Unloading of juvenile muscle results in a reduced muscle size 9 wk after reloading

The concept of skeletal muscle memory: Evidence from animal and human studies

Within the current paradigm of the myonuclear domain theory, it is postulated that a linear relationship exists between muscle fibre size and myonuclear content. The myonuclear domain is kept (relatively) constant by adding additional nuclei (supplied by muscle satellite cells) during muscle fibre hypertrophy and nuclear loss (by apoptosis) during muscle fibre atrophy. However, data from recent animal studies suggest that myonuclei that are added to support muscle fibre hypertrophy are not lost within various muscle atrophy models. Such myonuclear permanence has been suggested to constitute a mechanism allowing the muscle fibre to (re)grow more efficiently during retraining, a phenomenon referred to as "muscle memory." The concept of "muscle memory by myonuclear permanence" has mainly been based on data attained from rodent experimental models. Whether the postulated mechanism also holds true in humans remains largely ambiguous. Nevertheless, there are several studies in humans that provide evidence to potentially support or contradict (parts of) the muscle memory hypothesis. The goal of the present review was to discuss the evidence for the existence of "muscle memory" in both animal and human models of muscle fibre hypertrophy as well as atrophy. Furthermore, to provide additional insight in the potential presence of muscle memory by myonuclear permanence in humans, we present new data on previously performed exercise training studies. Finally, suggestions for future research are provided to establish whether muscle memory really exists in humans.

Satellite Cells Contribution to Exercise Mediated Muscle Hypertrophy and Repair

Satellite cells (SCs) are the most abundant skeletal muscle stem cells. They are widely recognized for their contributions to maintenance of muscle mass, regeneration and hypertrophy during the human life span. These cells are good candidates for cell therapy due to their self-renewal capabilities and presence in an undifferentiated form. Presently, a significant gap exists between our knowledge of SCs behavior and their application as a means for human skeletal muscle tissue repair and regeneration. Both physiological and pathological stimuli potentially affect SCs activation, proliferation, and terminal differentiation the former category being the focus of this article. Activation of SCs occurs following exercise, post-training micro-injuries, and electrical stimulation. Exercise, as a potent and natural stimulus, is at the center of numerous studies on SC activation and relevant fields. According to research, different exercise modalities end with various effects. This review article attempts to picture the state of the art of the SCs life span and their engagement in muscle regeneration and hypertrophy in exercise.

Genetic Evidence That Captured Retroviral Envelope syncytins Contribute to Myoblast Fusion and Muscle Sexual Dimorphism in Mice

Syncytins are envelope genes from endogenous retroviruses, “captured” for a role in placentation. They mediate cell-cell fusion, resulting in the formation of a syncytium (the syncytiotrophoblast) at the fetomaternal interface. These genes have been found in all placental mammals in which they have been searched for. Cell-cell fusion is also pivotal for muscle fiber formation and repair, where the myotubes are formed from the fusion of mononucleated myoblasts into large multinucleated structures. Here we show, taking advantage of mice knocked out for syncytins, that these captured genes contribute to myoblast fusion, with a >20% reduction in muscle mass, mean muscle fiber area and number of nuclei per fiber in knocked out mice for one of the two murine syncytin genes. Remarkably, this reduction is only observed in males, which subsequently show muscle quantitative traits more similar to those of females. In addition, we show that syncytins also contribute to muscle repair after cardiotoxin-induced injury, with again a male-specific effect on the rate and extent of regeneration. Finally, ex vivo experiments carried out on murine myoblasts demonstrate the direct involvement of syncytins in fusion, with a >40% reduction in fusion index upon addition of siRNA against both syncytins. Importantly, similar effects are observed with primary myoblasts from sheep, dog and human, with a 20–40% reduction upon addition of siRNA against the corresponding syncytins. Altogether, these results show a direct contribution of the fusogenic syncytins to myogenesis, with a demonstrated male-dependence of the effect in mice, suggesting that these captured genes could be responsible for the muscle sexual dimorphism observed in placental mammals.

Syncytins are “captured” genes of retroviral origin, corresponding to the fusogenic envelope gene of endogenized retroviruses. They are present in all placental mammals in which they have been searched for, where they play an essential role in placentation via their cell-cell fusion activity. Here we show that they also contribute to myoblast fusion and muscle formation in development and repair after injury, using both in vivo knock-out mouse models and ex vivo primary myoblast cell cultures from several mammals, including humans, carnivores and ruminants. Interestingly, the effects observed in mice are sex-dependent, thus suggesting that the added “collateral” effect of syncytins on myogenesis could be responsible for the muscle sexual dimorphism observed in placental mammals.

In ovo L-arginine supplementation stimulates myoblast differentiation but negatively affects muscle development of broiler chicken after hatching

In this study, we tested the hypothesis that in ovo feeding (IOF) of L-arginine (L-Arg) enhances nitric oxide (NO) production, stimulates the process of myogenesis, and regulates post-hatching muscle growth. Different doses of L-Arg were injected into the amnion of chicken embryos at embryonic day (ED) 16. After hatching, the body weight of individual male chickens was recorded weekly for 3 weeks. During in vitro experiments, myoblasts of the pectoralis major (PM) were extracted at ED16 and were incubated in medium containing 0.01 mm L-Arg, 0.05 mm L-Arg, and (or) 0.05 mm L-nitro-arginine-methyl-ester (L-NAME), an inhibitor of nitric oxide synthase (NOS). When 25 mg/kg L-Arg/initial egg weight was injected, no difference was observed in body weight at hatch, but a significant decrease was found during the following 3 weeks compared to that of the non-injected and saline-injected control, and this also affected the growth of muscle mass. L-NAME inhibited gene expression of myogenic differentiation antigen (MyoD), myogenin, NOS, and follistatin, decreased the cell viability, and increased myostatin (MSTN) gene expression. 0.05 mm L-Arg stimulated myogenin gene expression but also depressed muscle cell viability. L-NAME blocked the effect of 0.05 mm L-Arg on myogenin mRNA levels when co-incubated with 0.05 mm L-Arg. L-Arg treatments had no significant influence on NOS mRNA gene expression, but had inhibiting effect on follistatin gene expression, while L-NAME treatments had effects on both. These results suggested that L-Arg stimulated myoblast differentiation, but the limited number of myoblasts would form less myotubes and then less myofibers, while the latter limited the growth of muscle mass.

Intermittent application of hypergravity by centrifugation attenuates disruption of rat gait induced by 2 weeks of simulated microgravity

The effects of intermittent hypergravity on gait alterations and hindlimb muscle atrophy in rats induced by 2 weeks of simulated microgravity were investigated. Rats were submitted to hindlimb unloading for 2 weeks (unloading period), followed by 2 weeks of reloading (recovery period). During the unloading period, animals were subjected to the following treatments: (1) free in cages (Control); (2) continuous unloading (UL); (3) released from unloading for 1 hour per day (UL+1G); (4) hypergravity for 1h per day using a centrifuge for small animals (UL+2G). The relative weights of muscles to the whole body weight and kinematics properties of hindlimbs during gait were evaluated. UL rats walked with their hindlimbs overextended, and the oscillation of their limb motion had become narrowed and forward-shifted after the unloading period, and this persisted for at least 2 weeks after the termination of unloading. However, these locomotor alterations were attenuated in rats subjected to UL+2G centrifugation despite minor systematic changes in muscle recovery. These findings indicate hypergravity application could counteract the adverse effects of simulated or actual microgravity environments.

