Noninvasive monitoring of muscle damage during reloading following limb disuse

Safety, feasibility, and efficacy of strengthening exercise in Duchenne muscular dystrophy

BACKGROUND

This two-part study explored the safety, feasibility, and efficacy of a mild-moderate resistance isometric leg exercise program in ambulatory boys with Duchenne muscular dystrophy (DMD).

METHODS

First, we used a dose escalation paradigm with varying intensity and frequency of leg isometric exercise to determine the dose response and safety in 10 boys. Second, we examined safety and feasibility of a 12-wk in-home, remotely supervised, mild-moderate intensity strengthening program in eight boys. Safety measures included T₂ MRI, creatine kinase levels, and pain. Peak strength and function (time to ascend/descend four stairs) were also measured.

RESULTS

Dose-escalation revealed no signs of muscle damage. Seven of the eight boys completed the 12-wk in-home program with a compliance of 84.9%, no signs of muscle damage, and improvements in strength (knee extensors P < .01; knee flexors P < .05) and function (descending steps P < .05).

CONCLUSIONS

An in-home, mild-moderate intensity leg exercise program is safe with potential to positively impact both strength and function in ambulatory boys with DMD.

An accumulation of muscle macrophages is accompanied by altered insulin sensitivity after reduced activity and recovery

BACKGROUND

Mechanisms underlying physical inactivity-induced insulin resistance are not well understood. In addition to a role in muscle repair, immune cell populations such as macrophages may regulate insulin sensitivity.

AIM

The aim of this study was to examine if the dynamic changes in insulin sensitivity during and after recovery from reduced physical activity corresponded to changes in skeletal muscle macrophages.

METHODS

In this prospective clinical study, we collected muscle biopsies from healthy older adults (70 ± 2 years, n = 12) before and during a hyperinsulinaemic-euglycaemic clamp and this occurred before (PRE) and after 2-week reduced physical activity (RA), and following 2-week of recovery (REC). Insulin sensitivity (hyperinsulinaemic-euglycaemic clamp), skeletal muscle mRNA expression of inflammatory markers, and immunofluorescent quantification of skeletal muscle macrophages, myofibre-specific satellite cell and capillary content were assessed.

RESULTS

Insulin sensitivity was decreased following reduced activity and rebounded following recovery above PRE levels. We observed an increase (P < 0.01) in muscle macrophages (CD68⁺ CD206⁺ : 190 [55, 324]; CD11b⁺ CD206⁺ : 117 [28, 205]% change from PRE) and CD68 (2.4 [1.4, 3.4]-fold) and CCL2 (1.9 [1.3, 2.5]-fold) mRNA following RA concurrent with increased (P < 0.03) satellite cells (55 [6, 104]%) in slow-twitch myofibres. Moreover, the distance of satellite cells to the nearest capillary was increased 7.7 (1.7, 13.7) µm in fast-twitch myofibres at RA (P = 0.007). Changes in macrophages were positively associated with increased insulin sensitivity following RA (R > 0.57, P < 0.05).

CONCLUSION

These findings suggested that a dynamic response of skeletal muscle macrophages following acute changes in physical activity in healthy older adults is related to insulin sensitivity.

Effects of hindlimb suspension and reloading on gastrocnemius and soleus muscle mass and function in geriatric mice

Reloading of atrophied muscles after hindlimb suspension (HLS) can induce muscle injury and prolong recovery after disuse in old rats, especially in fast contracting muscles. Less is known about the responses in mice and whether fast and slow muscles from geriatric mice will respond in a similar fashion to HLS unloading and recovery (HLS + R). Furthermore, while slow muscles undergo atrophy with disuse, they typically are more resistant to sarcopenia than fast contracting muscles. Geriatric (28 mo. of age) male C57BL/6 mice were randomly placed into 3 groups. These included HLS for 14 days n = 9, and HLS followed by 14 days of reloading recovery (HLS + R; n = 9), or normal ambulatory cage controls (n = 9). Control mice were not exposed to unloading. Electrically evoked maximal muscle function was assessed in vivo in anesthetized mice at baseline, after 14 days of HLS or HLS + R. As expected, HLS significantly reduced body weight, wet weight of gastrocnemius and soleus muscles and in vivo maximal force. There were no differences in vivo fatigability of the plantar flexor muscles and overall fiber size. There were only minor fiber type distribution and frequency distribution of fiber sizes that differ between HLS + R and control gastrocnemius and soleus muscles. Soleus muscle wet weight had recovered to control levels after reloading, but type I/IIA fibers in the soleus muscles were significantly smaller after HLS + R than control muscles. In contrast, gastrocnemius muscle wet weight did not recover to control levels after reloading. Plantar flexion muscle force (primarily influenced by the gastrocnemius muscles) did not recover in HLS + R conditions as compared to HLS conditions and both were lower than control force production signaling for apoptosis, autophagy and anabolic markers were not different between control and HLS + R gastrocnemius and soleus muscles in geriatric mice. These results suggest that molecular signaling does not explain attenuated ability to regain muscle wet weight, fiber size or muscle force production after HLS in geriatric mice. It is possible that fluid shifts, reduced blood flow, or shortened muscle fibers which failed to regain control lengths contributed to the attenuation of muscle wet weight after HLS and reloading and this affected force production. Further work is needed to determine if altered/loss of neural activity contributed to the inability of geriatric mice to regain gastrocnemius muscle weight and function after HLS and reloading.

