Muscle Fiber Function Research Articles

Additive Phenotype of MG53 and Dysferlin Deficiencies in Membrane Repair Function of Skeletal Muscle Fibers

Dysferlin and MG53 have been implicated as members of the sarcolemma repair machinery. Skeletal muscle fibers isolated from mice lacking expression of one or the other protein display defective membrane repair process following acute injuries. Our previous studies show that MG53 can nucleate the assembly of the membrane repair machinery by facilitating translocation of intracellular vesicles to membrane injury sites. Dysferlin appears to participate in Ca2+-dependent fusion of these vesicles for formation of a repair patch, but dysferlin itself cannot translocate to the injury site in the absence of MG53. To test the complementary function of MG53 and dysferlin in cell membrane repair, we generated a double knockout mouse lacking both MG53 and dysferlin. While serum creatine kinase (CK) levels of dysferlin-/-, mg53-/-, and wild-type mice were not significantly different from each other, CK levels of the mg53-/-dysferlin-/- mice were significantly elevated at resting condition. This suggests a more severe membrane repair defect. Plasma membrane targeted UV-irradiation of isolated skeletal muscle fibers in the presence of a membrane impermeant dye (FM1-43) was used to directly assay membrane repair. This experimental procedure was modified from previously established protocols for a quantitative assessment of the muscle membrane repair capacity. Compared with dysferlin-/- and mg53-/- fibers, significant elevated entry of FM1-43 dye was observed in mg53-/-dysferlin-/- muscle following UV-irradiation. These results suggest that the involvement of these two proteins in the membrane repair mechanism is more complicated than that as two points along a linear pathway. The additive effects of MG53 and dysferlin in muscle membrane repair suggest the possibility that these two proteins may either sense different membrane injury signals, or that they may act at different compartments of the sarcolemma membrane (caveolae or transverse-tubule network).

Mechanisms underlying ICU muscle wasting and effects of passive mechanical loading

IntroductionCritically ill ICU patients commonly develop severe muscle wasting and impaired muscle function, leading to delayed recovery, with subsequent increased morbidity and financial costs, and decreased quality of life for survivors. Critical illness myopathy (CIM) is a frequently observed neuromuscular disorder in ICU patients. Sepsis, systemic corticosteroid hormone treatment and post-synaptic neuromuscular blockade have been forwarded as the dominating triggering factors. Recent experimental results from our group using a unique experimental rat ICU model show that the mechanical silencing associated with CIM is the primary triggering factor. This study aims to unravel the mechanisms underlying CIM, and to evaluate the effects of a specific intervention aiming at reducing mechanical silencing in sedated and mechanically ventilated ICU patients.MethodsMuscle gene/protein expression, post-translational modifications (PTMs), muscle membrane excitability, muscle mass measurements, and contractile properties at the single muscle fiber level were explored in seven deeply sedated and mechanically ventilated ICU patients (not exposed to systemic corticosteroid hormone treatment, post-synaptic neuromuscular blockade or sepsis) subjected to unilateral passive mechanical loading for 10 hours per day (2.5 hours, four times) for 9 ± 1 days.ResultsThese patients developed a phenotype considered pathognomonic of CIM; that is, severe muscle wasting and a preferential myosin loss (P < 0.001). In addition, myosin PTMs specific to the ICU condition were observed in parallel with an increased sarcolemmal expression and cytoplasmic translocation of neuronal nitric oxide synthase. Passive mechanical loading for 9 ± 1 days resulted in a 35% higher specific force (P < 0.001) compared with the unloaded leg, although it was not sufficient to prevent the loss of muscle mass.ConclusionMechanical silencing is suggested to be a primary mechanism underlying CIM; that is, triggering the myosin loss, muscle wasting and myosin PTMs. The higher neuronal nitric oxide synthase expression found in the ICU patients and its cytoplasmic translocation are forwarded as a probable mechanism underlying these modifications. The positive effect of passive loading on muscle fiber function strongly supports the importance of early physical therapy and mobilization in deeply sedated and mechanically ventilated ICU patients.

