The role of neural and mechanical influences in maintaining normal fast and slow muscle properties

Morphological examination of pelvic floor muscles in a rat model of vaginal delivery

OBJECTIVE

This study investigated morphological changes in the composition of the pelvic floor muscles, degree of atrophy, and urethral function in a rat of simulated birth trauma induced by vaginal distension (VD) model.

METHODS

Female Sprague-Dawley rats were classified into four groups: a sham group, and 1, 2, and 4 weeks post-VD (1 W, 2 W, and 4 W, respectively) groups. We measured the amplitude of urethral response to electrical stimulation (A-URE) to evaluate urethral function. After measuring the muscle wet weight of the pubococcygeus (Pcm) and iliococcygeus (Icm) muscles, histochemical staining was used to classify muscle fibers into Types I, IIa, and IIb, and the occupancy and cross-sectional area of each muscle fiber were determined.

RESULTS

There were 24 Sprague-Dawley rats used. A-URE was significantly lower in the 1 W group versus the other groups. Muscle wet weight was significantly lower in the VD groups versus the sham group for Pcm. The cross-sectional area of Type I Pcm and Icm was significantly lower in the VD groups versus the sham group. Type I muscle fiber composition in Pcm was significantly lower in the VD groups versus the sham groupand lowest in the 2 W group. Type I muscle fiber composition in Icm was significantly lower in the 2 and 4 W groups versus the sham group.

CONCLUSION

Muscle atrophy and changes in muscle composition in the pelvic floor muscles were observed even after improvements in urethral function. These results may provide insight into the pathogenesis of stress urinary incontinence after VD.

Prochlorperazine Withdraws the Delayed Onset Tonic Activity of Unloaded Rat Soleus Muscle: A Pilot Study

A gradual increase in rat soleus muscle electromyographic (EMG) activity is known to occur after 3–4 days of hindlimb suspension/unloading (HS). The physiological significance and mechanisms of such activity of motoneurons under unloading conditions are currently unclear. Since hyperactivity of motoneurons and muscle spasticity after spinal cord injury are associated with KCC2 downregulation, we hypothesized that a decrease in potassium (K⁺)/chloride (Cl⁻) co-transporter 2 (KCC2) in motoneurons would be responsible for an increase in soleus muscle EMG activity during HS. We aimed to investigate the effect of prochlorperazine (KCC2 activator) on the electrical activity of rat soleus muscle under HS. Wistar rats were divided into the following groups: (1) vivarium control (C), (2) 7-day HS group (7HS) and (3) 7-day HS group plus intraperitoneal injections of prochlorperazine (10 mg/kg, daily) (7HS + P). Expression of proteins in the motoneurons of the lumbar spinal cord was determined by Western blotting. An electromyogram of the rat soleus muscle was recorded using intramuscular electrodes. KCC2 content after 7-day HS significantly decreased by 34% relative to the control group. HS-induced decrease in KCC2 protein content was prevented by prochlorperazine administration. HS also induced a significant 80% decrease in KCC2 Ser940 phosphorylation; however prochlorperazine did not affect KCC2 phosphorylation. The treatment of the rats with prochlorperazine prevented a HS-induced increase in Na(+)/K(+)/(Cl−) co-transporter 1 (KCC2 antagonist) protein content. In parallel with the restoration of KCC2 content, prochlorperazine administration during HS partially prevented an increase in the soleus muscle tonic EMG activity. Thus, prochlorperazine administration during 7-day HS prevents a decrease in KCC2 protein expression in motoneurons and significantly reduces the level of HS-induced soleus muscle electrical activity.

Chest Wall Reconstruction in Male Poland Syndrome Patients with Endoscopic-Assisted Latissimus Dorsi Muscle Flap Transfer

BACKGROUND

Pectoralis major absence generates chest wall deformity and always requires surgical intervention. This study aimed to introduce a technique to reconstruct the chest wall for male Poland Syndrome patients with endoscopic latissimus dorsi muscle (ELDM) flap via a single transverse axillary incision and evaluate its safety and effectiveness.

METHODS

A prospective study was designed to recruit male Poland Syndrome candidates for ELDM chest reconstruction. By performing a short and hidden transaxillary incision, we created anterior chest wall pocket and transferred the latissimus dorsi muscle (LDM) flap to recontour the chest wall. Data for patient demographics, LDM flap dimension, operative time, and complications were collected. Upper extremity functional disabilities were evaluated by the disabilities of the arm, shoulder and hand (DASH) outcome questionnaire. Satisfaction with the outcome was measured by satisfaction with outcome subscale of the BREAST-Q questionnaire.

RESULTS

This study recruited 11 eligible patients to receive ELDM chest wall reconstruction. ELDM flap harvesting averagely consumed 79.9 minutes. Without significant complications, all patients recovered uneventfully. Chest wall anomalies of different severity were corrected safely and effectively. The score of DASH was 3.7±3.3 preoperatively and 4.0±3.7 postoperatively with no statistically significant difference. The score of satisfaction with the outcome was 84.8±14.3.

CONCLUSIONS

For chest wall anomalies of different severity in male Poland Syndrome patients, the ELDM technique provides a safe and efficient way to reconstruct the chest wall with a better aesthetic outcome, high satisfaction rate, and satisfactory upper limb function.