Effects of the Inertia Barbell Training on Lumbar Muscle T2 Relaxation Time

Sun, M-Y, Lu, J-Q, Ma, Z-C, Lü, J-J, Huang, Q, Sun, Y-N, and Liü, Y. Effects of the inertia barbell training on lumbar muscle T2 relaxation time. J Strength Cond Res 34(12): 3454-3462, 2020-The purpose of this study was to investigate variations in T2 relaxation time in normal human lumbar muscles caused by inertia barbell training. Thirty undergraduate healthy men (mean age = 19 ± 1.2 years, body mass = 72 ± 10.0 kg, and height = 1.78 ± 0.1 m) were recruited to participate in this study. Subjects were randomly assigned into 2 groups: an inertia barbell training group (IBTG) (n = 15) and a normal barbell-training group (NBTG) (n = 15). All subjects participated in lumbar flexion and extension muscle strength training for 1 hour per time, 3 times per week for a total of 8 weeks. The lumbar area of each subject was scanned before and after the experiment using a 3.0T superconductive magnetic resonance imaging system. The T2 values measured after intervention were significantly different compared with the T2 values measured before the experiment in both the IBTG and NBTG groups (p < 0.001). After intervention, there was no significant difference in T2 values between the IBTG and NBTG groups (p = 0.17). The ([INCREMENT]T2)/T2 percentage was significantly different in the IBTG group (p < 0.01). This study demonstrated that 8 weeks of strength training led to significant improvements in the values for T2 relaxation time of the lumbar muscles. Furthermore, the ([INCREMENT]T2)/T2 percentage for IBTG was higher than that for NBTG, which suggested that lumbar muscle activity increased more with inertial barbell training.

Skeletal muscle water T2 as a biomarker of disease status and exercise effects in patients with Duchenne muscular dystrophy

The purpose of this study was to examine exercise effects on muscle water T₂ in patients with Duchenne muscular dystrophy (DMD). In 12 DMD subjects and 19 controls, lower leg muscle fat (%) was measured by Dixon and muscle water T₂ and R₂ (1/T₂) by the tri-exponential model. Muscle water R₂ was measured again at 3 hours after an ankle dorsiflexion exercise. The muscle fat fraction was higher in DMD participants than in controls (p < .001) except in the tibialis posterior muscle. Muscle water T₂ was measured independent of the degree of fatty degeneration in DMD muscle. At baseline, muscle water T₂ was higher in all but the extensor digitorum longus muscles of DMD participants than controls (p < .001). DMD participants had a lower muscle torque (p < .001) and exerted less power (p < .01) during exercise than controls. Nevertheless, muscle water R₂ decreased (T₂ increased) after exercise from baseline in DMD subjects and controls with greater changes in the target muscles of the exercise than in ankle plantarflexor muscles. Skeletal muscle water T₂ is a sensitive biomarker of the disease status in DMD and of the exercise response in DMD patients and controls.

Near-Infrared Optical Imaging Noninvasively Detects Acutely Damaged Muscle

Muscle damage is currently assessed through methods such as muscle biopsy, serum biomarkers, functional testing, and imaging procedures, each with its own inherent limitations, and a pressing need for a safe, repeatable, inexpensive, and noninvasive modality to assess the state of muscle health remains. Our aim was to develop and assess near-infrared (NIR) optical imaging as a novel noninvasive method of detecting and quantifying muscle damage. An immobilization-reambulation model was used for inducing muscle damage and recovery in the lower hindlimbs in mice. Confirmation of muscle damage was obtained using in vivo indocyanine green-enhanced NIR optical imaging, magnetic resonance imaging, and ex vivo tissue analysis. The soleus of the immobilized-reambulated hindlimb was found to have a greater amount of muscle damage compared to that in the contralateral nonimmobilized limb, confirmed by in vivo indocyanine green-enhanced NIR optical imaging (3.86-fold increase in radiant efficiency), magnetic resonance imaging (1.41-fold increase in T2), and an ex vivo spectrophotometric assay of indocyanine green uptake (1.87-fold increase in normalized absorbance). Contrast-enhanced NIR optical imaging provides a sensitive, rapid, and noninvasive screening method that can be used for imaging and quantifying muscle damage and recovery in vivo.

Age-related T2 changes in hindlimb muscles of mdx mice

INTRODUCTION

Magnetic resonance imaging (MRI) was used to monitor changes in the transverse relaxation time constant (T2) in lower hindlimb muscles of mdx mice at different ages.

METHODS

Young (5 weeks), adult (44 weeks), and old mdx (96 weeks), and age-matched control mice were studied. Young mdx mice were imaged longitudinally, whereas adult and old mdx mice were imaged at a single time-point.

RESULTS

Mean muscle T2 and percent of pixels with elevated T2 were significantly different between mdx and control mice at all ages. In young mdx mice, mean muscle T2 peaked at 7-8 weeks and declined at 9-11 weeks. In old mdx mice, mean muscle T2 was decreased compared with young and adult mice, which could be attributed to fibrosis.

