100 ng of each primer (specific primers are listed in Table 1), and
2.5 units of Taq DNA polymerase (Sigma
Chemical, St. Louis, MO) in a total volume of 50 µl. GAPDH was
amplified in the same PCRs and used as an internal control to verify
that equal amounts of cDNA were used in PCRs and that equal amounts of
PCR products were loaded onto agarose gels. Annealing temperatures
given in Table 1 for desmin, muscle creatine kinase, and AChR were used
in the PCRs. The resulting signals were visualized after separation on
1.5% agarose gels with use of SYBR Green I nucleic acid gel stain
(Molecular Probes, Eugene, OR), scanned using an optical scanner (model
840, Molecular Dynamics, Sunnyvale, CA), and then quantitated using
ImageQuant software. Preliminary experiments were conducted to ensure
that PCR was performed within the linear range. Control RT reactions
were PCR amplified to confirm that DNA did not contaminate the RNA; no
bands were detected when these reactions were visualized on agarose
gels with SYBR Green I staining. In addition, no extraneous bands were
detected in any of the PCRs.
DNA analyses. DNA was extracted from the same Pat and ALD muscle homogenates from which the RNA was isolated. The extraction was done according to the directions provided with the TriReagent. After precipitation, DNA was solubilized in 40 mM NaOH and quantitated in duplicate by absorbance at 260 nm.
Statistical analyses.
MRF data are expressed as percent differences of MRF-to-GAPDH ratios
between contralateral control and stretch-overloaded muscles. Values
are means ± SE. One-way ANOVA (age) and Tukey post hoc tests were
used to determine whether differences between basal MRF and GAPDH mRNA
levels occurred. Two-way ANOVA (age × time) and Tukey post hoc
tests were used to determine whether differences between the means for
percent changes in nucleic acid contents and MRF mRNA levels occurred.
An -level of 0.05 was used for all tests.
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RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Nucleic acid content of ALD muscles.
RNA content of ALD muscles from adult quails was increased after 6, 24, and 72 h of stretch overload compared with contralateral muscles
(P 
0.013; Fig.
1) and was increased in
muscles from senescent quails after 24 and 72 h of stretch overload
(P 0.04). RNA content increases in
72-h stretch-overloaded ALD muscles were affected by age
(P = 0.001; Fig. 1). RNA content of
control ALD muscles was not different between adult and senescent
quails: 32 ± 2 and 34 ± 2 µg/muscle, respectively
(P = 0.58).
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0.17; Fig. 1). DNA
content was greater in 72-h stretch-overloaded ALD muscles than in
contralateral control muscles (P 0.003), and the increase was affected by age
(P = 0.002; Fig. 1). DNA content of
control ALD muscles was not different between adult and senescent
quails: 21 ± 2 and 23 ± 2 µg/muscle, respectively
(P = 0.52).
Nucleic acid content of Pat muscles.
After 6 h of stretch overload, RNA content was increased in Pat muscles
from adult animals (P = 0.01; Fig.
2), and after 24 h of stretch overload, RNA
content was increased in muscles from adult, middle-aged, and aged
animals (P 0.04). RNA
content of Pat muscles from quails of all ages was increased after 72 h
of stretch overload compared with contralateral muscles
(P 0.01). Increases in RNA content
after 24 and 72 h of stretch overload were less in Pat muscles from
senescent quails than in those from younger quails
(P = 0.04; Fig. 2). RNA content of
control Pat muscles was not different among adult, middle-aged, aged,
and senescent quails: 122 ± 9, 127 ± 5, 132 ± 6, and 118 ± 4 µg/muscle, respectively (P = 0.35).
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0.19;
Fig. 2). DNA contents were increased after 72 h of stretch overload in
muscles from quails of all ages (P 0.04), and the increases were affected by age
(P = 0.01; Fig. 2). DNA content of
control Pat muscles was not different among adult, middle-aged, aged,
and senescent quails: 79 ± 8, 84 ± 11, 90 ± 13, and 83 ± 11 µg/muscle, respectively (P = 0.65).
RPAs. All five riboprobes were used and detected simultaneously in each RPA (Fig. 3). Protected fragments of MRF4, MyoD, and GAPDH were detected in all muscle samples. Myf5 was detected in ~60% of the ALD muscles, and myogenin was detected only in 72-h stretch-overloaded ALD muscles. Fewer than 20% of the Pat muscles expressed detectable levels of myogenin or Myf5 mRNA, but when they were detected it was always in stretch-overloaded muscle (Fig. 3).