Effects of carnosine on contractile apparatus Ca2+sensitivity and sarcoplasmic reticulum Ca2+release in human skeletal muscle fibers

There is considerable interest in potential ergogenic and therapeutic effects of increasing skeletal muscle carnosine content, although its effects on excitation-contraction (EC) coupling in human muscle have not been defined. Consequently, we sought to characterize what effects carnosine, at levels attained by supplementation, has on human muscle fiber function, using a preparation with all key EC coupling proteins in their in situ positions. Fiber segments, obtained from vastus lateralis muscle of human subjects by needle biopsy, were mechanically skinned, and their Ca(2+) release and contractile apparatus properties were characterized. Ca(2+) sensitivity of the contractile apparatus was significantly increased by 8 and 16 mM carnosine (increase in pCa(50) of 0.073 ± 0.007 and 0.116 ± 0.006 pCa units, respectively, in six type I fibers, and 0.063 ± 0.018 and 0.103 ± 0.013 pCa units, respectively, in five type II fibers). Caffeine-induced force responses were potentiated by 8 mM carnosine in both type I and II fibers, with the potentiation in type II fibers being entirely explicable by the increase in Ca(2+) sensitivity of the contractile apparatus caused by carnosine. However, the potentiation of caffeine-induced responses caused by carnosine in type I fibers was beyond that expected from the associated increase in Ca(2+) sensitivity of the contractile apparatus and suggestive of increased Ca(2+)-induced Ca(2+) release. Thus increasing muscle carnosine content likely confers benefits to muscle performance in both fiber types by increasing the Ca(2+) sensitivity of the contractile apparatus and possibly also by aiding Ca(2+) release in type I fibers, helping to lessen or slow the decline in muscle performance during fatiguing stimulation.

Differential requirements for Myocyte Enhancer Factor-2 during adult myogenesis in Drosophila

Identifying the genetic program that leads to formation of functionally and morphologically distinct muscle fibers is one of the major challenges in developmental biology. In Drosophila, the Myocyte Enhancer Factor-2 (MEF2) transcription factor is important for all types of embryonic muscle differentiation. In this study we investigated the role of MEF2 at different stages of adult skeletal muscle formation, where a diverse group of specialized muscles arises. Through stage- and tissue-specific expression of Mef2 RNAi constructs, we demonstrate that MEF2 is critical at the early stages of adult myoblast fusion: mutant myoblasts are attracted normally to their founder cell targets, but are unable to fuse to form myotubes. Interestingly, ablation of Mef2 expression at later stages of development showed MEF2 to be more dispensable for structural gene expression: after myoblast fusion, Mef2 knockdown did not interrupt expression of major structural gene transcripts, and myofibrils were formed. However, the MEF2-depleted fibers showed impaired integrity and a lack of fibrillar organization. When Mef2 RNAi was induced in muscles following eclosion, we found no adverse effects of attenuating Mef2 function. We conclude that in the context of adult myogenesis, MEF2 remains an essential factor, participating in control of myoblast fusion, and myofibrillogenesis in developing myotubes. However, MEF2 does not show a major requirement in the maintenance of muscle structural gene expression. Our findings point to the importance of a diversity of regulatory factors that are required for the formation and function of the distinct muscle fibers found in animals.

Effects of ageing on single muscle fibre contractile function following short‐term immobilisation

Very little attention has been given to the combined effects of healthy ageing and short-term disuse on the contractile function of human single muscle fibres. Therefore, the present study investigated the effects of 2 weeks of lower limb cast immobilisation (i.e. disuse) on selected contractile properties of single muscle fibres (n = 378) from vastus lateralis of nine young (24 ± 1 years) and eight old (67 ± 2 years) healthy men with comparable levels of physical activity. Prior to immobilisation, MHC IIa fibres produced higher maximum Ca(2+)-activated force (approx. 32%) and specific force (approx. 33%) and had lower Ca(2+) sensitivity than MHC I fibres (P < 0.05), with no differences between young and old. After immobilisation, the decline in single fibre force (MHC I: young 21% and old 22%; MHC IIa: young 22% and old 30%; P < 0.05) as well as specific force (MHC I: young 14% and old 13%; MHC IIa: young 18% and old 25%; P < 0.05) was more pronounced in MHC IIa fibres compared to MHC I fibres (P < 0.05), with no differences between young and old. Notably, there was a selective decrease in Ca(2+) sensitivity in MHC IIa fibres of young (P < 0.05) and in MHC I fibres of old individuals (P < 0.05), respectively. In conclusion, 2 weeks of lower limb immobilisation caused greater impairments in single muscle fibre force and specific force in MHC IIa than MHC I fibres independently of age. In contrast, immobilisation-induced changes in Ca(2+) sensitivity that were dependent on age and MHC isoform.