The Effect of Reloading on Disuse Muscle Atrophy: Time Course of Hypertrophy and Regeneration Focusing on the Myofiber Cross-sectional Area and Myonuclear Change

The purpose of this study was to investigate the effect of reloading on atrophied muscle and the time course of hypertrophy and regeneration. Forty-nine male Wistar rats were randomly assigned to groups for hindlimb suspension (HS), hindlimb suspension and reloading (R), or control (C0). Rats in the HS group were suspended for 14 days. Rats in the R group were randomly divided into five subgroups for different post-hindlimb-suspension recovery times. Briefly, each subgroup was suspended for 14 days and given 1 day of reloading (R1), 3 days of reloading (R3), 7 days of reloading (R7), 10 days of reloading (R10), or 14 days of reloading (R14). Myonuclear numbers were significantly decreased in the groups with hindlimb suspension and 1 day and 3 days of reloading compared with that in the control group. We focused on the processes of change of mean myofiber cross-sectional area and myonuclear domain size; the degrees of increase of both indexes were limited until 3 days of reloading, and significantly increased after 7 days of reloading. An important finding of the current study was that the processes of muscle hypertrophy and regeneration did not show uniform change. In addition, there were differences in the ratio of increase among the stages of hypertrophy and regeneration. Therefore, consideration of the duration and method of physiotherapeutic intervention for atrophied muscle on the basis of the process of hypertrophy and regeneration is needed to provide more effective physiotherapy.

Discordance in recovery between altered locomotion and muscle atrophy induced by simulated microgravity in rats

Exposure to a microgravity environment leads to adverse effects in motion and musculoskeletal properties. However, few studies have investigated the recovery of altered locomotion and muscle atrophy simultaneously. The authors investigated altered locomotion in rats submitted to simulated microgravity by hindlimb unloading for 2 weeks. Motion deficits were characterized by hyperextension of the knees and ankle joints and forward-shifted limb motion. Furthermore, these locomotor deficits did not revert to their original form after a 2-week recovery period, although muscle atrophy in the hindlimbs had recovered, implying discordance in recovery between altered locomotion and muscle atrophy, and that other factors such as neural drives might control behavioral adaptations to microgravity.

Macrophage deficiency in osteopetrotic (op/op) mice inhibits activation of satellite cells and prevents hypertrophy in single soleus fibers

Effects of macrophage on the responses of soleus fiber size to hind limb unloading and reloading were studied in osteopetrotic homozygous (op/op) mice with inactivated mutation of macrophage colony-stimulating factor (M-CSF) gene and in wild-type (+/+) and heterozygous (+/op) mice. The basal levels of mitotically active and quiescent satellite cell (-46 and -39% vs. +/+, and -40 and -30% vs. +/op) and myonuclear number (-29% vs. +/+ and -28% vs. +/op) in fibers of op/op mice were significantly less than controls. Fiber length and sarcomere number in op/op were also less than +/+ (-22%) and +/op (-21%) mice. Similar trend was noted in fiber cross-sectional area (CSA, -15% vs. +/+, P = 0.06, and -14% vs. +/op, P = 0.07). The sizes of myonuclear domain, cytoplasmic volume per myonucleus, were identical in all types of mice. The CSA, length, and the whole number of sarcomeres, myonuclei, and mitotically active and quiescent satellite cells, as well as myonuclear domain, in single muscle fibers were decreased after 10 days of unloading in all types of mice, although all of these parameters in +/+ and +/op mice were increased toward the control values after 10 days of reloading. However, none of these levels in op/op mice were recovered. Data suggest that M-CSF and/or macrophages are important to activate satellite cells, which cause increase of myonuclear number during fiber hypertrophy. However, it is unclear why their responses to general growth and reloading after unloading are different.

Muscle progenitor cell regenerative capacity in the torn rotator cuff

Chronic rotator cuff (RC) tears affect a large portion of the population and result in substantial upper extremity impairment, shoulder weakness, pain, and limited range of motion. Regardless of surgical or conservative treatment, persistent atrophic muscle changes limit functional restoration and may contribute to surgical failure. We hypothesized that deficits in the skeletal muscle progenitor (SMP) cell pool could contribute to poor muscle recovery following tendon repair. Biopsies were obtained from patients undergoing arthroscopic RC surgery. The SMP population was quantified, isolated, and assayed in culture for its ability to proliferate and fuse in vitro and in vivo. The SMP population was larger in muscles from cuffs with partial tears compared with no tears or full thickness tears. However, SMPs from muscles in the partial tear group also exhibited reduced proliferative ability. Cells from all cuff states were able to fuse robustly in culture and engraft when injected into injured mouse muscle, suggesting that when given the correct signals, SMPs are capable of contributing to muscle hypertrophy and regeneration regardless of tear severity. The fact that this does not appear to happen in vivo helps focus future therapeutic targets for promoting muscle recovery following rotator cuff repairs and may help improve clinical outcomes.

Fitness and physical activity in youth with type 1 diabetes mellitus in good or poor glycemic control

BACKGROUND

Patients with type 1 diabetes mellitus (T1DM) may experience poor muscle health as a result of chronic hyperglycemia. Despite this, muscle function in children with T1DM with good or poor glycemic control has yet to be examined in detail.

OBJECTIVE

To assess differences in muscle-related fitness variables in children with T1DM with good glycemic control (T1DM-G), as well as those with poor glycemic control (T1DM-P), and non-diabetic, healthy controls.

SUBJECTS

Eight children with T1DM-G [glycosylated hemoglobin (HbA1c) ≤ 7.5% for 9 months], eight children with T1DM-P (HbA1c ≥ 9.0% for 9 months), and eight healthy controls completed one exercise session.

METHODS

Anaerobic and aerobic muscle functions were assessed with a maximal isometric grip strength test, a Wingate test, and an incremental continuous cycling test until exhaustion. Blood samples were collected at rest to determine HbA1c at the time of testing. Physical activity was monitored over 7 d using accelerometry.

RESULTS

Children with T1DM-P displayed lower peak oxygen consumption (VO2peak ) values (mL/kg/min) compared to healthy controls (T1DM-P: 33.2 ± 5.6, controls: 43.5 ± 6.3, p < 0.01), while T1DM-G (43.5 ± 6.3) had values similar to controls and T1DM-P. There was a negative relationship between VO2peak and HbA1c% (r = -0.54, p < 0.01). All groups were similar in all other fitness variables. There were no group differences in physical activity variables.

CONCLUSION

Children with T1DM-G did not display signs of impaired muscle function, while children with T1DM-P have signs of altered aerobic muscle capacity.

Skeletal muscle wasting with disuse atrophy is multi-dimensional: the response and interaction of myonuclei, satellite cells and signaling pathways

Maintenance of skeletal muscle is essential for health and survival. There are marked losses of skeletal muscle mass as well as strength and physiological function under conditions of low mechanical load, such as space flight, as well as ground based models such as bed rest, immobilization, disuse, and various animal models. Disuse atrophy is caused by mechanical unloading of muscle and this leads to reduced muscle mass without fiber attrition. Skeletal muscle stem cells (satellite cells) and myonuclei are integrally involved in skeletal muscle responses to environmental changes that induce atrophy. Myonuclear domain size is influenced differently in fast and slow twitch muscle, but also by different models of muscle wasting, a factor that is not yet understood. Although the myonuclear domain is 3-dimensional this is rarely considered. Apoptosis as a mechanism for myonuclear loss with atrophy is controversial, whereas cell death of satellite cells has not been considered. Molecular signals such as myostatin/SMAD pathway, MAFbx, and MuRF1 E3 ligases of the ubiquitin proteasome pathway and IGF1-AKT-mTOR pathway are 3 distinctly different contributors to skeletal muscle protein adaptation to disuse. Molecular signaling pathways activated in muscle fibers by disuse are rarely considered within satellite cells themselves despite similar exposure to unloading or low mechanical load. These molecular pathways interact with each other during atrophy and also when various interventions are applied that could alleviate atrophy. Re-applying mechanical load is an obvious method to restore muscle mass, however how nutrient supplementation (e.g., amino acids) may further enhance recovery (or reduce atrophy despite unloading or ageing) is currently of great interest. Satellite cells are particularly responsive to myostatin and to growth factors. Recently, the hibernating squirrel has been identified as an innovative model to study resistance to atrophy.