Early Lipid Raft-Related Changes: Interplay between Unilateral Denervation and Hindlimb Suspension

Muscle disuse and denervation leads to muscle atrophy, but underlying mechanisms can be different. Previously, we have found ceramide (Cer) accumulation and lipid raft disruption after acute hindlimb suspension (HS), a model of muscle disuse. Herein, using biochemical and fluorescent approaches the influence of unilateral denervation itself and in combination with short-term HS on membrane-related parameters of rat soleus muscle was studied. Denervation increased immunoexpression of sphingomyelinase and Cer in plasmalemmal regions, but decreased Cer content in the raft fraction and enhanced lipid raft integrity. Preliminary denervation suppressed (1) HS-induced Cer accumulation in plasmalemmal regions, shown for both nonraft and raft-fractions; (2) HS-mediated decrease in lipid raft integrity. Similar to denervation, inhibition of the sciatic nerve afferents with capsaicin itself increased Cer plasmalemmal immunoexpression, but attenuated the membrane-related effects of HS. Finally, both denervation and capsaicin treatment increased immunoexpression of proapoptotic protein Bax and inhibited HS-driven increase in antiapoptotic protein Bcl-2. Thus, denervation can increase lipid raft formation and attenuate HS-induced alterations probably due to decrease of Cer levels in the raft fraction. The effects of denervation could be at least partially caused by the loss of afferentation. The study points to the importance of motor and afferent inputs in control of Cer distribution and thereby stability of lipid rafts in the junctional and extrajunctional membranes of the muscle.

Testosterone suppression does not exacerbate disuse atrophy and impairs muscle recovery that is not rescued by high protein

Androgen deprivation therapy (ADT) decreases muscle mass, force, and physical activity levels, but it is unclear whether disuse atrophy and testosterone suppression are additive. Additionally, conflicting reports exist on load-mediated hypertrophy during ADT and if protein supplementation offsets these deficits. This study sought to determine the role of testosterone suppression and a high-protein diet on 1) immobilization-induced atrophy and 2) muscle regrowth during reloading. Eight-week-old male Fischer 344 rats underwent sham surgery (Sham), castration surgery (ORX), or ORX and a high-casein diet supplemented with branched-chain amino acids (BCAA) (ORX+CAS/AA) followed by 10 days of unilateral immobilization (IMM) and 0, 6, or 14 days of reloading. With IMM, body mass gains were ~8% greater than ORX and ORX+CAS/AA that increased to 15% during reloading (both P < 0.01). IMM reduced muscle mass by 11-34% (all P < 0.01) and extensor digitorum longus and soleus (SOL) force by 21% and 49% (both P < 0.01), respectively, with no group differences. During reloading, castration reduced gastrocnemius mass (~12%) at 6 days and SOL mass (~20%) and SOL force recovery (~46%) at 14 days relative to Sham (all P < 0.05). Specific force reduced castration deficits, indicating that muscle atrophy was a key contributor. IMM decreased SOL cross-sectional area by 30.3% (P < 0.001), with a trend for reduced regrowth in ORX and ORX+CAS/AA following reloading (P = 0.083). Castration did not exacerbate disuse atrophy but may impair recovery of muscle function, with no benefit from a CAS/AA diet during reloading. Examining functional outcomes in addition to muscle mass during dietary interventions provides novel insights into muscle regrowth during ADT.NEW & NOTEWORTHY Low testosterone levels during skeletal muscle disuse did not worsen declines in muscle mass and function, although hypogonadism may attenuate recovery during subsequent reloading. Diets high in casein did not improve outcomes during immobilization or reloading. Practical strategies are needed that do not compromise caloric intake yet provide effective protein doses to augment these adverse effects.

Human skeletal muscle fiber type percentage and area after reduced muscle use: A systematic review and meta-analysis

The main objective of this systematic review was to examine the effect of reduced muscle activity on the relative number of type 1 muscle fibers (%) in the human vastus lateralis muscle. Other objectives were changes in type 2A and 2X percentages and muscle fiber cross-sectional area. We conducted systematic literature searches in eight databases and included studies assessing type 1 fiber percentage visualized by ATPase or immunohistochemical staining before and after a period (≥14 days) of reduced muscle activity. The reduced muscle activity models were detraining, leg unloading, and bed rest. Forty-two studies comprising 451 participants were included. Effect sizes were calculated as the mean difference between baseline and follow-up and Generic Inverse Variance tests with random-effects models were used for the weighted summary effect size. Overall, the mean type 1 muscle fiber percentage was significantly reduced after interventions (-1.94%-points, 95% CI [-3.37, -0.51], P = .008), with no significant differences between intervention models (P = .86). Meta-regression showed no effect of study duration on type 1 fiber percentage (P = .98). Conversely, the overall type 2X fiber percentage increased after reduced muscle activity (P < .001). The CSA of the muscle fiber types decreased after the study period (all P-values < 0.001) with greater reductions in type 2 than type 1 fibers (P < .001). The result of this meta-analysis display that the type 1 muscle fiber percentage decrease as a result of reduced muscle activity, although the effect size is relatively small.

Muscle atrophy and regeneration associated with behavioural loss and recovery of function after sciatic nerve crush

AIM

To resolve timing and coordination of denervation atrophy and the re-innervation recovery process to discern correlations indicative of common programs governing these processes.

METHODS

Female Sprague-Dawley (SD) rats had a unilateral sciatic nerve crush. Based on longitudinal behavioural observations, the triceps surae muscle was analysed at different time points post-lesion.

RESULTS

Crush results in a loss of muscle function and mass (-30%) followed by a recovery to almost pre-lesion status at 30 days post-crush (dpc). There was no loss of fibres nor any significant change in the number of nuclei per fibre but a shift in fibres expressing myosins I and II that reverted back to control levels at 30 dpc. A residual was the persistence of hybrid fibres. Early on a CHNR -ε to -γ switch and a re-expression of embryonic MyHC showed as signs of denervation. Foxo1, Smad3, Fbxo32 and Trim63 transcripts were upregulated but not Myostatin, InhibinA and ActivinR2B. Combined this suggests that the mechanism instigating atrophy provides a selectivity of pathway(s) activated. The myogenic differentiation factors (MDFs: Myog, Myod1 and Myf6) were upregulated early on suggesting a role also in the initial atrophy. The regulation of these transcripts returned towards baseline at 30 dpc. The examined genes showed a strong baseline covariance in transcript levels which dissolved in the response to crush driven mainly by the MDFs. At 30 dpc the naïve expression pattern was re-established.