CONCLUSIONS

MRI captured longitudinal changes in skeletal muscle integrity of mdx mice. This information will be valuable for pre-clinical testing of potential therapeutic interventions for muscular dystrophy.

Myopathic changes in murine skeletal muscle lacking synemin

Diseases of striated muscle linked to intermediate filament (IF) proteins are associated with defects in the organization of the contractile apparatus and its links to costameres, which connect the sarcomeres to the cell membrane. Here we study the role in skeletal muscle of synemin, a type IV IF protein, by examining mice null for synemin (synm-null). Synm-null mice have a mild skeletal muscle phenotype. Tibialis anterior (TA) muscles show a significant decrease in mean fiber diameter, a decrease in twitch and tetanic force, and an increase in susceptibility to injury caused by lengthening contractions. Organization of proteins associated with the contractile apparatus and costameres is not significantly altered in the synm-null. Elastimetry of the sarcolemma and associated contractile apparatus in extensor digitorum longus myofibers reveals a reduction in tension consistent with an increase in sarcolemmal deformability. Although fatigue after repeated isometric contractions is more marked in TA muscles of synm-null mice, the ability of the mice to run uphill on a treadmill is similar to controls. Our results suggest that synemin contributes to linkage between costameres and the contractile apparatus and that the absence of synemin results in decreased fiber size and increased sarcolemmal deformability and susceptibility to injury. Thus synemin plays a moderate but distinct role in fast twitch skeletal muscle.

High-field (11.75T) multimodal MR imaging of exercising hindlimb mouse muscles using a non-invasive combined stimulation and force measurement device

We have designed and constructed an experimental set-up allowing electrical stimulation of hindlimb mouse muscles and the corresponding force measurements at high-field (11.75T). We performed high-resolution multimodal MRI (including T2 -weighted imaging, angiography and diffusion) and analysed the corresponding MRI changes in response to a stimulation protocol. Mice were tested twice over a 1-week period to investigate the reliability of mechanical measurements and T2 changes associated with the stimulation protocol. Additionally, angiographic images were obtained before and immediately after the stimulation protocol. Finally, multislice diffusion imaging was performed before, during and immediately after the stimulation session. Apparent diffusion coefficient (ADC) maps were calculated on the basis of diffusion weighted images (DWI). Both force production and T2 values were highly reproducible as illustrated by the low coefficient of variation (<8%) and high intraclass correlation coefficient (≥0.75) values. Maximum intensity projection angiographic images clearly showed a strong vascular effect resulting from the stimulation protocol. Although a motion sensitive imaging sequence was used (echo planar imaging) and in spite of the strong muscle contractions, motion artifacts were minimal for DWI recorded under exercising conditions, thereby underlining the robustness of the measurements. Mean ADC values increased under exercising conditions and were higher during the recovery period as compared with the corresponding control values. The proposed experimental approach demonstrates accurate high-field multimodal MRI muscle investigations at a preclinical level which is of interest for monitoring the severity and/or the progression of neuromuscular diseases but also for assessing the efficacy of potential therapeutic interventions.

Multi-parametric MRI characterization of inflammation in murine skeletal muscle

Myopathies often display a common set of complex pathologies that include muscle weakness, inflammation, compromised membrane integrity, fat deposition, and fibrosis. Multi-parametric, quantitative, non-invasive imaging approaches may be able to resolve these individual pathological components. The goal of this study was to use multi-parametric MRI to investigate inflammation as an isolated pathological feature. Proton relaxation, diffusion tensor imaging (DTI), quantitative magnetization transfer (qMT-MRI), and dynamic contrast enhanced (DCE-MRI) parameters were calculated from data acquired in a single imaging session conducted 6-8 hours following the injection of λ-carrageenan, a local inflammatory agent. T2 increased in the inflamed muscle and transitioned to bi-exponential behavior. In diffusion measurements, all three eigenvalues and the apparent diffusion coefficient increased, but λ3 had the largest relative change. Analysis of the qMT data revealed that the T1 of the free pool and the observed T1 both increased in the inflamed tissue, while the ratio of exchanging spins in the solid pool to those in the free water pool (the pool size ratio) significantly decreased. DCE-MRI data also supported observations of an increase in extracellular volume. These findings enriched the understanding of the relation between multiple quantitative MRI parameters and an isolated inflammatory pathology, and may potentially be employed for other single or complex myopathy models.