Diabetic myopathy: impact of diabetes mellitus on skeletal muscle progenitor cells

Diabetes mellitus is defined as a group of metabolic diseases that are associated with the presence of a hyperglycemic state due to impairments in insulin release and/or function. While the development of each form of diabetes (Type 1 or Type 2) drastically differs, resultant pathologies often overlap. In each diabetic condition, a failure to maintain healthy muscle is often observed, and is termed diabetic myopathy. This significant, but often overlooked, complication is believed to contribute to the progression of additional diabetic complications due to the vital importance of skeletal muscle for our physical and metabolic well-being. While studies have investigated the link between changes to skeletal muscle metabolic health following diabetes mellitus onset (particularly Type 2 diabetes mellitus), few have examined the negative impact of diabetes mellitus on the growth and reparative capacities of skeletal muscle that often coincides with disease development. Importantly, evidence is accumulating that the muscle progenitor cell population (particularly the muscle satellite cell population) is also negatively affected by the diabetic environment, and as such, likely contributes to the declining skeletal muscle health observed in diabetes mellitus. In this review, we summarize the current knowledge surrounding the influence of diabetes mellitus on skeletal muscle growth and repair, with a particular emphasis on the impact of diabetes mellitus on skeletal muscle progenitor cell populations.

Effects of aging on thyroarytenoid muscle regeneration

OBJECTIVES/HYPOTHESIS

Regenerative properties of age-associated changes in the intrinsic laryngeal muscles following injury are unclear. The purpose of this study was to investigate the regenerative properties of the thyroarytenoid (TA) muscle in an aging rat model. The hypothesis was that following myotoxic injury, old animals would exhibit a decrease in mitotic activities of muscle satellite cells when compared with younger rats, suggesting reduced regenerative potential in the aging rat TA muscle.

STUDY DESIGN

Animal group comparison.

METHODS

Regeneration responses following injury to the TA muscle were examined in 18 young adult, middle-aged, and old Fischer 344/Brown Norway rats. TA muscle fiber cross-sectional area (CSA), satellite cell mitosis (number/fiber), and regeneration index (CSA injured side/CSA noninjured side) were measured and compared across age groups.

RESULTS

Young adult animals had a significantly higher regeneration index than the middle-aged and old groups. Within the lateral region of the TA muscle (LTA), the regeneration index was significantly higher in the young adult animals than in the middle-aged and old animals. The regeneration index of the medial TA was significantly higher than the LTA across all age groups.

CONCLUSIONS

The regenerative capacity of the TA muscle is impaired with increasing age.

Overexpression of inducible 70-kDa heat shock protein in mouse improves structural and functional recovery of skeletal muscles from atrophy

Heat shock proteins play a key regulatory role in cellular defense. To investigate the role of the inducible 70-kDa heat shock protein (HSP70) in skeletal muscle atrophy and subsequent recovery, soleus (SOL) and extensor digitorum longus (EDL) muscles from overexpressing HSP70 transgenic mice were immobilized for 7 days and subsequently released from immobilization and evaluated after 7 days. Histological analysis showed that there was a decrease in cross-sectional area of type II myofiber from EDL and types I and II myofiber from SOL muscles at 7-day immobilization in both wild-type and HSP70 mice. At 7-day recovery, EDL and SOL myofibers from HSP70 mice, but not from wild-type mice, recovered their size. Muscle tetanic contraction decreased only in SOL muscles from wild-type mice at both 7-day immobilization and 7-day recovery; however, it was unaltered in the respective groups from HSP70 mice. Although no effect in a fatigue protocol was observed among groups, we noticed a better contractile performance of EDL muscles from overexpressing HSP70 groups as compared to their matched wild-type groups. The number of NCAM positive-satellite cells reduced after immobilization and recovery in both EDL and SOL muscles from wild-type mice, but it was unchanged in the muscles from HSP70 mice. These results suggest that HSP70 improves structural and functional recovery of skeletal muscle after disuse atrophy, and this effect might be associated with preservation of satellite cell amount.

Voluntary resistance wheel exercise during post-natal growth in rats enhances skeletal muscle satellite cell and myonuclear content at adulthood

AIM

To determine whether voluntary free wheel (FW) or resistance wheel (RW) exercise or reduced muscle activity would influence maturational increases in muscle mass and the number of satellite cells (SCs) and myonuclei (MN) accrued by adulthood.

METHODS

Hind limb muscles of male rats housed with, or without, FWs from 4 to 5, 7 or 10 weeks of age, and rats housed with RWs from 4 to 10 week of age, were evaluated. To assess the effect of reduced muscle activity, gastrocnemius muscles of 4-week-old rats were injected with botulinum toxin (Btx) and collected at 7 weeks of age. Muscle fibre size and the frequency of Pax7-positive SCs and MN were determined in 7- and 10-week-old muscles via immunohistochemical methods.

RESULTS

Free wheel exercise enhanced muscle growth and the frequency of SCs in the medial gastrocnemius (MG) (threefold) and vastus lateralis (VL) (twofold) of rats at 10 week of age. Resistance wheel exercise increased the number of SCs and MN (22-30%), with more muscle fibre nuclei being associated with larger fibre size, in the soleus, MG and VL muscles. Btx impaired the normal increases in muscle fibre size and the accrual of MN but not SCs.

CONCLUSION

A greater volume of exercise during maturational growth was important for enhancing SC numbers, whereas their conversion to MN required higher-intensity exercise. The enhanced muscle fibre nuclear populations may influence the capacity of the muscle to adapt to exercise, injury or disuse in later adulthood.

Inhibition of plasminogen activator inhibitor-1 restores skeletal muscle regeneration in untreated type 1 diabetic mice

OBJECTIVE

Type 1 diabetes leads to impairments in growth, function, and regenerative capacity of skeletal muscle; however, the underlying mechanisms have not been clearly defined.

RESEARCH DESIGN AND METHODS

With the use of Ins2(WT/C96Y) mice (model of adolescent-onset type 1 diabetes), muscle regeneration was characterized in terms of muscle mass, myofiber size (cross-sectional area), and protein expression. Blood plasma was analyzed for glucose, nonesterified fatty acids, insulin, and plasminogen activator inhibitor-1 (PAI-1). PAI-039, an effective inhibitor of PAI-1, was orally administered to determine if PAI-1 was attenuating muscle regeneration in Ins2(WT/C96Y) mice.

RESULTS

Ins2(WT/C96Y) mice exposed to 1 or 8 weeks of untreated type 1 diabetes before chemically induced muscle injury display significant impairments in their regenerative capacity as demonstrated by decreased muscle mass, myofiber cross-sectional area, myogenin, and Myh3 expression. PAI-1, a physiologic inhibitor of the fibrinolytic system and primary contributor to other diabetes complications, was more than twofold increased within 2 weeks of diabetes onset and remained elevated throughout the experimental period. Consistent with increased circulating PAI-1, regenerating muscles of diabetic mice exhibited excessive collagen levels at 5 and 10 days postinjury with concomitant decreases in active urokinase plasminogen activator and matrix metalloproteinase-9. Pharmacologic inhibition of PAI-1 with orally administered PAI-039 rescued the early regenerative impairments in noninsulin-treated Ins2(WT/C96Y) mice.