CONCLUSION

Peripheral nerve crush offers an excellent model to assess and interfere with muscle adaptions to denervation and re-innervation.

AMPK Mediates Muscle Mass Change But Not the Transition of Myosin Heavy Chain Isoforms during Unloading and Reloading of Skeletal Muscles in Mice

5′AMP-activated protein kinase (AMPK) plays an important role in the regulation of skeletal muscle mass and fiber-type distribution. However, it is unclear whether AMPK is involved in muscle mass change or transition of myosin heavy chain (MyHC) isoforms in response to unloading or increased loading. Here, we checked whether AMPK controls muscle mass change and transition of MyHC isoforms during unloading and reloading using mice expressing a skeletal-muscle-specific dominant-negative AMPKα1 (AMPK-DN). Fourteen days of hindlimb unloading reduced the soleus muscle weight in wild-type and AMPK-DN mice, but reduction in the muscle mass was partly attenuated in AMPK-DN mice. There was no difference in the regrown muscle weight between the mice after 7 days of reloading, and there was concomitantly reduced AMPKα2 activity, however it was higher in AMPK-DN mice after 14 days reloading. No difference was observed between the mice in relation to the levels of slow-type MyHC I, fast-type MyHC IIa/x, and MyHC IIb isoforms following unloading and reloading. The levels of 72-kDa heat-shock protein, which preserves muscle mass, increased in AMPK-DN-mice. Our results indicate that AMPK mediates the progress of atrophy during unloading and regrowth of atrophied muscles following reloading, but it does not influence the transition of MyHC isoforms.

Influence of acute exercise on renalase and its regulatory mechanism

AIMS

Renalase expression in the kidneys and liver is regulated by nuclear factor (NF)-κB, Sp1, and hypoxia-inducible factor (HIF)-1α. The dynamics of renalase expression in acute exercise, and its mechanism and physiological effects are unclear. We evaluated the effect of different exercise intensities on renalase expression and examined its mechanism and physiological effects.

MAIN METHODS

21 male Wistar rats ran for 30 min on a treadmill after resting for 15 min. The sedentary group rested on the treadmill while the exercise group ran for 30 min at 10 or 30 m/min. Skeletal muscles, the kidney, heart, liver, and blood samples were collected after exercise. The expression of renalase and phosphate IkB-α and Akt was measured by western blotting, while HIF-1α, Sp1, MuRF-1, and MAFbx were measured in the skeletal muscle by real-time RT-PCR.

KEY FINDINGS

Renalase expression in skeletal muscles increased after acute exercise, while its expression in the kidneys, heart, and liver decreased. NF-κB regulated renalase expression in the plantaris muscle and that of HIF-1α in the soleus muscle. Phosphate Akt in the plantaris muscle significantly increased in the 30 m/min group compared with that in the sedentary group. MuRF-1 in the plantaris did not change between these groups.

SIGNIFICANCE

Renalase expression in skeletal muscles increased after acute exercise but decreased in other tissues. This increase may be a response to exercise-induced oxidative stress. Furthermore, NF-κB in the plantaris muscle may mainly regulate renalase expression, and support a relationship with the cell protective effects of renalase.

Muscle Atrophy Induced by Mechanical Unloading: Mechanisms and Potential Countermeasures

Prolonged periods of skeletal muscle inactivity or mechanical unloading (bed rest, hindlimb unloading, immobilization, spaceflight and reduced step) can result in a significant loss of musculoskeletal mass, size and strength which ultimately lead to muscle atrophy. With advancement in understanding of the molecular and cellular mechanisms involved in disuse skeletal muscle atrophy, several different signaling pathways have been studied to understand their regulatory role in this process. However, substantial gaps exist in our understanding of the regulatory mechanisms involved, as well as their functional significance. This review aims to update the current state of knowledge and the underlying cellular mechanisms related to skeletal muscle loss during a variety of unloading conditions, both in humans and animals. Recent advancements in understanding of cellular and molecular mechanisms, including IGF1-Akt-mTOR, MuRF1/MAFbx, FOXO, and potential triggers of disuse atrophy, such as calcium overload and ROS overproduction, as well as their role in skeletal muscle protein adaptation to disuse is emphasized. We have also elaborated potential therapeutic countermeasures that have shown promising results in preventing and restoring disuse-induced muscle loss. Finally, identified are the key challenges in this field as well as some future prospectives.

The Molecular Mechanisms and Prevention Principles of Muscle Atrophy in Aging

Muscle atrophy in aging is characterized by progressive loss of muscle mass and function. Muscle mass is determined by the balance of synthesis and degradation of protein, which are regulated by several signaling pathways such as ubiquitin-proteasome system, autophagy-lysosome systems, oxidative stress, proinflammatory cytokines, hormones, and so on. Sufficient nutrition can enhance protein synthesis, while exercise can improve the quality of life in the elderly. This chapter will discuss the epidemiology, pathogenesis, as well as the current treatment for aging-induced muscular atrophy.

Tendon Length, Calf Muscle Atrophy, and Strength Deficit After Acute Achilles Tendon Rupture: Long-Term Follow-up of Patients in a Previous Study

BACKGROUND

In this prospective study, we used magnetic resonance imaging (MRI) to assess long-term Achilles tendon length, calf muscle volume, and muscle fatty degeneration after surgery for acute Achilles tendon rupture.

METHODS

From 1998 to 2001, 60 patients at our center underwent surgery for acute Achilles tendon rupture followed by early functional postoperative rehabilitation. Fifty-five patients were reexamined after a minimum duration of follow-up of 13 years (mean, 14 years), and 52 of them were included in the present study. Outcome measures included Achilles tendon length, calf muscle volume, and fatty degeneration measured with MRI of both the affected and the uninjured leg. The isokinetic plantar flexion strength of both calves was measured and was correlated with the structural findings.