Diaphragm muscle fiber function and structure in humans with hemidiaphragm paralysis

Recent studies proposed that mechanical inactivity of the human diaphragm during mechanical ventilation rapidly causes diaphragm atrophy and weakness. However, conclusive evidence for the notion that diaphragm weakness is a direct consequence of mechanical inactivity is lacking. To study the effect of hemidiaphragm paralysis on diaphragm muscle fiber function and structure in humans, biopsies were obtained from the paralyzed hemidiaphragm in eight patients with hemidiaphragm paralysis. All patients had unilateral paralysis of known duration, caused by en bloc resection of the phrenic nerve with a tumor. Furthermore, diaphragm biopsies were obtained from three control subjects. The contractile performance of demembranated muscle fibers was determined, as well as fiber ultrastructure and morphology. Finally, expression of E3 ligases and proteasome activity was determined to evaluate activation of the ubiquitin-proteasome pathway. The force-generating capacity, as well as myofibrillar ultrastructure, of diaphragm muscle fibers was preserved up to 8 wk of paralysis. The cross-sectional area of slow fibers was reduced after 2 wk of paralysis; that of fast fibers was preserved up to 8 wk. The expression of the E3 ligases MAFbx and MuRF-1 and proteasome activity was not significantly upregulated in diaphragm fibers following paralysis, not even after 72 and 88 wk of paralysis, at which time marked atrophy of slow and fast diaphragm fibers had occurred. Diaphragm muscle fiber atrophy and weakness following hemidiaphragm paralysis develops slowly and takes months to occur.

Myosin‐actin cross‐bridge kinetics explain variation in single skeletal muscle fiber function in humans

Our objective was to determine if myosin-actin cross-bridge kinetics, measured in their intact myofilament lattice, could account for variation in skeletal muscle fiber type specific performance among healthy and diseased humans. We applied small amplitude sinusoidal analysis for the first time to human skeletal muscle fibers obtained from 9 heart failure patients and 6 controls of similar age and activity level. In both groups, myosin heavy chain (MHC) isoform contractile velocity (I<IIA<IIX) was inversely related to myosin-actin attachment time (ton). The rate of myosin transition between weakly- and strongly-bound states (2πb) increased with MHC velocity, suggesting the myosin-actin detachment time (toff) decreases with faster contractions. These results suggest that total myosin-actin cycle time (ton + toff) decreases with contractile velocity due to reductions in both ton and toff in humans. Heart failure patients had an increased ton, which compensated for their reduced cross-bridge number due to MHC content loss, resulting in similar isometric tensions compared to healthy controls. Collectively, these results indicate the usefulness of cross-bridge kinetics and highlight their potential use for examining the effects of disease, muscle activity and aging on single fiber function. Support: NIH HL-077418 and AG-031303

Physiological Characterization of Muscle Strength With Variable Levels of Dystrophin Restoration in mdx Mice Following Local Antisense Therapy

Antisense-induced exon skipping can restore the open reading frame, and thus correct the dystrophin deficiency that causes Duchenne muscular dystrophy (DMD), a lethal muscle wasting condition. Successful proof-of-principle in preclinical models has led to human clinical trials. However, it is still not known what percentage of dystrophin-positive fibers and what level of expression is necessary for functional improvement. This study directly address these key questions in the mdx mouse model of DMD. To achieve a significant variation in dystrophin expression, we locally administered into tibialis anterior muscles various doses of a phosphorodiamidate morpholino oligomer (PMO) designed to skip the mutated exon 23 from the mRNA of murine dystrophin. We found a highly significant correlation between the number of dystrophin-positive fibers and resistance to contraction-induced injury, with a minimum of 20% of dystrophin-positive fibers required for meaningful improvement. Furthermore, our results also indicate that a relatively low level of dystrophin expression in muscle fibers may have significant clinical benefits. In contrast, improvements in muscle force were not correlated with either the number of positive fibers or total dystrophin levels, which highlight the need to conduct appropriate functional assessments in preclinical testing using the mdx mouse.

Differential Effect of Calsequestrin Ablation on Structure and Function of Fast and Slow Skeletal Muscle Fibers

We compared structure and function of EDL and Soleus muscles in adult (4–6 m) mice lacking both Calsequestrin (CASQ) isoforms, the main SR Ca2+-binding proteins. Lack of CASQ induced ultrastructural alterations in ~30% of Soleus fibers, but not in EDL. Twitch time parameters were prolonged in both muscles, although tension was not reduced. However, when stimulated for 2 sec at 100 hz, Soleus was able to sustain contraction, while in EDL active tension declined by 70–80%. The results presented in this paper unmask a differential effect of CASQ1&2 ablation in fast versus slow fibers. CASQ is essential in EDL to provide large amount of Ca2+ released from the SR during tetanic stimulation. In contrast, Soleus deals much better with lack of CASQ because slow fibers require lower Ca2+ amounts and slower cycling to function properly. Nevertheless, Soleus suffers more severe structural damage, possibly because SR Ca2+ leak is more pronounced.