CONCLUSIONS

Taken together, these data illustrate that the pharmacologic inhibition of elevated PAI-1 restores the early impairments in skeletal muscle repair observed in type 1 diabetes and suggests that early interventional studies targeting PAI-1 may be warranted to ensure optimal growth and repair in adolescent diabetic skeletal muscle.

Impaired growth and force production in skeletal muscles of young partially pancreatectomized rats: a model of adolescent type 1 diabetic myopathy?

This present study investigated the temporal effects of type 1 diabetes mellitus (T1DM) on adolescent skeletal muscle growth, morphology and contractile properties using a 90% partial pancreatecomy (Px) model of the disease. Four week-old male Sprague-Dawley rats were randomly assigned to Px (n = 25) or Sham (n = 24) surgery groups and euthanized at 4 or 8 weeks following an in situ assessment of muscle force production. Compared to Shams, Px were hyperglycemic (>15 mM) and displayed attenuated body mass gains by days 2 and 4, respectively (both P<0.05). Absolute maximal force production of the gastrocnemius plantaris soleus complex (GPS) was 30% and 50% lower in Px vs. Shams at 4 and 8 weeks, respectively (P<0.01). GP mass was 35% lower in Px vs Shams at 4 weeks (1.24±0.06 g vs. 1.93±0.03 g, P<0.05) and 45% lower at 8 weeks (1.57±0.12 vs. 2.80±0.06, P<0.05). GP fiber area was 15–20% lower in Px vs. Shams at 4 weeks in all fiber types. At 8 weeks, GP type I and II fiber areas were ∼25% and 40% less, respectively, in Px vs. Shams (group by fiber type interactions, P<0.05). Phosphorylation states of 4E-BP1 and S6K1 following leucine gavage increased 2.0- and 3.5-fold, respectively, in Shams but not in Px. Px rats also had impaired rates of muscle protein synthesis in the basal state and in response to gavage. Taken together, these data indicate that exposure of growing skeletal muscle to uncontrolled T1DM significantly impairs muscle growth and function largely as a result of impaired protein synthesis in type II fibers.

Are human and mouse satellite cells really the same?

Satellite cells are quiescent cells located under the basal lamina of skeletal muscle fibers that contribute to muscle growth, maintenance, repair, and regeneration. Mouse satellite cells have been shown to be muscle stem cells that are able to regenerate muscle fibers and self-renew. As human skeletal muscle is also able to regenerate following injury, we assume that the human satellite cell is, like its murine equivalent, a muscle stem cell. In this review, we compare human and mouse satellite cells and highlight their similarities and differences. We discuss gaps in our knowledge of human satellite cells, compared with that of mouse satellite cells, and suggest ways in which we may advance studies on human satellite cells, particularly by finding new markers and attempting to re-create the human satellite cell niche in vitro.

Myofibrillar protein and gene expression in acute quadriplegic myopathy

The dramatic muscle wasting, preferential loss of myosin and impaired muscle function in intensive care unit (ICU) patients with acute quadriplegic myopathy (AQM) have traditionally been suggested to be the result of proteolysis via specific proteolytic pathways. In this study we aim to investigate the mechanisms underlying the preferential loss of thick vs. thin filament proteins and the reassembly of the sarcomere during the recovery process in muscle samples from ICU patients with AQM. Quantitative and qualitative analyses of myofibrillar protein and mRNA expression were analyzed using SDS-PAGE, confocal microscopy, histochemistry and real-time PCR. The present results demonstrate that the transcriptional regulation of myofibrillar protein synthesis plays an important role in the loss of contractile proteins, as well as the recovery of protein levels during clinical improvement, myosin in particular, presumably in concert with proteolytic pathways, but the mechanisms are specific to the different thick and thin filament proteins studied.

Diabetic myopathy differs between Ins2Akita+/- and streptozotocin-induced Type 1 diabetic models

Mechanistic studies examining the effects of Type 1 diabetes mellitus (T1DM) on skeletal muscle have largely relied on streptozotocin-induced diabetic (STZ) rodents. Unfortunately, characterization of diabetic myopathy in this model is confounded by the effects of streptozotocin on skeletal muscle independent of the diabetic phenotype. Here we define adolescent diabetic myopathy in a novel, genetic model of T1DM, Ins2(Akita+/-) mice, and contrast these findings with STZ mice. Eight weeks of diabetes resulted in significantly reduced gastrocnemius-plantaris-soleus mass (control: 0.16 +/- 0.005 g; Ins2(Akita+/-): 0.12 +/- 0.003 g; STZ: 0.12 +/- 0.01g) and IIB/D fiber area in Ins2(Akita+/-) (1,294 +/- 94 microm(2)) and STZ (1,768 +/- 163 microm(2)) compared with control (2,241 +/- 144 microm(2)). Conversely, STZ type I fibers (1,535 +/- 165 microm(2)) were significantly larger than Ins2(Akita+/-) (915 +/- 76 microm(2)) but not control (1,152 +/- 86 microm(2)). Intramyocellular lipid increased in STZ (122.9 +/- 3.6% of control) but not Ins2(Akita+/-) likely resultant from depressed citrate synthase (control: 6.2 +/- 1.2 micromol.s(-1).mg(-1); Ins2(Akita+/-): 5.2 +/- 0.8 micromol.s(-1).mg(-1); STZ: 2.8 +/- 0.5 micromol.s(-1).mg(-1)) and 3-beta-hydroxyacyl coenzyme-A dehydrogenase (control: 4.2 +/- 0.6 nmol.s(-1).mg(-1); Ins2(Akita+/-): 5.0 +/- 0.6 nmol.s(-1).mg(-1); STZ: 2.7 +/- 0.6 nmol.s(-1).mg(-1)) enzyme activity in STZ muscle. In situ muscle stimulation revealed lower absolute peak tetanic force in Ins2(Akita+/-) (70.2 +/- 8.2% of control) while STZ exhibited an insignificant decrease (87.6 +/- 7.9% of control). Corrected for muscle mass, no force loss was observed in Ins2(Akita+/-), while STZ was significantly elevated vs. control and Ins2(Akita+/-). These results demonstrate that atrophy and specific fiber-type loss in Ins2(Akita+/-) muscle did not affect contractile properties (relative to muscle mass). Furthermore, we demonstrate distinctive contractile, metabolic, and phenotypic properties in STZ vs. Ins2(Akita+/-) diabetic muscle despite similarity in hyperglycemia/hypoinsulinemia, raising concerns of our current state of knowledge regarding the effects of T1DM on skeletal muscle.

Essential role of satellite cells in the growth of rat soleus muscle fibers

Effects of gravitational loading or unloading on the growth-associated increase in the cross-sectional area and length of fibers, as well as the total fiber number, in soleus muscle were studied in rats. Furthermore, the roles of satellite cells and myonuclei in growth of these properties were also investigated. The hindlimb unloading by tail suspension was performed in newborn rats from postnatal day 4 to month 3 with or without 3-mo reloading. The morphological properties were measured in whole muscle and/or single fibers sampled from tendon to tendon. Growth-associated increases of soleus weight and fiber cross-sectional area in the unloaded group were approximately 68% and 69% less than the age-matched controls. However, the increases of number and length of fibers were not influenced by unloading. Growth-related increases of the number of quiescent satellite cells and myonuclei were inhibited by unloading. And the growth-related decrease of mitotically active satellite cells, seen even in controls (20%, P > 0.05), was also stimulated (80%). The increase of myonuclei during 3-mo unloading was only 40 times vs. 92 times in controls. Inhibited increase of myonuclear number was not related to apoptosis. The size of myonuclear domain in the unloaded group was less and that of single nuclei, which was decreased by growth, was larger than controls. However, all of these parameters, inhibited by unloading, were increased toward the control levels generally by reloading. It is suggested that the satellite cell-related stimulation in response to gravitational loading plays an essential role in the cross-sectional growth of soleus muscle fibers.