RESULTS

The Achilles tendon was, on average, 12 mm (95% confidence interval [CI] = 8.6 to 15.6 mm; p < 0.001) longer (6% longer) in the affected leg than in the uninjured leg. The mean volumes of the soleus and medial and lateral gastrocnemius muscles were 63 cm (13%; p < 0.001), 30 cm (13%; p < 0.001), and 16 cm (11%; p < 0.001) lower in the affected leg than in the uninjured leg, whereas the mean volume of the flexor hallucis longus (FHL) was 5 cm (5%; p = 0.002) greater in the affected leg, indicating FHL compensatory hypertrophy. The median plantar flexion strength for the whole range of motion ranged from 12% to 18% less than that on the uninjured side. Finally, the side-to-side difference in Achilles tendon length correlated substantially with the strength deficit (ρ = 0.51, p < 0.001) and with medial gastrocnemius (ρ = 0.46, p = 0.001) and soleus (ρ = 0.42, p = 0.002) muscle atrophy.

CONCLUSIONS

Increased Achilles tendon length is associated with smaller calf muscle volumes and persistent plantar flexion strength deficits after surgical repair of Achilles tendon rupture. Strength deficits and muscle volume deficits are partly compensated for by FHL hypertrophy, but 11% to 13% deficits in soleus and gastrocnemius muscle volumes and 12% to 18% deficits in plantar flexion strength persist even after long-term follow-up.

LEVEL OF EVIDENCE

Therapeutic Level II. See Instructions for Authors for a complete description of levels of evidence.

Soleus Atrophy Is Common After the Nonsurgical Treatment of Acute Achilles Tendon Ruptures: A Randomized Clinical Trial Comparing Surgical and Nonsurgical Functional Treatments

BACKGROUND

It remains controversial whether nonsurgical or surgical treatment provides better calf muscle strength recovery after an acute Achilles tendon rupture (ATR). Recent evidence has suggested that surgery might surpass nonsurgical treatment in restoring strength after an ATR.

PURPOSE

To assess whether magnetic resonance imaging (MRI) findings could explain calf muscle strength deficits and the difference between nonsurgical and surgical treatments in restoring calf muscle strength.

STUDY DESIGN

Randomized controlled trial; Level of evidence, 1.

METHODS

From 2009 to 2013, 60 patients with acute ATRs were randomized to surgery or nonsurgical treatment with an identical rehabilitation protocol. The primary outcome measure was the volume of calf muscles assessed using MRI at 3 and 18 months. The secondary outcome measures included fatty degeneration of the calf muscles and length of the affected Achilles tendon. Additionally, isokinetic plantarflexion strength was measured in both legs.

RESULTS

At 3 months, the study groups showed no differences in muscle volumes or fatty degeneration. However, at 18 months, the mean differences between affected and healthy soleus muscle volumes were 83.2 cm³ (17.7%) after surgery and 115.5 cm³ (24.8%) after nonsurgical treatment (difference between means, 33.1 cm³; 95% CI, 1.3-65.0; P = .042). The study groups were not substantially different in the volumes or fatty degeneration of other muscles. From 3 to 18 months, compensatory hypertrophy was detected in the flexor hallucis longus (FHL) and deep flexors in both groups. In the nonsurgical treatment group, the mean difference between affected and healthy FHL muscle volumes was -9.3 cm³ (12%) and in the surgical treatment group was -8.4 cm³ (10%) ( P ≤ .001). At 18 months, Achilles tendons were, on average, 19 mm longer in patients treated nonsurgically compared with patients treated surgically ( P < .001). At 18 months, surgically treated patients demonstrated 10% to 18% greater strength results ( P = .037). Calf muscle isokinetic strength deficits for the entire range of ankle motion correlated with soleus atrophy (ρ = 0.449-0.611; P < .001).

CONCLUSION

Treating ATRs nonsurgically with a functional rehabilitation protocol resulted in greater soleus muscle atrophy compared with surgical treatment. The mean Achilles tendon length was 19 mm longer after nonsurgical treatment than after the surgical treatment of ATRs. These structural changes partly explained the 10% to 18% greater calf muscle strength observed in patients treated with surgery compared with those treated nonsurgically. Registration: NCT02012803 ( ClinicalTrials.gov identifier).

Heat shock transcription factor 1-associated expression of slow myosin heavy chain in mouse soleus muscle in response to unloading with or without reloading

AIM

The effects of heat shock transcription factor 1 (HSF1) deficiency on the fibre type composition and the expression level of nuclear factor of activated T cells (NFAT) family members (NFATc1, NFATc2, NFATc3 and NFATc4), phosphorylated glycogen synthase kinase 3α (p-GSK3α) and p-GSK3β, microRNA-208b (miR-208b), miR-499 and slow myosin heavy chain (MyHC) mRNAs (Myh7 and Myh7b) of antigravitational soleus muscle in response to unloading with or without reloading were investigated.

METHODS

HSF1-null and wild-type mice were subjected to continuous 2-week hindlimb suspension followed by 2- or 4-week ambulation recovery.

RESULTS

In wild-type mice, the relative population of slow type I fibres, the expression level of NFATc2, p-GSK3 (α and β), miR-208b, miR-499 and slow MyHC mRNAs (Myh7 and Myh7b) were all decreased with hindlimb suspension, but recovered after it. Significant interactions between train and time (the relative population of slow type I fibres; P = 0.01, the expression level of NFATc2; P = 0.001, p-GSKβ; P = 0.009, miR-208b; P = 0.002, miR-499; P = 0.04) suggested that these responses were suppressed in HSF1-null mice.

CONCLUSION

HSF1 may be a molecule in the regulation of the expression of slow MyHC as well as miR-208b, miR-499, NFATc2 and p-GSK3 (α and β) in mouse soleus muscle.