Time course of diaphragm function recovery after controlled mechanical ventilation in rats

Controlled mechanical ventilation (CMV) is known to result in rapid and severe diaphragmatic dysfunction, but the recovery response of the diaphragm to normal function after CMV is unknown. Therefore, we examined the time course of diaphragm function recovery in an animal model of CMV. Healthy rats were submitted to CMV for 24-27 h (n = 16), or to 24-h CMV followed by either 1 h (CMV + 1 h SB, n = 9), 2 h (CMV + 2 h SB, n = 9), 3 h (CMV + 3 h SB, n = 9), or 4-7 h (CMV + 4-7 h SB, n = 9) of spontaneous breathing (SB). At the end of the experiment, the diaphragm muscle was excised for functional and biochemical analysis. The in vitro diaphragm force was significantly improved in the CMV + 3 h SB and CMV + 4-7 h SB groups compared with CMV (maximal tetanic force: +27%, P < 0.05, and +59%, P < 0.001, respectively). This was associated with an increase in the type IIx/b fiber dimensions (P < 0.05). Neutrophil influx was increased in the CMV + 4-7 h SB group (P < 0.05), while macrophage numbers remained unchanged. Markers of protein synthesis (phosphorylated Akt and eukaryotic initiation factor 4E binding protein 1) were significantly increased (±40%, P < 0.001, and ±52%, P < 0.01, respectively) in the CMV + 3 h SB and CMV + 4-7 h SB groups and were positively correlated with diaphragm force (P < 0.05). Finally, also the maximal specific force generation of skinned single diaphragm fibers was increased in the CMV + 4-7 h SB group compared with CMV (+45%, P < 0.05). In rats, reloading the diaphragm for 3 h after CMV is sufficient to improve diaphragm function, while complete recovery occurs after longer periods of reloading. Enhanced muscle fiber dimensions, increased protein synthesis, and improved intrinsic contractile properties of diaphragm muscle fibers may have contributed to diaphragm function recovery.

Loss of the inducible Hsp70 delays the inflammatory response to skeletal muscle injury and severely impairs muscle regeneration

Skeletal muscle regeneration following injury is a highly coordinated process that involves transient muscle inflammation, removal of necrotic cellular debris and subsequent replacement of damaged myofibers through secondary myogenesis. However, the molecular mechanisms which coordinate these events are only beginning to be defined. In the current study we demonstrate that Heat shock protein 70 (Hsp70) is increased following muscle injury, and is necessary for the normal sequence of events following severe injury induced by cardiotoxin, and physiological injury induced by modified muscle use. Indeed, Hsp70 ablated mice showed a significantly delayed inflammatory response to muscle injury induced by cardiotoxin, with nearly undetected levels of both neutrophil and macrophage markers 24 hours post-injury. At later time points, Hsp70 ablated mice showed sustained muscle inflammation and necrosis, calcium deposition and impaired fiber regeneration that persisted several weeks post-injury. Through rescue experiments reintroducing Hsp70 intracellular expression plasmids into muscles of Hsp70 ablated mice either prior to injury or post-injury, we confirm that Hsp70 optimally promotes muscle regeneration when expressed during both the inflammatory phase that predominates in the first four days following severe injury and the regenerative phase that predominates thereafter. Additional rescue experiments reintroducing Hsp70 protein into the extracellular microenvironment of injured muscles at the onset of injury provides further evidence that Hsp70 released from damaged muscle may drive the early inflammatory response to injury. Importantly, following induction of physiological injury through muscle reloading following a period of muscle disuse, reduced inflammation in 3-day reloaded muscles of Hsp70 ablated mice was associated with preservation of myofibers, and increased muscle force production at later time points compared to WT. Collectively our findings indicate that depending on the nature and severity of muscle injury, therapeutics which differentially target both intracellular and extracellular localized Hsp70 may optimally preserve muscle tissue and promote muscle functional recovery.

Diaphragm and ventilatory dysfunction during cancer cachexia

Cancer cachexia is characterized by a continuous loss of locomotor skeletal muscle mass, which causes profound muscle weakness. If this atrophy and weakness also occurs in diaphragm muscle, it could lead to respiratory failure, which is a major cause of death in patients with cancer. Thus, the purpose of the current study was to determine whether colon-26 (C-26) cancer cachexia causes diaphragm muscle fiber atrophy and weakness and compromises ventilation. All diaphragm muscle fiber types were significantly atrophied in C-26 mice compared to controls, and the atrophy-related genes, atrogin-1 and MuRF1, significantly increased. Maximum isometric specific force of diaphragm strips, absolute maximal calcium activated force, and maximal specific calcium-activated force of permeabilized diaphragm fibers were all significantly decreased in C-26 mice compared to controls. Further, isotonic contractile properties of the diaphragm were affected to an even greater extent than isometric function. Ventilation measurements demonstrated that C-26 mice have a significantly lower tidal volume compared to controls under basal conditions and, unlike control mice, an inability to increase breathing frequency, tidal volume, and, thus, minute ventilation in response to a respiratory challenge. These data demonstrate that C-26 cancer cachexia causes profound respiratory muscle atrophy and weakness and ventilatory dysfunction.