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0.16 for
ALD muscles and P 0.25 for Pat
muscles). No difference in GAPDH levels was detected between ALD
muscles from adult and senescent quails
(P = 0.53). However, levels of GAPDH
mRNA were ~25% greater in Pat muscles from adult and middle-aged
animals than in Pat muscles from aged and senescent animals
(P = 0.02). Because GAPDH was used as
an internal control in the RPAs and the MRF data (relative to GAPDH) were expressed as within-animal differences, the change in GAPDH mRNA
with age was not a confounding factor.
Basal MRF levels.
Control muscle MRF mRNA levels (basal levels) were not different among
age groups in ALD muscles (P 0.48),
but a difference was detected in Pat muscles
(P 0.02). MRF4 and MyoD mRNA levels in Pat muscles from senescent quails were 48 and 50% greater, respectively, than levels from adult quails. In Pat muscles from middle-aged and aged quails, MRF4 and MyoD mRNA basal levels were intermediate to those from adult and senescent quails but not significantly different from any group.
Stretch overload-induced MRF changes in ALD muscles.
Initially, only ALD muscles from adult and senescent quails were
investigated. The time of stretch overload did not affect increases in
MRF4, MyoD, or Myf5 mRNA levels
(P 0.13). When collapsed across
time, MRF4, MyoD, and Myf5 mRNA levels were significantly elevated in response to stretch overload in ALD muscles from adult and
senescent animals (P 0.03; Fig.
4). However, no differences in MRF4, MyoD,
or Myf5 mRNA increases between ALD muscles from adult and
senescent quails were found (P 0.38); therefore, ALD muscles from middle-aged and aged quails were not
studied.
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Stretch overload-induced MRF changes in Pat muscles.
MRF4 and MyoD mRNA increases were not different between Pat muscles
stretch overloaded for 6, 24, and 72 h within any age group
(P 0.34), and no interaction
between time of stretch overload and age was detected
(P 0.25). Therefore, data from the
three stretch-overload times were pooled for further analyses. MRF4 mRNA levels in stretch-overloaded Pat muscles (collapsed across time)
were significantly elevated above contralateral control levels at each
age (P 0.005). However,
MRF4 increases in muscles from older animals were less than those in
muscles from younger animals (P = 0.02; Fig. 5). MyoD mRNA was also increased
at each age after stretch overload (P 0.04), and the increase was significantly affected by age
(P = 0.02; Fig. 5).
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RT-PCR of transcripts whose genes are regulated by MRFs. Analyses of desmin, muscle creatine kinase, and AChR mRNA were done to determine whether changes in transcription of some genes regulated by MRFs occurred with stretch overload. Because quantities of RNA were limited, the sensitive method of RT-PCR was performed using the primers listed in Table 1 and RNA isolated from 72-h stretch-overloaded and control Pat and ALD muscles from adult and senescent animals. Stretch-overloaded muscles from adult and senescent quails had greater desmin signals than the contralateral control muscles (Fig. 6). The desmin signal increase in Pat muscle from the younger quails was greater than that from the older quails. No differences in stretch-induced desmin signal increases were observed between adult and senescent ALD muscles (Fig. 6). These same patterns of increases were observed with muscle creatine kinase (Fig. 7) and AChR signals (Fig. 8). A second set of 72-h stretch-overloaded and control Pat and ALD muscles from adult and senescent quails was analyzed to confirm the RT-PCR results shown in Figs. 6-8. The amplification results were analogous to those shown.
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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General. We have emphasized our findings in the Pat muscle over those in the ALD muscle because of their different fiber type compositions. The ALD muscle is composed of tonic fibers (2, 5), and thus its relevance to mammalian muscle is unclear. The Pat muscle, however, is a twitch muscle that is composed primarily of fibers similar to type II mammalian fibers (19), and so it is probable that findings in this muscle can be extrapolated more readily to mammalian muscle.
It is not surprising that MyoD transcripts were detected in all Pat muscles, because MyoD is prevalent in fast-twitch fibers (13, 35). Likewise, very few of these muscles showed transcripts of myogenin, the MRF that is prevalent in slow-twitch fibers (13, 35). We did not consistently detect myogenin mRNA in the slow-tonic ALD muscle, except after 72 h of stretch overload. This makes sense in light of the facts that 1) ALD muscle is a slow-tonic muscle and myogenin is more prevalent in slow-twitch muscle and 2) myogenin is primarily involved in differentiation and thus is normally expressed at time points later than Myf5 and MyoD (23). MRF4 is predominantly expressed in fully differentiated muscle fibers (12). Correspondingly, all muscles in our study had relatively high levels of MRF4 transcripts.Muscle hypertrophy. RNA content in adult ALD and Pat muscles was increased as early as 6 h after stretch overload, and DNA content was increased after 72 h. The increases indicate that the stretch overload did apply a hypertrophy stimulus to these muscles, and the data are comparable to other reports of nucleic acid content changes in chicken and rat muscles after stretch (10, 18). Earlier studies conducted in our laboratory have characterized muscle mass and fiber number increases that occur in ALD (1, 2, 5) and Pat (19) muscles from adult and aged quails after stretch overload.