Morphological and molecular aspects of immobilization-induced muscle atrophy in rats at different stages of postnatal development: the role of autophagy

Muscle loss occurs following injury and immobilization in adulthood and childhood, which impairs the rehabilitation process; however, far fewer studies have been conducted analyzing atrophic response in infants. This work investigated first the morphological and molecular mechanisms involved in immobilization-induced atrophy in soleus muscles from rats at different stages of postnatal development [i.e., weanling (WR) and adult (AR) rats] and, second, the role of autophagy in regulating muscle plasticity during immobilization. Hindlimb immobilization for 10 days reduced muscle mass and fiber cross-sectional area, with more pronounced atrophy in WR, and induced slow-to-fast fiber switching. These effects were accompanied by a decrease in markers of protein synthesis and an increase in autophagy. The ubiquitin (Ub)-ligase MuRF1 and the ubiquitinated proteins were upregulated by immobilization in AR while the autolyzed form of μ-calpain was increased in WR. To further explore the role of autophagy in muscle abnormalities, AR were concomitantly immobilized and treated with colchicine, which blocks autophagosome-lysosome fusion. Colchicine-treated immobilized muscles had exacerbated atrophy and presented degenerative features. Despite Igf1/Akt signaling was downregulated in immobilized muscles from both age groups, Foxo1 and 4 phosphorylation was increased in WR. In the same group of animals, Foxo1 acetylation and Foxo1 and 4 content was increased and decreased, respectively. Our data show that muscle disorders induced by 10-day-immobilization occur in both age-dependent and -independent manners, an understanding that may optimize treatment outcomes in infants. We also provide further evidence that the strong inhibition of autophagy may be ineffective for treating muscle atrophy.

In Vivo Rodent Models of Skeletal Muscle Adaptation to Decreased Use

Skeletal muscle possesses plasticity and adaptability to external and internal physiological changes. Due to these characteristics, skeletal muscle shows dramatic changes depending on its response to stimuli such as physical activity, nutritional changes, disease status, and environmental changes. Modulation of the rate of protein synthesis/degradation plays an important role in atrophic responses. The purpose of this review is to describe different features of skeletal muscle adaptation with various models of deceased use. In this review, four models were addressed: immobilization, spinal cord transection, hindlimb unloading, and aging. Immobilization is a form of decreased use in which skeletal muscle shows electrical activity, tension development, and motion. These results differ by muscle group. Spinal cord transection was selected to simulate spinal cord injury. Similar to the immobilization model, dramatic atrophy occurs in addition to fiber type conversion in this model. Despite the fact that electromyography shows unremarkable changes in muscle after hindlimb unloading, decreased muscle mass and contractile force are observed. Lastly, aging significantly decreases the numbers of muscle fibers and motor units. Skeletal muscle responses to decreased use include decreased strength, decreased fiber numbers, and fiber type transformation. These four models demonstrated different changes in the skeletal muscle. This review elucidates the different skeletal muscle adaptations in these four decreased use animal models and encourages further studies.

Differences in histone modifications between slow- and fast-twitch muscle of adult rats and following overload, denervation, or valproic acid administration

Numerous studies have reported alterations in skeletal muscle properties and phenotypes in response to various stimuli such as exercise, unloading, and gene mutation. However, a shift in muscle fiber phenotype from fast twitch to slow twitch is not completely induced by stimuli. This limitation is hypothesized to result from the epigenetic differences between muscle types. The main purpose of the present study was to identify the differences in histone modification for the plantaris (fast) and soleus (slow) muscles of adult rats. Genome-wide analysis by chromatin immunoprecipitation followed by DNA sequencing revealed that trimethylation at lysine 4 and acetylation of histone 3, which occurs at transcriptionally active gene loci, was less prevalent in the genes specific to the slow-twitch soleus muscle. Conversely, gene loci specific to the fast-twitch plantaris muscle were associated with the aforementioned histone modifications. We also found that upregulation of slow genes in the plantaris muscle, which are related to enhanced muscular activity, is not associated with activating histone modifications. Furthermore, silencing of muscle activity by denervation caused the displacement of acetylated histone and RNA polymerase II (Pol II) in 5' ends of genes in plantaris, but minor effects were observed in soleus. Increased recruitment of Pol II induced by forced acetylation of histone was also suppressed in valproic acid-treated soleus. Our present data indicate that the slow-twitch soleus muscle has a unique set of histone modifications, which may relate to the preservation of the genetic backbone against physiological stimuli.

Disuse-induced muscle wasting

Loss of skeletal muscle mass occurs frequently in clinical settings in response to joint immobilization and bed rest, and is induced by a combination of unloading and inactivity. Disuse-induced atrophy will likely affect every person in his or her lifetime, and can be debilitating especially in the elderly. Currently there are no good therapies to treat disuse-induced muscle atrophy, in part, due to a lack of understanding of the cellular and molecular mechanisms responsible for the induction and maintenance of muscle atrophy. Our current understanding of disuse atrophy comes from the investigation of a variety of models (joint immobilization, hindlimb unloading, bed rest, spinal cord injury) in both animals and humans. Under conditions of unloading, it is widely accepted that there is a decrease in protein synthesis, however, the role of protein degradation, especially in humans, is debated. This review will examine the current understanding of the molecular and cellular mechanisms regulating muscle loss under disuse conditions, discussing the similarities and areas of dispute between the animal and human literature. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Muscle weakness in the elderly: role of sarcopenia, dynapenia, and possibilities for rehabilitation

Aging is a multifactorial process leading to changes in skeletal muscle quantity and quality, which cause muscle weakness and disability in the aging population. This paper discusses the reasons for muscle weakness—and its biological and physiological mechanisms—in the elderly and describes the role of sarcopenia and dynapenia, and the possibilities to modify the age-associated decline in muscle function and decelerate the development of muscle weakness and disability. Resistance and endurance training are effective measures of exercise therapy in the elderly, which improve muscle metabolism and thereby muscle function and life quality.