Overexpression of insulin-like growth factor-1 attenuates skeletal muscle damage and accelerates muscle regeneration and functional recovery after disuse

Skeletal muscle is a highly dynamic tissue that responds to endogenous and external stimuli, including alterations in mechanical loading and growth factors. In particular, the antigravity soleus muscle experiences significant muscle atrophy during disuse and extensive muscle damage upon reloading. Given that insulin-like growth factor-1 (IGF-1) has been implicated as a central regulator of muscle repair and modulation of muscle size, we examined the effect of virally mediated overexpression of IGF-1 on the soleus muscle following hindlimb cast immobilization and upon reloading. Recombinant IGF-1 cDNA virus was injected into one of the posterior hindlimbs of the mice, while the contralateral limb was injected with saline (control). At 20 weeks of age, both hindlimbs were immobilized for 2 weeks to induce muscle atrophy in the soleus and ankle plantarflexor muscle group. Subsequently, the mice were allowed to reambulate, and muscle damage and recovery were monitored over a period of 2-21 days. The primary finding of this study was that IGF-1 overexpression attenuated reloading-induced muscle damage in the soleus muscle, and accelerated muscle regeneration and force recovery. Muscle T2 assessed by magnetic resonance imaging, a non-specific marker of muscle damage, was significantly lower in IGF-1-injected compared with contralateral soleus muscles at 2 and 5 days reambulation (P<0.05). The reduced prevalence of muscle damage in IGF-1-injected soleus muscles was confirmed on histology, with a lower fractional area of abnormal muscle tissue in IGF-1-injected muscles at 2 days reambulation (33.2±3.3 versus 54.1±3.6%, P<0.05). Evidence of the effect of IGF-1 on muscle regeneration included timely increases in the number of central nuclei (21% at 5 days reambulation), paired-box transcription factor 7 (36% at 5 days), embryonic myosin (37% at 10 days) and elevated MyoD mRNA (7-fold at 2 days) in IGF-1-injected limbs (P<0.05). These findings demonstrate a potential role of IGF-1 in protecting unloaded skeletal muscles from damage and accelerating muscle repair and regeneration.

Magnitude of spinal muscle damage is not statistically associated with exercise-induced low back pain intensity

BACKGROUND CONTEXT

Findings on imaging of noncontractile anatomic abnormalities and the intensity of low back pain have weak associations because of false-positive rates among asymptomatic individuals. This association might be stronger for contractile tissues.

PURPOSE

The purpose of this study was to examine the relationship between location and reports of pain intensity in the low back and exercise-induced muscle damage to the lumbar paraspinal muscles.

STUDY DESIGN

Nondiagnostic observational study in a laboratory setting.

METHODS

Delayed onset muscle soreness was induced in the low back of healthy pain-free volunteers. Measures of pain intensity (100-mm visual analog scale [VAS]) and location (area on the pain diagram) were taken before and 48 hours after exercise. Muscle damage was quantified using mechanical pain thresholds, motor performance deficits, and transverse relaxation time (T2)-weighted magnetic resonance imaging (MRI). Changes pre- to postexercise in signal intensity on T2-weighted imaging within the erector spinae, pain intensity, pain area, mechanical pain threshold, and isometric torque were assessed using paired t tests. Bivariate correlations were conducted to assess associations among muscle damage, pain intensity, and pain drawing area.

RESULTS

Twenty participants volunteered (11 women; average age, 22.3 years; average body mass index, 23.5) for study participation. Reports of pain intensity at 48 hours ranged from 0 to 59 mm on the VAS. Muscle damage was confirmed by reductions in mechanical threshold (p=.011) and motor performance (p<.001) and by changes in T2-weighted MRI (p=.007). This study was powered to find an association of at least r=0.5 to be statistically significant. Correlations of continuous variables revealed no significant correlations between pain intensity and measures of muscle damage (ranging between -0.075 and 0.151). There was a significant association between the remaining torque deficit at 48 hours and pain area.

CONCLUSIONS

The results of this study indicate that there was no association between the magnitude of muscle damage in the lumbar erector spinae and reported pain intensity in the low back. In future studies, larger cohorts may report statistically significant associations, but our data suggest that there will be low magnitude potentially indicating limited clinical relevance.

Diffusion tensor MRI to assess damage in healthy and dystrophic skeletal muscle after lengthening contractions

The purpose of this study was to determine if variables calculated from diffusion tensor imaging (DTI) would serve as a reliable marker of damage after a muscle strain injury in dystrophic (mdx) and wild type (WT) mice. Unilateral injury to the tibialis anterior muscle (TA) was induced in vivo by 10 maximal lengthening contractions. High resolution T1- and T2-weighted structural MRI, including T2 mapping and spin echo DTI was acquired on a 7T small animal MRI system. Injury was confirmed by a significant loss of isometric torque (85% in mdx versus 42% in WT). Greater increases in apparent diffusion coefficient (ADC), axial, and radial diffusivity (AD and RD) of the injured muscle were present in the mdx mice versus controls. These changes were paralleled by decreases in fractional anisotropy (FA). Additionally, T2 was increased in the mdx mice, but the spatial extent of the changes was less than those in the DTI parameters. The data suggest that DTI is an accurate indicator of muscle injury, even at early time points where the MR signal changes are dominated by local edema.

Factors controlling T2 mapping from partially spoiled SSFP sequence: optimization for skeletal muscle characterization

A fast and robust methodology for in vivo T(2) mapping is presented. The approach is based on the partially spoiled steady state free precession technique recently proposed by Bieri et al. (Magn Reson Med 2011). The accuracy of this method was demonstrated in simulations and phantom experiments. Variations in skeletal muscle T(2) relaxation time have been correlated with cell damage and inflammatory response. Nonetheless, the lack of easily implementable, fast, accurate and reproducible methods has hampered the adoption of T(2) measurement as a noninvasive tool for skeletal muscle characterization. The applicability of the partially spoiled steady state free precession method for tissue characterization in muscle disease is illustrated in this work by several examples. Quantitative MRI, in particular T(2) mapping based on partially spoiled steady state free precession acquisitions, might provide objective markers of muscle damage and degenerative changes, and an alternative to serial muscle biopsies.