Lack of galectin-1 results in defects in myoblast fusion and muscle regeneration

Galectin-1 has been implicated in the development of skeletal muscle, being maximally expressed at the time of myofiber formation. Furthermore, in the presence of exogenous galectin-1, mononuclear myoblasts show increased fusion in vitro. In the current study, we have used the galectin-1 null mouse to elucidate the role of galectin-1 in skeletal muscle development and regeneration. Myoblasts derived from the galectin-1 mutant showed a reduced ability to fuse in vitro. In galectin-1 null mutants, there was evidence of a delay in muscle fiber development at the neonatal stage and muscle fiber diameter was reduced when compared with wild-type at the adult stage. Muscle regeneration was also compromised in the galectin-1 mutant with the process being delayed and a reduced fiber size being maintained. These results, therefore, show a definitive role for galectin-1 in fusion of myoblasts both in vitro, in vivo, and in regeneration after recovery from induced injury.

Differential effects of post-natal development, animal strain and long term recovery on the restoration of neuromuscular function after neuromyotoxic injury in rat

We have analysed the effect of long term recovery, post-natal development and animal strain on the extent of restoration of neuromuscular function after neuromyotoxic injury in the rat (Rattus norvegicus). Muscle isometric contractile properties of soleus muscle in response to nerve stimulation were measured in situ in snake venom injured muscles and compared to contralateral uninjured muscles. We show here that neuromuscular function was not fully recovered until 24 weeks after injury in young adult (2-3 month old) Wistar rats. Moreover, the level of functional recovery 3 weeks after injury induced in juvenile rats (1 month old) was not globally different from that in younger adult, adult (10 month old) and older adult (24 month old) Wistar rats. Furthermore, the level of recovery of some contractile parameters differed between Wistar and Sprague-Dawley strains 3 weeks after injury. In conclusion, a very long time (>12 weeks) is required for full neuromuscular recovery following neuromyotoxic injury of young adult rats. Moreover, neuromuscular recovery during post-natal development is not markedly different from that during adult stage in the Wistar rat strain. Finally, some rat strain differences are observed in the recovery after injury of young adult rats.

Skeletal muscle atrophy increases cell proliferation in mice gastrocnemius during the first week of hindlimb suspension

The comprehension of the cellular mechanisms underlying skeletal muscle atrophy has been the aim of several experimental studies. However, the majority of them focused on alterations of the myocytes induced by different experimental conditions yet disregarding the contribution of other cells such as endothelial cells and fibroblasts. In this sense, 70 Charles River CD1 male mice were randomly assigned to seven groups (n=10 per group): control and 6, 12, 24, 48, 72 h and 1 week with respect to the period of hindlimb suspension. Forty-eight hours before sacrifice, the animals were injected with bromodeoxyuridine (BrdU) in order to identify proliferating cells. Immunohistochemistry and south-western blotting techniques were used to evaluate across the whole gastrocnemius muscle BrdU incorporation into the different proliferating cells. The contribution of the apoptotic response was also measured in order to ascertain whether the balance between cell survival and death was preserved. The results observed during 1 week of unloading-induced atrophy evidenced an intense peak of proliferating activity only after 6 h, mainly due to the duplication of satellite cells. Consequently to this unexpected activation of satellite cells, the addition of nuclei to the fibre syncytium was recognized at 12 h of unloading. After 48 h of weightlessness, the proliferating activity observed was largely due to an interstitial fibrosis. According to the apoptotic index profile observed during the analysed unloading period, this general proliferative activity was balanced by apoptosis, which strongly suggests the existence of a regulatory feedback response between anabolic and catabolic events in unloading-induced skeletal muscle atrophy.

Mechanical load-dependent regulation of satellite cell and fiber size in rat soleus muscle

The effects of mechanical unloading and reloading on the properties of rat soleus muscle fibers were investigated in male Wistar Hannover rats. Satellite cells in the fibers of control rats were distributed evenly throughout the fiber length. After 16 days of hindlimb unloading, the number of satellite cells in the central, but not the proximal or distal, region of the fiber was decreased. The number of satellite cells in the central region gradually increased during the 16-day period of reloading. The mean sarcomere length in the central region of the fibers was passively shortened during unloading due to the plantarflexed position at the ankle joint: sarcomere length was maintained at <2.1 μm, which is a critical length for tension development. Myonuclear number and domain size, fiber cross-sectional area, and the total number of mitotically active and quiescent satellite cells of whole muscle fibers were lower than control fibers after 16 days of unloading. These values then returned to control values after 16 days of reloading. These results suggest that satellite cells play an important role in the regulation of muscle fiber properties. The data also indicate that the satellite cell-related regulation of muscle fiber properties is dependent on the level of mechanical loading, which, in turn, is influenced by the mean sarcomere length. However, it is still unclear why the region-specific responses, which were obvious in satellite cells, were not induced in myonuclear number and fiber cross-sectional area.

Early post-hatch fasting induces satellite cell self-renewal

Early post-hatch satellite cell kinetics are an important aspect of muscle development, and understanding the interplay between fasting and muscle development will lead to improvements in muscle mass following an illness, and optimal meat production. The objective of this experiment was to test the influence of immediate post-hatch fasting on satellite cells in the poult. Male Nicholas poults (Meleagris gallopavo) were placed into two treatments: a fed treatment with immediate access to feed and water upon placement and a fasted treatment without access to feed and water for the first three days post-hatch. 5-bromo-2'-deoxyuridine (BrdU) was injected intra-abdominally in all poults to label mitotically active satellite cells. The pectoralis thoracicus muscle was harvested two hours following the BrdU injection. Immunohistochemistry for BrdU, Pax7, Bcl-2, Pax7 with BrdU, and determining myofiber cross-sectional area along with computer-based image analysis was used to study muscle development. Fed poults had higher body masses throughout the experiment (P< or =0.01), and they had higher pectoralis thoracicus muscle mass (P< or =0.01) at ten days of age than the fasted poults. Fed poults had higher satellite cell mitotic activity at three days and four days of age (P< or =0.01) compared to the fasted poults. However, Pax7 labeling index was higher in the fasted poults (P< or =0.01) at three days, four days, and five days post-hatch than the fed group. Similarly Bcl-2 labeling was higher in the fasted than in the fed group at three days post-hatch. Therefore, fasting depleted proliferating satellite cells indicated by the lower BrdU labeling in the fasted poults compared to the fed poults, and conserved the satellite cell proliferative reserve indicated by the higher level of Pax7 labeling for the fasted poults compared to the fed poults.