Effects of hindlimb unloading on neurogenesis in the hippocampus of newly weaned rats

Effects of hindlimb suspension (HS) and ambulation recovery on hippocampal neurogenesis of newly weaned rats were studied by using immunohistochemical techniques. The number of proliferating cell nuclear antigen-positive (PCNA(+)) cells in the subgranular zone (SGZ) markedly decreased during normal growth. However, neither HS nor subsequent recovery caused additional changes in the number of PCNA(+) cells. The number of doublecortin-positive (DCX(+)) neurons decreased gradually during normal growth. HS resulted in a further decrease in these neurons. However, DCX(+) cell numbers became identical to the levels in age-matched controls after 14 days of recovery. PCNA and DCX-double positive cells in the SGZ were also observed, and their cell numbers were not affected by HS and 14-day ambulation. Thus, HS suppressed the generation of DCX(+) neurons without affecting PCNA(+) cells in the SGZ of weaned rats. Taken together, hippocampal neurogenesis in weaned rats was not severely affected by HS while it decreased significantly as they had grown.

The role of cremaster muscle in testicular descent in humans and animal models

Testicular descent is a complex developmental process involving anatomical and hormonal regulation. The gubernaculum undergoes a "swelling reaction" during the transabdominal phase and is mainly under the control of Insulin-Like Peptide 3 (INSL-3) and Mullerian Inhibitory Substance/Anti-Mullerian Hormone (MIS/AMH). The second phase of testicular descent is regulated by androgens and calcitonin gene-related peptide (CGRP) release from the sensory nucleus of the genitofemoral nerve (GFN). In rodents, the active proliferation of the gubernacular tip and cremaster muscle, its rhythmic contraction, as well as the chemotactic gradient provided by the CGRP result in eventual migration of the testis into the scrotum. This review illustrates the structural aspects and hormonal control of cremaster muscle development to better understand the mechanism of testicular descent in normal rodents and humans, compared to diseased rodent models. The analysis showed the cremaster muscle is formed from mesenchymal differentiation of the gubernacular tip and is not a direct passive extension of internal oblique muscle. Cremaster muscle matures slower than other body muscles, and the persistence of immature myogenic proteins seen in cardiac muscle allows rhythmic contraction to guide the testis into the scrotum. Finally, remodelling of the cremaster muscle enables gubernacular eversion. Further understanding of the molecular regulators governing the structural and hormonal changes in the cremaster muscle may lead to new advances in the treatment of undescended testes.

HSP25 can modulate myofibrillar desmin cytoskeleton following the phosphorylation at Ser15 in rat soleus muscle

The main purpose of the present study was to investigate the role(s) of 25-kDa heat shock protein (HSP25) in the regulation and integration of myofibrillar Z-disc structure during down- or upregulation of the size in rat soleus muscle fibers. Hindlimb unloading by tail suspension was performed in adult rats for 7 days, and reloading was allowed for 5 days after the termination of suspension. Interaction of HSP25 and Z-disc proteins, phosphorylation status, distribution, and complex formation of HSP25 were investigated. Non- and single-phosphorylated HSP25s were generally expressed in the cytoplasmic fraction of normal muscle. The level of total HSP25, as well as the phosphorylation ratio, did not change significantly in response to atrophy. Increased expressions of HSP25, phosphorylated at serine 15 (p-Ser15) and dual-phosphorylated form, were noted, when atrophied muscles were reloaded. Myofibrillar HSP25 was also noted in reloaded muscle. Histochemical analysis further indicated the localization of p-Ser15 in the regions with disorganization of Z-disc structure in reloaded muscle fibers. HSP25 formed a large molecular complex in the cytoplasmic fraction of normal muscle, whereas dissociation of free HSP25 with Ser15 phosphorylation was noted in reloaded muscle. The interaction of p-Ser15 with desmin and actinin was detected in Z-discs by proximity ligation assay. Strong interaction between p-Ser15 and desmin, but not actinin, was noted in the disorganized areas. These results indicated that HSP25 contributed to the desmin cytoskeletal organization following the phosphorylation at Ser15 during reloading and regrowing of soleus muscle.

Na v1.4 and Na v1.5 are modulated differently during muscle immobilization and contractile phenotype conversion

Muscle immobilization leads to modification in its fast/slow contractile phenotype. Since the properties of voltage-gated sodium channels (Na(v)) are different between "fast" and "slow" muscles, we studied the effects of immobilization on the contractile properties and the Na(v) of rat peroneus longus (PL). The distal tendon of PL was cut and fixed to the adjacent bone at neutral muscle length. After 4 or 8 wk of immobilization, the contractile and the Na(v) properties were studied and compared with muscles from control animals (Student's t-test). After 4 wk of immobilization, PL showed a faster phenotype with a rightward shift of the force-frequency curve and a decrease in both the Burke's index of fatigability and the tetanus-to-twitch ratio. These parameters showed opposite changes between 4 and 8 wk of immobilization. The maximal sodium current in 4-wk immobilized fibers was higher compared with that of control fibers (11.5 ± 1.2 vs. 7.8 ± 0.8 nA, P = 0.008), with partial recovery to the control values in 8-wk immobilized fibers (8.6 ± 0.7 nA, P = 0.48). In the presence of tetrodotoxin, the maximal residual sodium current decreased continuously throughout immobilization. Using the Western blot analysis, Na(v)1.4 expression showed a transient increase in 4-wk muscle, whereas Na(v)1.5 expression decreased during immobilization. Our results indicate that a muscle immobilized at optimal functional length with the preservation of neural inputs exhibits a transient fast phenotype conversion. Na(v)1.4 expression and current are related to the contractile phenotype variation.