Changes in muscle T2 and tissue damage after downhill running in mdx mice

INTRODUCTION

In this study we compared the effects of downhill or horizontal treadmill running on the magnetic resonance imaging (MRI) transverse relaxation time constant (T(2)) in mdx mice.

METHODS

Mice underwent either downhill (n = 11 mdx, n = 6 controls) or horizontal running (n = 9, mdx only) on a treadmill. MRI was conducted prior to exercise, immediately afterward (∽20 minutes), and then 24 and 48 hours after exercise.

RESULTS

A higher percentage of pixels with elevated T(2) in the lower hindlimb muscles was observed in the mdx mice compared with controls both pre-exercise (P < 0.001) and at each time-point after downhill running (P < 0.05), but not with horizontal running. The medial compartment muscles appeared to be the most susceptible to increased T(2).

CONCLUSIONS

Downhill running provides a stimulus for inducing acute changes in muscle T(2) in mdx mice. MRI is a non-invasive approach for examining acute muscle damage and recovery in multiple muscle groups simultaneously.

Physiology, structure, and susceptibility to injury of skeletal muscle in mice lacking keratin 19-based and desmin-based intermediate filaments

Intermediate filaments, composed of desmin and of keratins, play important roles in linking contractile elements to each other and to the sarcolemma in striated muscle. Our previous results show that the tibialis anterior (TA) muscles of mice lacking keratin 19 (K19) lose costameres, accumulate mitochondria under the sarcolemma, and generate lower specific tension than controls. Here we compare the physiology and morphology of TA muscles of mice lacking K19 with muscles lacking desmin or both proteins [double knockout (DKO)]. K19-/- mice and DKO mice showed a threefold increase in the levels of creatine kinase (CK) in the serum. The absence of desmin caused a larger change in specific tension (-40%) than the absence of K19 (-19%) and played the predominant role in contractile function (-40%) and decreased tolerance to exercise in the DKO muscle. By contrast, the absence of both proteins was required to obtain a significantly greater loss of contractile torque after injury (-48%) compared with wild type (-39%), as well as near-complete disruption of costameres. The DKO muscle also showed a significantly greater misalignment of myofibrils than either mutant alone. In contrast, large subsarcolemmal gaps and extensive accumulation of mitochondria were only seen in K19-null TA muscles, and the absence of both K19 and desmin yielded milder phenotypes. Our results suggest that keratin filaments containing K19- and desmin-based intermediate filaments can play independent, complementary, or antagonistic roles in the physiology and morphology of fast-twitch skeletal muscle.

A strictly noninvasive MR setup dedicated to longitudinal studies of mechanical performance, bioenergetics, anatomy, and muscle recruitment in contracting mouse skeletal muscle

MR techniques have proven their ability to investigate skeletal muscle function in situ. Their benefit in terms of noninvasiveness is, however, lost in animal research, given that muscle stimulation and force output measurements are usually achieved using invasive surgical procedures, thereby excluding repeated investigations in the same animal. This study describes a new setup allowing strictly noninvasive investigations of mouse gastrocnemius muscle function using (1)H-MRI and (31)P-MR spectroscopy. Its originality is to integrate noninvasive systems for inducing muscle contraction through transcutaneous stimulation and for measuring mechanical performance with a dedicated ergometer. In order to test the setup, muscle function was investigated using a fatiguing stimulation protocol (6 min of repeated isometric contractions at 1.7 Hz). T(2)-weighted imaging demonstrated that transcutaneous stimulation mainly activated the gastrocnemius. Moreover, investigations repeated twice with a 7-day interval between bouts did show a high reproducibility in measurements with regard to changes in isometric force and energy metabolism. In conclusion, this setup enables us for the first time to access mechanical performance, energy metabolism, anatomy, and physiology strictly noninvasively in contracting mouse skeletal muscle. The possibility for implementing longitudinal studies opens up new perspectives in many research areas, including ageing, pharmaceutical research, and gene and cell therapy.

Influence of fixed muscle length and contractile properties on atrophy and subsequent recovery in the rat soleus and plantaris muscles

This study examined muscular atrophy and the recovery process induced by hindlimb unloading and joint immobilization in the rat soleus and plantaris muscles. Rats were divided into control, hindlimb unloading (HU), hindlimb unloading with ankle joint immobilization at the maximum dorsiflexion (HUD), and maximum plantarflexion (HUP) groups. The hindlimb was reloaded after fourteen days of unloading, and muscle atrophy and walking ability were assessed at 0, 3, and 7 days of reloading. A cross sectional area of muscle fibers in the soleus muscle on day 0 of reloading revealed sizes in order from the control, HUD, HUP down to the HU group, indicating that the HU group was the most atrophied among the four groups. These values in the plantaris muscle ranged in order from the control, HU, HUD, to HUP groups, the HUP group being the most atrophied among the four groups. These muscles recovered from atrophy in the same descending order, and the values in the HUD and HUP groups slowly recovered during the reloading periods. The HUD and HUP groups showed a central core lesion and reloading-induced lesions in some type I muscle fibers after the immobilization and reloading, one possible reason for the delayed recovery in these groups. The muscle atrophy in the HU, HUD, and HUP groups remained at day 7 although the walking ability appeared to be normal. Accordingly, further rehabilitation therapy might be necessary even if the functional ability appears to be normal.