Stretch-induced myoblast proliferation is dependent on the COX2 pathway

Skeletal muscle increases in size due to weight bearing loads or passive stretch. This growth response is dependent in part upon myoblast proliferation. Although skeletal muscles are responsive to mechanical forces, the effect on myoblast proliferation remains unknown. To investigate the effects of mechanical stretch on myoblast proliferation, primary myoblasts isolated from Balb/c mice were subjected to 25% cyclical uniaxial stretch for 5 h at 0.5 Hz. Stretch stimulated myoblast proliferation by 32% and increased cell number by 41% 24 and 48 h after stretch, respectively. COX2 mRNA increased 3.5-fold immediately poststretch. Prostaglandin E2 and F2alpha increased 2.4- and 1.6-fold 6 h after stretch, respectively. Because COX2 has been implicated in regulating muscle growth and regeneration, we hypothesized that stretched myoblasts may proliferate via a COX2-dependent mechanism. We employed two different models to disrupt COX2 activity: (1) treatment with a COX2-selective drug, and (2) transgenic mice null for COX2. Treating myoblasts with a COX2-specific inhibitor blocked stretch-induced proliferation. Likewise, stretched COX2-/- myoblasts failed to proliferate compared to controls. However, supplementing stretched, COX2-/- myoblasts with prostaglandin E2 or fluprostenol increased proliferation. These data suggest that the COX2 pathway is critical for myoblast proliferation in response to stretch.

The effect of early nutrition on satellite cell dynamics in the young turkey

Early posthatch satellite cell mitotic activity is an important aspect of muscle development. An understanding of the interplay between nutrition and satellite cell mitotic activity will lead to more efficient meat production. The objective of this study was to test the influence of the leucine metabolite, beta-hydroxy beta-methylbutyrate (HMB), and feed deprivation on muscle development in the early posthatch poult. Male Nicholas poults were placed on 1 of 4 treatments: immediately fed a starter diet with 0.1% HMB (IF-HMB), immediately fed a starter diet containing 0.1% Solka-Floc for a control (IF-No HMB), feed and water withheld for 48 h immediately posthatch and then fed the HMB diet (WF-HMB), and feed and water withheld for 48 h immediately posthatch and then fed the control starter diet (WF-No HMB). 5-bromo-2'-deoxyuridine (BrdU) was injected intra-abdominally into all poults to label mitotically active satellite cells. The pectoralis thoracicus was harvested 2 h after the BrdU injection. Immunohistochemistry for BrdU, Pax7, and laminin along with computer-based image analysis was used to study muscle development. IF-HMB poults had higher body weights (P < 0.01) at 48 h and 1 wk of age and had higher satellite cell mitotic activity at 48 h of age (P < 0.01) compared with the IF-No HMB and WF poults. Therefore, dietary supplementation of HMB may have an anabolic effect on early posthatch muscle.

Skeletal muscle atrophy leads to loss and dysfunction of muscle precursor cells

Atrophy of skeletal muscle leads to decreases in myofiber size and nuclear number; however, the effects of atrophic conditions on muscle precursor cells (MPC) are largely unknown. MPC lie outside myofibers and represent the main source of additional myonuclei necessary for muscle growth and repair. In the present study, we examined the properties of MPC after hindlimb suspension (HS)-induced atrophy and subsequent recovery of the mouse hindlimb muscles. We demonstrated that the number of MPC in atrophied muscles was decreased. RT-PCR analysis of cells isolated from atrophied muscles indicated that several mRNA characteristic of the myogenic program in MPC were absent. Cells isolated from atrophied muscles failed to properly proliferate and undergo differentiation into multinucleated myotubes. Thus atrophy led to a decrease in MPC and caused dysfunction in those MPC that remained. Upon regrowth of the atrophied muscles, these deleterious effects were reversed. Our data suggest that preventing loss or dysfunction of MPC may be a new pharmacological target during muscle atrophy.

Regrowth of skeletal muscle atrophied from inactivity

The current state of knowledge regarding regrowth of skeletal muscle after inactivity-induced atrophy is reviewed. Muscle regrowth is incomplete after hindlimb suspension in juvenile rats and after limb immobilization in old animals. The process of regrowth from immobilization-induced atrophy likely involves the reversal of directional changes in molecules producing muscle loss while initiating anabolic processes for regrowth of muscle mass. Unfortunately, the molecular mechanisms responsible for successful, or failed, muscle regrowth are not well understood. The purpose of the review is to provide current knowledge about the biology of muscle regrowth from inactivity-induced atrophy.

Satellite cell proliferation, reinnervation, and revascularization in human free microvascular muscle flaps

BACKGROUND

Satellite cell proliferation, reinnervation, and revascularization were studied in human nonreinnervated free microvascular muscle flaps to characterize mechanisms of muscle regeneration after flap surgery.

MATERIALS AND METHODS

Patient biopsies (n = 19) were taken at operation and five timepoints up to 9 months after operation, and corresponding clinical data were obtained. Immunohistochemistry for Ki-67 was used to detect proliferating satellite cells, CD-31 to identify endothelial cells, and S-100 and PGP 9.5 proteins to detect reinnervation.

RESULTS

Two weeks after operation, the expression of PGP 9.5 and S-100 had virtually disappeared in all larger nerve fibers and half of smaller nerve fibers. By 6 months, however, a strong expression of PGP 9.5 and S-100 had reappeared in larger nerve fibers in three of four flaps, suggesting that reinnervation had taken place. The number of mitotic satellite cells already peaked at 2 weeks, indicating onset of muscle regeneration. The number of intramuscular capillaries first increased but later decreased to lower than original level. Flaps with more muscle volume showed more reinnervation and satellite cell mitotic activity. In cases of a delay occurring in reconstructive surgery, a low level of reinnervation was seen.

CONCLUSION

Three patients of four showed spontaneous muscle reinnervation in microvascular free flaps with satellite cell activation followed by restored morphology. Late reconstruction and obesity lead to poor reinnervation, placing emphasis on timing of surgery and patient selection.

Contraction-induced injury run amok: an introduction

Skeletal muscle has an amazing capacity to adapt to increased levels of physical activity. Adaptation is often preceded by contraction-induced injury. In most cases, the damage is repaired quickly, the muscle adapts, and becomes stronger and less fatigable. Diseased or deconditioned muscle is an exception; the response to increased functional demand, and the associated injury can be incomplete or even maladaptive. When and why is an adaptive response limited? This question will be addressed in the symposium papers following this brief introduction. The papers will discuss cellular, molecular, and immunological mechanisms that may be involved in impaired muscle adaptation.

Myonuclear apoptosis occurs during early posthatch starvation

Apoptosis is a naturally occurring process; it is important for the final shape and size of developing tissues, and it is characterized by some morphological features such as plasma membrane blebbing, nuclear breakdown, chromosomal fragmentation and apoptotic bodies followed by phagocytosis. The objective of the study was to evaluate the occurrence of apoptosis in chickens immediately posthatch under fed and starved conditions. Male broiler chickens were or were not provided feed for the first 3 days posthatch. Chickens were killed immediately after hatch, at 1 day of age, at 2 days of age and at 3 days of age. The Pectoralis thoracicus was removed, fixed, dehydrated, cleared and embedded in paraffin. Muscle sections were labeled using terminal deoxynucleotidyl transferase (TdT)-mediated dUTP Nick-End Labeling (TUNEL) for detection of apoptotic nuclei. Body weights were lower (P<0.05) in the starved compared to the fed group at 2 and 3 days posthatch. Myofiber cross-sectional area was only smaller (P<0.05) in the starved compared to the fed birds at 3 days posthatch. TUNEL-positive nuclei were present at all days for the fed and starved groups. The proportion of TUNEL-positive nuclei was higher (P<0.05) for the starved group at day 2 and day 3 posthatch compared to the fed group at 3 days posthatch. Apoptosis is a mechanism that contributes to the smaller myofiber size observed at 3 days posthatch.