Effects of botulinum toxin-induced paralysis on postnatal development of the supraspinatus muscle

The mechanical environment plays an important role in musculoskeletal tissue development. The present study characterized changes in supraspinatus muscle due to removal of mechanical cues during postnatal development. An intramuscular injection of botulinum toxin type A (BTX) was used to induce and maintain paralysis in the left shoulders of mice since birth while the right shoulders received saline and served as contralateral controls. A separate group of animals was allowed to develop normally without any injections. Muscles were examined postnatally at various time points. The maximum isometric tetanic force generated by the muscle was significantly reduced in the BTX group compared to saline and normal groups. The paralyzed muscles were smaller and showed significant muscle atrophy and fat accumulation on histologic evaluation. Myogenic genes myogenin, myoD1, myf5, myf6, and fast type II myosin heavy chain (MHC) isoform were significantly upregulated while slow type I MHC isoform was significantly downregulated in the BTX group. Adipogenic genes C/EBPα, PPARγ2, leptin, and lipoprotein lipase were significantly upregulated in the BTX group. Results indicate that reduced muscle loading secondary to BTX-induced paralysis leads to fat accumulation and muscle degeneration in the developing muscle. Understanding the molecular and compositional changes in developing supraspinatus muscles may be useful for identifying and addressing the pathological changes that occur in shoulder injuries such as neonatal brachial plexus palsy.

Effects of recession versus tenotomy surgery without recession in adult rabbit extraocular muscle

PURPOSE

Surgical recession of an extraocular muscle (EOM) posterior to its original insertion is a common form of strabismus surgery, weakening the rotational force exerted by the muscle on the globe and improving eye alignment. The purpose of this study was to assess myosin heavy chain (MyHC) isoform expression and satellite cell activity as defined by Pax7 expression in recessed EOMs of adult rabbits compared with that in muscles tenotomized but not recessed and with that in normal control muscles.

METHODS

The scleral insertion of the superior rectus muscle was detached and sutured either 7 mm posterior to its original insertion site (recession surgery) or at the same site (tenotomy). One day before euthanization, the rabbits received bromodeoxyuridine (BrdU) injections. After 7 and 14 days, selected EOMs from both orbits were examined for changes in fast, slow, neonatal, and developmental MyHC isoform expression, Pax7 expression, and BrdU incorporation.

RESULTS

Recession and tenotomy surgery resulted in similar changes in the surgical EOMs. These included a decreased proportion of fast MyHC myofibers, an increased proportion of slow MyHC myofibers, and increased BrdU-positive satellite cells. Similar changes were seen in the non-operated contralateral superior rectus muscles. The ipsilateral inferior rectus showed reciprocal changes to the surgical superior rectus muscles.

CONCLUSIONS

The EOMs are extremely adaptive to changes induced by recession and tenotomy surgery, responding with modulations in fiber remodeling and myosin expression. These adaptive responses could be manipulated to improve surgical success rates.

Effects of baclofen on motor units paralysed by chronic cervical spinal cord injury

Baclofen, a gamma-aminobutyric acid receptor(B) agonist, is used to reduce symptoms of spasticity (hyperreflexia, increases in muscle tone, involuntary muscle activity), but the long-term effects of sustained baclofen use on skeletal muscle properties are unclear. The aim of our study was to evaluate whether baclofen use and paralysis due to cervical spinal cord injury change the contractile properties of human thenar motor units more than paralysis alone. Evoked electromyographic activity and force were recorded in response to intraneural stimulation of single motor axons to thenar motor units. Data from three groups of motor units were compared: 23 paralysed units from spinal cord injured subjects who take baclofen and have done so for a median of 7 years, 25 paralysed units from spinal cord injured subjects who do not take baclofen (median: 10 years) and 45 units from uninjured control subjects. Paralysed motor unit properties were independent of injury duration and level. With paralysis and baclofen, the median motor unit tetanic forces were significantly weaker, twitch half-relaxation times longer and half maximal forces reached at lower frequencies than for units from uninjured subjects. The median values for these same parameters after paralysis alone were comparable to control data. Axon conduction velocities differed across groups and were slowest for paralysed units from subjects who were not taking baclofen and fastest for units from the uninjured. Greater motor unit weakness with long-term baclofen use and paralysis will make the whole muscle weaker and more fatigable. Significantly more paralysed motor units need to be excited during patterned electrical stimulation to produce any given force over time. The short-term benefits of baclofen on spasticity (e.g. management of muscle spasms that may otherwise hinder movement or social interactions) therefore have to be considered in relation to its possible long-term effects on muscle rehabilitation. Restoring the strength and speed of paralysed muscles to pre-injury levels may require more extensive therapy when baclofen is used chronically.

Contractile properties of the denervated external anal sphincter

OBJECTIVE

The purpose of this study was to evaluate the effect of denervation on contractile properties of the external anal sphincter (EAS) of the female rat.

STUDY DESIGN

Sham operation, pudendal nerve transection, pelvic neurectomy, or combined pudendal nerve transection/pelvic neurectomy was performed in young female rats. Contractile function of the EAS was determined after 2 weeks.

RESULTS

Maximal force-generating capacity of the EAS was not impaired by bilateral pudendal denervation or pelvic neurectomy. Twitch tension, however, was decreased, and fatigability increased after pelvic neurectomy. Combined bilateral pudendal nerve-transection plus pelvic neurectomy resulted in compromised force-generating capacity, decreased twitch tension, and increased fatigability of the anal sphincter.

CONCLUSION

Subtle changes in EAS function are detectable after pelvic neurectomy, but not pudendal denervation. In contrast, combination pudendal and pelvic neurectomy resulted in severe compromise of EAS function. These data suggest that EAS function is relatively preserved unless injury occurs to > 1 source of innervation.