The roles of MRFs in mature skeletal muscle have not been delineated, despite the finding that each has a unique role during embryonic skeletal muscle development (23). In our study, MyoD and MRF4 mRNA levels were elevated and myogenin and Myf5 mRNAs were induced after stretch overload, indicating that they all take part in the response to a hypertrophy stimulus. MyoD and MRF4 mRNAs were elevated by 100-500% after 6-72 h of stretch overload in adult ALD and Pat muscles. Although we observed no increase of any MRF mRNA in quail Pat muscle after 0.5 h of stretch in situ (unpublished observations), the MRF mRNA responses after 6-72 h of stretch overload are similar in magnitude to those observed in stretched ALD muscle of adult chickens (8) and adult rat hindlimb muscles after the muscles were casted for 48 h in a stretched position (22). We did not determine MRF levels between 0.5 and 6 h, because our goal was to determine a time at which MRFs were elevated in stretch-overloaded muscle of adult animals and then to find whether the same increases occurred in muscles of older animals. We extended our experiments to time points beyond 6 h to ensure that the attenuated response we found in Pat muscle from aged and senescent quails was not simply a lag in the response time of the older muscle. Studies conducted on regenerating muscle suggest that MRF mRNA increases occur in satellite cell nuclei, because the time of MRF increases coincides with the initiation of satellite cell proliferation (11, 15). Our data obtained from 72-h stretch-overloaded muscles agree with this suggestion. These muscles had elevated levels of DNA, indicating that satellite cells were proliferating, and MRF mRNA levels were also increased at this time point. However, Jacobs-El et al. (14) suggested that myonuclei express elevated levels of MRF mRNA 1) on the basis of observations that MRF increases were detected as early as 2 h after stimulation and 2) because Myf5 and MRF4 in situ hybridization signals in stimulated rat muscles spread into the cytoplasm of some mature fibers. Our data also support the suggestion of an elevation of MRF mRNA in the myonuclei, because these levels were increased after just 6 h of stretch overload, and it has been shown that satellite cells in quail ALD muscle are not activated until after 24 h of stretch overload (37). In addition, a previous study in our laboratory showed that 3-14 days of stretch overload did not result in satellite cell proliferation in quail Pat muscle (19). Therefore, the increases in MRF mRNA we observed in quail Pat muscle were likely derived from myofiber nuclei. Additional studies are necessary to localize MRF transcript increases, i.e., myofiber nuclei vs. satellite cell nuclei, especially after periods of stretch overload that induce satellite cell proliferation.Aging.
Only two previous studies have reported MRF expression in muscles from
aged animals (27, 29). Musaro et al. (29) showed that basal mRNA levels
of MyoD, myogenin, and Myf5, but not MRF4, were higher in hindlimb
muscles of aged than adult mice. In agreement with this, we found
higher basal levels of MyoD mRNA in Pat muscles from senescent than
adult quails. Additionally, we found that basal MRF4 mRNA levels were
elevated in Pat muscles from older quails. Marsh et al. (27) also
reported elevated levels of MyoD, myogenin, and MRF4 mRNA with aging.
In the present study, basal levels of MyoD and MRF4 in muscles from
middle-aged and aged animals were intermediate to those from adult and
senescent animals, indicating that the increases occur gradually during
aging. It is puzzling why MRF mRNA basal levels are higher in muscles
of older animals, because basal rates of protein synthesis are lower in
muscles of aged animals than younger animals (24). One possible
explanation for the higher basal MRF mRNA levels in muscle of older
animals may be the loss of -motor neurons that occurs with aging
(3). It has been demonstrated that MyoD and myogenin transcripts are elevated in denervated muscle (16, 35) and that the extent of the
elevation depends on the severity of the denervation (16). Muscle of
aged animals may be considered mildly denervated, and this could
influence basal expression of the MRFs.
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ACKNOWLEDGEMENTS |
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We thank Dr. Charles Emerson for providing the qmf1, qmf2, and qmf3 cDNA, Dr. Duane Hinton for assistance with the subcloning, and Paul Llobet for assistance with the DNA analyses.
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FOOTNOTES |
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This research was supported by National Institute on Aging Grant AG-10871.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests: S. E. Alway, Dept. of Anatomy, MDC #6, 12901 Bruce B. Downs Bl., University of South Florida, Tampa, FL 33612.
Received 4 February 1998; accepted in final form 31 March 1998.
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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