Effect of artificial rearing on the contractile properties and myosin heavy chain isoforms of developing rat tongue musculature

This study's purpose was to examine the influence of an altered activity level, via artificial rearing, on the contractile properties, myosin heavy chain phenotypes (MHC), and muscle fiber sizes of the developing rat tongue retractor musculature. Artificially reared rat pups were fed through a gastric cannula, eliminating nutritive suckling from postnatal day 4 to postnatal day 14. Rat pups were observed immediately following artificial rearing (postnatal day 14) and after a 1-mo resumption of function (postnatal day 42). The contractile characteristics of the tongue retractor musculature were measured in response to stimulation of the hypoglossal nerve. At postnatal day 14, artificially reared rat pups demonstrated significantly longer twitch half-decay times, lower fusion frequencies, and a marked decrease in fatigue resistance. These contractile speed and fatigue characteristics were fully recovered following a 1-mo resumption of function. MHC phenotypes of the styloglossus muscle (a tongue retractor) were determined by gel electrophoresis. At postnatal day 14, artificial rearing had not altered the MHC phenotype or muscle fiber sizes of the styloglossus muscle. However, following a 1-mo resumption of function artificially reared rat pups demonstrated a small but significant increase in MHCIIa expression and decrease in MHCIIb expression compared with dam-reared rats. These results support artificial rearing as a useful model for altering the activity level of the tongue and suggest that normal suckling behavior is necessary for the normal postnatal development of the tongue retractor musculature. This may also be the case for premature infants necessarily fed artificially.

Glyceraldehyde-3-phosphate dehydrogenase expression varies with age and nutrition status

OBJECTIVE

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a key enzyme in the glycolytic pathway, and it is a popular internal standard for northern blot analysis. We examined GAPDH expression early in life when feed is either provided or not provided to animals.

METHODS

Male broiler chickens were provided a standard starter diet plus Oasis nutritional supplement (fed group; Novus International, St. Louis, MO, USA) or no feed (starved group) for the first 3 d posthatch. Subsequently, the standard starter diet was provided to all chickens between 3 and 7 d posthatch. RNA was extracted from the pectoralis thoracicus, and GAPDH expression was evaluated with quantitative northern analysis.

RESULTS

GAPDH expression was significantly (P < 0.05) higher in the fed than in the starved group at 3 d posthatch, suggesting that nutritional manipulations can alter GAPDH transcription. Similarly, GAPDH mRNA levels were significantly (P < 0.05) higher at 7 d posthatch compared with all younger animals, suggesting that GAPDH is developmentally upregulated with advancing age.

CONCLUSION

GAPDH expression changes with age and nutrition status in the early posthatch chick, suggesting that GAPDH is not a proper internal standard for muscle studies using quantitative northern analysis.

Protein turnover in atrophying muscle: from nutritional intervention to microarray expression analysis

PURPOSE OF REVIEW

In response to decreased usage, skeletal muscle undergoes adaptive reductive remodeling due to the decrease in tension on the weight bearing components of the musculo-skeletal system. This response occurs with uncomplicated disuse (e.g. bed rest, space flight), as a secondary consequence of several widely prevalent chronic diseases for which activity is reduced (e.g. chronic obstructive pulmonary disease and chronic heart failure) and is part of the aging process. The problem is therefore one of considerable clinical importance.

RECENT FINDINGS

The impaired function and exercise intolerance is related more to the associated muscle wasting rather than to the specific organ system primarily impacted by the disease. Progress has continued in describing the use of anabolic drugs and dietary manipulation. The major advance in the field has been: (i) the discovery of the atrogin-1 gene and (ii) the application of microarray expression analysis and proteomics with the objectives of obtaining comprehensive understanding of the pathways changed with disuse atrophy.

SUMMARY

Disuse atrophy is a common clinical problem. There is a need for therapeutic interventions that do not involve exercise. A better understanding of the changes, particularly at the molecular level, could indicate hitherto unsuspected sites for nutritional and pharmacological intervention.

The effect of early posthatch nutrition on satellite cell mitotic activity

Myofiber growth is dependent upon the contribution of new nuclei from the mitotically active satellite cell population. The objective of this study was to examine satellite cell mitotic activity in conjunction with different nutritional paradigms during the early posthatch period. Turkey poults were provided a standard turkey starter diet; the starter diet top-dressed with a hydrated low-fat, highly digestible protein and carbohydrate nutritional hatchling supplement, Oasis; the starter diet top-dressed with Solka-floc dyed green; or no food for the first 3 d posthatch. All birds were fed a standard starter diet during the experimental period. 5-Bromo-2'-deoxyuridine (BrdU) was continuously infused into all treatments (n = 5 all groups) between hatch and 3 d of age. A second group of identically treated poults housed in separate pens (n = 3 to 5) was continuously infused with BrdU between 2 and 9 d of age. Mitotically active satellite cells were identified in the pectoralis thoracicus and quantitated using BrdU immunohistochemistry in combination with computer-based image analysis. Satellite cell mitotic activity was significantly higher (P < or = 0.05) in the birds fed a standard starter diet compared to all other treatments at 3 d posthatch. However, there were no (P > or = 0.05) differences in satellite cell mitotic activity among treatments at 9 d posthatch. The results of the current study suggest that any improvements in meat yield through early nutritional supplementation do not appear to occur through a satellite cell pathway and that there is no compensatory response in the satellite cell population following refeeding after early posthatch starvation.

The effect of early posthatch starvation on calpain mRNA levels

The calpain system is a family of calcium activated proteases that degrade myofibrillar protein. Male broiler chickens (Ross) were provided a standard starter diet top-dressed with Oasis((R)) nutritional supplement (fed; Novus International, St. Louis, MO, USA), or they were not provided any feed (starved) for the first 3 days posthatch. Subsequently, the standard starter diet was provided to all chickens between 3 and 7 days posthatch. RNA was extracted from the Pectoralis thoracicus, and skeletal muscle-specific n-calpain-1 (p94) calpain, mu-calpain, and m-calpain expression was evaluated using quantitative Northern analysis. Early posthatch starvation did not (P>0.05) affect calpain mRNA levels on each day examined. Similarly, there were no (P>0.05) changes in mu-calpain or m-calpain mRNA levels between 0 and 7 days posthatch in fed birds. However, p94 calpain mRNA levels were significantly (P<0.05) lower at 7 days posthatch compared to 0 or 2 days posthatch. Therefore, in the early posthatch chicken, it appears that the calpain system may not be affected by the presence of oral nutrition, and that there is an age-related downregulation of p94 calpain mRNA expression.

Effects of botulinum toxin A injection and exercise on the growth of juvenile rat gastrocnemius muscle

Botulinum toxin A (Btx) injections and supervised exercise are often used concurrently to treat calf muscle spasticity in children. This study has analyzed the early effects of Btx-induced paralysis and increased activity by voluntary wheel running on cell growth-related processes in juvenile rat gastrocnemius muscle. Btx injection at 29 days of age prevented the normal increases in wet mass (50%) and fiber cross-sectional area (34%) seen by 36 days of age in control rats. Btx-injected vs. contralateral muscles had 22% fewer myonuclei per fiber length but greater than twofold the number of MyoD-positive nuclei at 36 days of age. The accretion of 5-bromo-2'-deoxyuridine-labeled newly produced myonuclei did not differ between limbs. Voluntary exercise during the 7 days increased the mass (18%) and fiber size (23%) of Btx-injected and contralateral muscles but did not affect any other variable. Thus Btx injection and exercise had early effects on muscle and fiber size without consistently associated changes in myonuclear production or number. This suggests the presence of noncontractile activity-dependent, growth-promoting cytoplasmic events in juvenile muscle.