Location of myofiber damage in skeletal muscle after lengthening contractions

High-force lengthening contractions are associated with muscle damage and pain, and the muscle-tendon junction is commonly cited as the primary area where myofiber damage occurs. We induced injury in the rat tibialis anterior muscle and acquired magnetic resonance imaging (MRI) images postinjury. We also assayed membrane damage and quantified the number of centrally nucleated myofibers throughout the injured muscles. Results suggest that myofiber injury occurs primarily in the middle portion of the muscle, with interstitial edema in the middle and distal portions.

Non-invasive assessment of lower extremity muscle composition after incomplete spinal cord injury

STUDY DESIGN

Cross-sectional study.

OBJECTIVE

(1) To quantify intramyocellular lipid (IMCL) content of the soleus muscle. (2) To assess the T(2) relaxation rates in the lower extremity skeletal muscles in persons with incomplete spinal cord injury (SCI).

SETTING

Academic Institution, Florida.

METHODS

Eight subjects (42+/-10 years old; 70+/-12 kg; 176+/-10 cm) with chronic (17+/-9 months post injury) motor SCI (C4-T12; ASIA C or D) and eight matched healthy controls were tested. Localized unsuppressed proton spectroscopy (H-MRS) was performed to estimate total lipid content and individual lipid components; IMCL and extramyocellular lipid (EMCL) from the soleus muscle. T(2)-weighted imaging of lower extremity muscles yielded muscle T(2) rates.

RESULTS

The IMCL content of the soleus muscle was 3.3 times higher in the patient group as compared to controls (P=0.002; 0.0401 (0.0234-0.0849) versus 0.0123 (0.0090-0.0175)). Similarly, EMCL measures were 4.5 times higher as compared to the controls (P=0.002). Significant differences were observed in the T(2) relaxation times of the soleus and gastrocnemius muscles (P<0.05).

CONCLUSION

The increased levels of IMCL might interfere with the glucose uptake in skeletal muscle; potentially predisposing persons with incomplete SCI to the development of peripheral insulin resistance. Marked elevations in the T(2) relaxation times of the locomotor muscles are reflective of an altered muscle composition.

Alterations in inorganic phosphate in mouse hindlimb muscles during limb disuse

Muscle disuse induces a wide array of structural, biochemical, and neural adaptations in skeletal muscle, which can affect its function. We recently demonstrated in patients with an orthopedic injury that cast immobilization alters the resting P(i) content of skeletal muscle, which may contribute to loss of specific force. The goal of this study was to determine the direct effect of disuse on the basal phosphate content in skeletal muscle in an animal model, avoiding the confounding effects of injury/surgery. (31)P and (1)H MRS data were acquired from the gastrocnemius muscle of young adult mice (C57BL6 female, n = 8), at rest and during a reversible ischemia experiment, before and after 2 weeks of cast immobilization. Cast immobilization resulted in an increase in resting P(i) content (75%; p < 0.001) and the P(i) to phosphocreatine (PCr) ratio (P(i)/PCr; 80%, p < 0.001). The resting concentrations of ATP, PCr and total creatine (PCr + creatine) and the intracellular pH were not significantly different after immobilization. During ischemia (30 min), PCr concentrations decreased to 54 +/- 2% and 52 +/- 6% of the resting values in pre-immobilized and immobilized muscles, respectively, but there were no detectable differences in the rates of P(i) increase or PCr depletion (0.55 +/- 0.01 mM min(-1) and 0.52 +/- 0.03 mM min(-1) before and after immobilization, respectively; p = 0.78). At the end of ischemia, immobilized muscles had a twofold higher phosphorylation potential ([ADP][P(i)]/[ATP]) and intracellular buffering capacity (3.38 +/- 0.54 slykes vs 6.18 +/- 0.57 slykes). However, the rate of PCr resynthesis (k(PCr)) after ischemia, a measure of in vivo mitochondrial function, was significantly lower in the immobilized muscles (0.31 +/- 0.04 min(-1)) than in pre-immobilized muscles (0.43 +/- 0.04 min(-1)). In conclusion, our findings indicate that 2 weeks of cast immobilization, independent of injury-related alterations, leads to a significant increase in the resting P(i) content of mouse skeletal muscle. The increase in P(i) with muscle disuse has a significant effect on the cytosolic phosphorylation potential during transient ischemia and increases the intracellular buffering capacity of skeletal muscle.