Effects of 14 days of microgravity on fast hindlimb and diaphragm muscles of the rat

The purpose of the present study was to determine the effects of 14 days of microgravity on specific rat fast-twitch muscles, and to compare these data with previous data from rat fast-twitch muscles exposed to microgravity for 10 days (Kraemer et al. 2000). Hindlimb muscles containing predominately fast fibers [extensor digitorum longus (EDL), superficial "white" (GSW) and deep "red" (GDR) gastrocnemius] and the diaphragm (DIA) were removed from flight and ground-based control animals and analyzed for: muscle mass, fiber type distribution, cross-sectional area, and myosin heavy chain (MHC) isoform content. Gravitational unloading for 14 days caused significant decreases in muscle mass (8-9%) and cross-sectional area of almost all fiber types (10-35%) from both EDL and gastrocnemius muscles. However, microgravity had little effect on fiber type composition in these muscles with significant changes occurring only in the EDL type IID fiber population (9.5% decrease). Similarly, relative MHC isoform content was only slightly altered by exposure to microgravity (increased content of MHCIIa in flight EDL). No changes in area, fiber type percentages, or MHC isoform content were detected in the DIA following the 14-day spaceflight. Similar to data gathered following a 10-day spaceflight (Kraemer et al. 2000), the 14-day flight did not appear to cause significant slow-to-fast (I --> IIA) or fast-to-faster (IIA --> IID --> IIB) transformations in hindlimb muscles containing predominantly fast-twitch fibers. However, the longer period of gravitational unloading did result in additional loss in muscle fiber cross-sectional area with involvement of more major fiber types.

Regeneration of reinnervated rat soleus muscle is accompanied by fiber transition toward a faster phenotype

The functional recovery of skeletal muscles after peripheral nerve transection and microsurgical repair is generally incomplete. Several reinnervation abnormalities have been described even after nerve reconstruction surgery. Less is known, however, about the regenerative capacity of reinnervated muscles. Previously, we detected remarkable morphological and motor endplate alterations after inducing muscle necrosis and subsequent regeneration in the reinnervated rat soleus muscle. In the present study, we comparatively analyzed the morphometric properties of different fiber populations, as well as the expression pattern of myosin heavy chain isoforms at both immunohistochemical and mRNA levels in reinnervated versus reinnervated-regenerated muscles. A dramatic slow-to-fast fiber type transition was found in reinnervated soleus, and a further change toward the fast phenotype was observed in reinnervated-regenerated muscles. These findings suggest that the (fast) pattern of reinnervation plays a dominant role in the specification of fiber phenotype during regeneration, which can contribute to the long-lasting functional impairment of the reinnervated muscle. Moreover, because the fast II fibers (and selectively, a certain population of the fast IIB fibers) showed better recovery than did the slow type I fibers, the faster phenotype of the reinnervated-regenerated muscle seems to be actively maintained by selective yet undefined cues.

Adaptation of muscle fibre types and capillary network to acute denervation and shortlasting reinnervation

We postulated that, in rat extensor digitorum longus muscle (EDL), the length of capillaries per fibre surface area (Lcap/Sfib) and per fibre volume (Lcap/Vfib) could reflect fibre-type transformations accompanied by changes in oxidative metabolic profile and selective fibre-type atrophy. We excised rat EDL muscle 2 weeks after the sciatic nerve was cut (acute denervation; DEDL) and 4 weeks after the nerve was crushed (early reinnervation; REDL) and characterised muscle fibre-type transformation by the expression of myosin heavy-chain isoforms and by succinate dehydrogenase (SDH) and nicotinoamide adenine dinucleotide-tetrazolium reductase (NADH-TR) reactions. The numerical percentage (N/N) and area percentage (A/A) of pure and hybrid fibres and their diameter were determined, as was the A/A of SDH- and NADH-TR-positive fibres. The length of capillaries per fibre length (Lcap/Lfib), Lcap/Sfib and Lcap/Vfib were estimated in REDL and Lcap/Vfib in DEDL. In DEDL, the type 2x and 2b fibres evidently atrophied, with the N/N of type 2x fibres being lower and that of hybrid fibres higher. In REDL, the N/N of hybrid fibres was even higher, consequent to a lower N/N of type 2b fibres; however, fibre diameters approached values of the control EDL. Compared with control EDL, denervated and reinnervated muscles exhibited a higher A/A of oxidative fibres. This is probably the result of fibre-type transformation and selective fibre atrophy. We conclude that capillary length does not change during acute denervation and early reinnervation. The obtained higher values of Lcap/Sfib and Lcap/Vfib are related to changes in muscle fibre cross-sectional area.

Role(s) of nucleoli and phosphorylation of ribosomal protein S6 and/or HSP27 in the regulation of muscle mass

Effects of 14 days of hindlimb unloading or synergist ablation-related overloading with or without deafferentation on the fiber cross-sectional area, myonuclear number, size, and domain, the number of nucleoli in a single myonucleus, and the levels in the phosphorylation of the ribosomal protein S6 (S6) and 27-kDa heat shock protein (HSP27) were studied in rat soleus. Hypertrophy of fibers (+24%), associated with increased nucleolar number (from 1–2 to 3–5) within a myonucleus and myonuclear domain (+27%) compared with the preexperimental level, was induced by synergist ablation. Such phenomena were associated with increased levels of phosphorylated S6 (+84%) and HSP27 (+28%). Fiber atrophy (−52%), associated with decreased number (−31%) and domain size (−28%) of myonuclei and phosphorylation of S6 (−98%) and HSP27 (−63%), and with increased myonuclear size (+19%) and ubiquitination of myosin heavy chain (+33%, P > 0.05), was observed after unloading, which inhibited the mechanical load. Responses to deafferentation, which inhibited electromyogram level (−47%), were basically similar to those caused by hindlimb unloading, although the magnitudes were minor. The deafferentation-related responses were prevented and nucleolar number was even increased (+18%) by addition of synergist ablation, even though the integrated electromyogram level was still 30% less than controls. It is suggested that the load-dependent maintenance or upregulation of the nucleolar number and/or phosphorylation of S6 and HSP27 plays the important role(s) in the regulation of muscle mass. It was also indicated that such regulation was not necessarily associated with the neural activity.