Skeletal muscle damage and recovery

Muscular strength is essential for recovery after an acute illness. Disuse atrophy of muscle begins within 4 hours of the start of bed rest resulting in decreases in muscle mass, muscle cell diameter, and the number of muscle fibers. Strenuous exercise of atrophic muscle can lead to muscle damage including sarcolemmal disruption, distortion of the myofibrils' contractile components, and cytoskeletal damage. Assessment of skeletal muscle for disuse atrophy is done clinically at the bedside through strength assessment. Examination of the muscle itself can be conducted through the use of nuclear magnetic resonance imaging, whereas muscle strength can be quantified with a computerized dynamometer. Biochemical markers, including creatine kinase and troponin, also are available for the assessment of skeletal muscle damage. Activity management in the critical care environment focuses on an individualized plan, developed in cooperation with the recovering patient, with the goal of preserving and improving atrophic skeletal muscle.

Early posthatch starvation induces myonuclear apoptosis in chickens

The effect of early posthatch starvation on myonuclear apoptosis was examined in chickens. Male broiler chickens were or were not provided feed for the first 3-d posthatch. Subsequently, all chickens were provided feed for an additional 4-d posthatch. Chickens were killed at 3- and 7-d posthatch, and the pectoralis thoracicus was harvested, fixed and embedded in paraffin. Muscle sections were labeled with the terminal deoxynucleotidyl transferase histochemical staining technique to identify apoptotic nuclei. At 3- and 7-d posthatch, there was a significantly (P < 0.05) smaller myofiber cross-sectional area for the starved compared with the fed chickens. A larger proportion (P < 0.05) of apoptotic nuclei relative to total nuclei was observed in the starved compared to the fed chickens killed at 3-d posthatch, but the proportion of apoptotic nuclei relative to total nuclei did not differ (P > 0.05) between the starved and fed chickens killed at 7-d posthatch. It appears that apoptosis is a mechanism contributing to the smaller myofiber size observed when feed is not provided early posthatch.

Myogenic satellite cells: physiology to molecular biology

Adult skeletal muscle has a remarkable ability to regenerate following myotrauma. Because adult myofibers are terminally differentiated, the regeneration of skeletal muscle is largely dependent on a small population of resident cells termed satellite cells. Although this population of cells was identified 40 years ago, little is known regarding the molecular phenotype or regulation of the satellite cell. The use of cell culture techniques and transgenic animal models has improved our understanding of this unique cell population; however, the capacity and potential of these cells remain ill-defined. This review will highlight the origin and unique markers of the satellite cell population, the regulation by growth factors, and the response to physiological and pathological stimuli. We conclude by highlighting the potential therapeutic uses of satellite cells and identifying future research goals for the study of satellite cell biology.

Muscle regeneration during hindlimb unloading results in a reduction in muscle size after reloading

The hindlimb-unloading model was used to study the ability of muscle injured in a weightless environment to recover after reloading. Satellite cell mitotic activity and DNA unit size were determined in injured and intact soleus muscles from hindlimb-unloaded and age-matched weight-bearing rats at the conclusion of 28 days of hindlimb unloading, 2 wk after reloading, and 9 wk after reloading. The body weights of hindlimb-unloaded rats were significantly (P < 0.05) less than those of weight-bearing rats at the conclusion of hindlimb unloading, but they were the same (P > 0.05) as those of weight-bearing rats 2 and 9 wk after reloading. The soleus muscle weight, soleus muscle weight-to-body weight ratio, myofiber diameter, number of nuclei per millimeter, and DNA unit size were significantly (P < 0.05) smaller for the injured soleus muscles from hindlimb-unloaded rats than for the soleus muscles from weight-bearing rats at each recovery time. Satellite cell mitotic activity was significantly (P < 0.05) higher in the injured soleus muscles from hindlimb-unloaded rats than from weight-bearing rats 2 wk after reloading, but it was the same (P > 0.05) as in the injured soleus muscles from weight-bearing rats 9 wk after reloading. The injured soleus muscles from hindlimb-unloaded rats failed to achieve weight-bearing muscle size 9 wk after reloading, because incomplete compensation for the decrease in myonuclear accretion and DNA unit size expansion occurred during the unloading period.

Regulation of the growth of multinucleated muscle cells by an NFATC2-dependent pathway

The nuclear factor of activated T cells (NFAT) family of transcription factors regulates the development and differentiation of several tissue types. Here, we examine the role of NFATC2 in skeletal muscle by analyzing adult NFATC2(-/)- mice. These mice exhibit reduced muscle size due to a decrease in myofiber cross-sectional area, suggesting that growth is blunted. Muscle growth was examined during regeneration after injury, wherein NFATC2-null myofibers form normally but display impaired growth. The growth defect is intrinsic to muscle cells, since the lack of NFATC2 in primary muscle cultures results in reduced cell size and myonuclear number in myotubes. Retroviral-mediated expression of NFATC2 in the mutant cells rescues this cellular phenotype. Myonuclear number is similarly decreased in NFATC2(-/)- mice. Taken together, these results implicate a novel role for NFATC2 in skeletal muscle growth. We demonstrate that during growth of multinucleated muscle cells, myoblasts initially fuse to form myotubes with a limited number of nuclei and that subsequent nuclear addition and increases in myotube size are controlled by a molecular pathway regulated by NFATC2.

Exercise-enhanced satellite cell proliferation and new myonuclear accretion in rat skeletal muscle

The effects of increased functional loading on early cellular regenerative events after exercise-induced injury in adult skeletal muscle were examined with the use of in vivo labeling of replicating myofiber nuclei and immunocyto- and histochemical techniques. Satellite cell proliferation in the soleus (Sol) of nonexercised rats (0.4 +/- 0.2% of fibers) was unchanged after an initial bout of declined treadmill exercise but was elevated after two (1.0 +/- 0.2%, P < or = 0.01), but not four or seven, daily bouts of the same task. Myonuclei produced over the 7-day period comprised 0.9-1.9% of myonuclei in isolated fibers of Sol, tibialis anterior, and vastus intermedius of nonexercised rats. The accretion of new myonuclei was enhanced (P < or = 0.05) in Sol and vastus intermedius by the initial exercise followed by normal activity (to 3.1-3.4% of myonuclei) and more so by continued daily exercise (4.2-5.3%). Observed coincident with a lower incidence of histological fiber injury and unchanged fiber diameter and myonuclei per millimeter, the greater new myonuclear accretion induced by continued muscle loading may contribute to an enhanced fiber repair and regeneration after exercise-induced injury.

Ontogenetic, gravity-dependent development of rat soleus muscle

We tested the hypothesis that rat soleus muscle fiber growth and changes in myosin phenotype during the postnatal, preweaning period would be largely independent of weight bearing. The hindlimbs of one group of pups were unloaded intermittently from postnatal day 4 to day 21: the pups were isolated from the dam for 5 h during unloading and returned for nursing for 1 h. Control pups were either maintained with the dam as normal or put on an alternating feeding schedule as described above. The enlargement of mass (approximately 3 times), increase in myonuclear number (approximately 1.6 times) and myonuclear domain (approximately 2.6 times), and transformation toward a slow fiber phenotype (from 56 to 70% fibers expressing type I myosin heavy chain) observed in controls were inhibited by hindlimb unloading. These properties were normalized to control levels or higher within 1 mo of reambulation beginning immediately after the unloading period. Therefore, chronic unloading essentially stopped the ontogenetic developmental processes of 1) net increase in DNA available for transcription, 2) increase in amount of cytoplasm sustained by that DNA pool, and 3) normal transition of myosin isoforms that occur in some fibers from birth to weaning. It is concluded that normal ontogenetic development of a postural muscle is highly dependent on the gravitational environment even during the early postnatal period, when full weight-bearing activity is not routine.