Long-term skeletal muscle protection after gene transfer in a mouse model of LGMD-2D

Limb girdle muscular dystrophy (LGMD) describes a group of inherited diseases resulting from mutations in genes encoding proteins involved in maintaining skeletal muscle membrane stability. LGMD type-2D is caused by mutations in alpha-sarcoglycan (sgca). Here we describe muscle-specific gene delivery of the human sgca gene into dystrophic muscle using an adeno-associated virus 1 (AAV1) capsid and creatine kinase promoter. Delivery of this construct to adult sgca(-/-) mice resulted in localization of the sarcoglycan complex to the sarcolemma and a reduction in muscle fiber damage. Sgca expression prevented disease progression as observed in vivo by T(2)-weighted magnetic resonance imaging (MRI) and confirmed in vitro by decreased Evan's blue dye accumulation. The ability of recombinant AAV-mediated gene delivery to restore normal muscle mechanical properties in sgca(-/-) mice was verified by in vitro force mechanics on isolated extensor digitorum longus (EDL) muscles, with a decrease in passive resistance to stretch as compared with untreated controls. In summary, AAV/AAV-sgca gene transfer provides long-term muscle protection from LGMD and can be non-invasively evaluated using magnetic resonance imaging.

Free radical scavengers are more effective than indomethacin in the prevention of experimentally induced heterotopic ossification

The pathogenesis of heterotopic ossification is still unclear and the preventive therapies are usually insufficient. The present study was designed to investigate the possible preventive effect of free radical scavengers on the development of experimentally induced heterotopic ossification in a rabbit model and to compare free radical scavengers with indomethacin to determine whether they act synergistically. A standard immobilization-manipulation model was used to induce heterotopic ossification in the hind legs of 40 1-year-old female New Zealand albino rabbits. The animals were divided into four groups and received daily either placebo, a free radical scavenger cocktail [allopurinol and N-acetylcysteine (A/A)], indomethacin or the combination of A/A and indomethacin in a randomized double-blind fashion. Every 4 days an X-ray was taken and the thickness and length of new bone formation was measured at the thigh. A marked statistically significant difference was found between the four groups. In the groups that received A/A, either alone or combined with indomethacin, an inhibition of bone growth, both in thickness and in length was demonstrated. In this experimental model free radical scavengers had a superior inhibitory effect on heterotopic ossification than indomethacin. Free radicals could play an important role in the pathogenesis of heterotopic ossification.

Changes in muscle T2 relaxation properties following spinal cord injury and locomotor training

Magnetic resonance (MR) is frequently used to study structural and biochemical properties of skeletal muscle. Changes in proton transverse relaxation (T2) properties have been used to study muscle cellular damage, as well as muscle activation during exercise protocols. In this study, we implemented MR imaging to characterize the T2 relaxation properties of rat hindlimb muscles following spinal cord injury (SCI) and locomotor training. After moderate midthoracic contusion SCI, Sprague-Dawley rats were assigned to either treadmill training, cycle training or an untrained group. T2 weighted images were obtained and mean muscle T2 times were calculated in the tibialis anterior, soleus, and gastrocnemius (GAS) muscles at pre-injury as well as at 1, 2, 4, 8, and 12 weeks post-injury. Following SCI, hindlimb muscles in untrained animals showed a significant increase in muscle T2, with the most dramatic shift (+5.46 ms) observed in soleus muscle at 1 week post-SCI. Subsequently, all muscle groups showed a spontaneous recovery in muscle T2 with normalized T2 values in the GAS and tibilias anterior muscles at 4 weeks and the soleus at 12 weeks post-SCI. Both training paradigms, treadmill and cycling training, accelerated the recovery of soleus muscle T2. As a result, soleus muscle T2 recovered back to pre-injury values within 3 weeks of training in both training groups. Finally, in vitro histological assessments of rat skeletal muscles demonstrated that there was no apparent muscle injury in any of the muscles studied at 1 week post-SCI.

Compression-induced deep tissue injury examined with magnetic resonance imaging and histology

The underlying mechanisms leading to deep tissue injury after sustained compressive loading are not well understood. It is hypothesized that initial damage to muscle fibers is induced mechanically by local excessive deformation. Therefore, in this study, an animal model was used to study early damage after compressive loading to elucidate on the damage mechanisms leading to deep pressure ulcers. The tibialis anterior of Brown-Norway rats was loaded for 2 h by means of an indenter. Experiments were performed in a magnetic resonance (MR)-compatible loading device. Muscle tissue was evaluated with transverse relaxation time (T2)-weighted MRI both during loading and up to 20 h after load removal. In addition, a detailed examination of the histopathology was performed at several time points (1, 4, and 20 h) after unloading. Results demonstrated that, immediately after unloading, T2-weighted MR images showed localized areas with increased signal intensity. Histological examination at 1 and 4 h after unloading showed large necrotic regions with complete disorganization of the internal structure of the muscle fibers. Hypercontraction zones were found bilateral to the necrotic zone. Twenty hours after unloading, an extensive inflammatory response was observed. The proposed relevance of large deformation was demonstrated by the location of damage indicated by T2-weighted MRI and the histological appearance of the compressed tissues. Differences in damage development distal and proximal to the indenter position suggested a contribution of perfusion status in the measured tissue changes that, however, appeared be to reversible.