The Recent Understanding of the Neurotrophin′s Role in Skeletal Muscle Adaptation

This paper summarizes the various effects of neurotrophins in skeletal muscle and how these proteins act as potential regulators of the maintenance, function, and regeneration of skeletal muscle fibers. Increasing evidence suggests that this family of neurotrophic factors influence not only the survival and function of innervating motoneurons but also the development and differentiation of myoblasts and muscle fibers. Muscle contractions (e.g., exercise) produce BDNF mRNA and protein in skeletal muscle, and the BDNF seems to play a role in enhancing glucose metabolism and may act for myokine to improve various brain disorders (e.g., Alzheimer's disease and major depression). In adults with neuromuscular disorders, variations in neurotrophin expression are found, and the role of neurotrophins under such conditions is beginning to be elucidated. This paper provides a basis for a better understanding of the role of these factors under such pathological conditions and for treatment of human neuromuscular disease.

HIGHLIGHTS FROM THE LITERATURE

HIGHLIGHTS FROM THE LITERATURE

Response of Alpha-actinin Isoforms to Mechanical Ventilation-induced Diaphragmatic Atrophy

Alpha-actinins are actin-binding proteins and important structural components of the Z-line. In mammalian skeletal muscle, the two isoforms are expressed, α-actinin-2 and α-actinin-3. α-actinin-2 is expressed in all skeletal muscles fibers, whereas α-actinin-3 expression is restricted to the fast fibers only, thus, α-actinin-3 is assumed to be associated with fast muscle fiber function. We previously examined α-actinin-3 level in the soleus muscle by using hindlimb unloading. This type of disuse induces the atrophy particularly in slow muscle fibers. In contrast, fast fibers are also atrophied by diaphragmatic inactivity, indicate that response of α-actinin-3 to atrophic stimuli with mechanical ventilation (MV) may differ from results of locomotor muscle disuse. PURPOSE: To examine the expression levels of α-actinin isoforms in rat diaphragm after MV. METHODS: Male Wistar rats were randomly divided into a control (n=5) or a 12h of MV (n=5) group. Mechanically ventilated animals were anesthetized, tracheostomized, and ventilated with 21% O2. Animals in the control group were acutely anesthetized but were not exposed to MV. After the treatment, diaphragms were removed, frozen with lipid nitrogen and stored at -80°C. Western blotting analysis was performed to Myosin heavy chain (MyHC) composition and expression of α-actinin isoforms. RESULTS: MV resulted in a decrease (14.0%, p < 0.01) in the cross-sectional area of diaphragm. MyHC composition did not differ between MV (I: 21.4, IIa: 19.3, IId/x: 49.2, IIb: 10.4%) and control (I: 22.2, IIa: 18.3, IId/x: 48.3, IIb: 11.2%) animals. Furthermore, there was no significant difference in expression levels of α-actinin isoforms between MV and control groups. CONCLUSION: The expression levels of α-actinin isoforms in rat diaphragm are not changed by the 12-h MV.

Prolonged space flight-induced alterations in the structure and function of human skeletal muscle fibres

The primary goal of this study was to determine the effects of prolonged space flight (180 days) on the structure and function of slow and fast fibres in human skeletal muscle. Biopsies were obtained from the gastrocnemius and soleus muscles of nine International Space Station crew members 45 days pre- and on landing day (R+0) post-flight. The main findings were that prolonged weightlessness produced substantial loss of fibre mass, force and power with the hierarchy of the effects being soleus type I > soleus type II > gastrocnemius type I > gastrocnemius type II. Structurally, the quantitatively most important adaptation was fibre atrophy, which averaged 20% in the soleus type I fibres (98 to 79 μm diameter). Atrophy was the main contributor to the loss of peak force (P(0)), which for the soleus type I fibre declined 35% from 0.86 to 0.56 mN. The percentage decrease in fibre diameter was correlated with the initial pre-flight fibre size (r = 0.87), inversely with the amount of treadmill running (r = 0.68), and was associated with an increase in thin filament density (r = 0.92). The latter correlated with reduced maximal velocity (V(0)) (r = 0.51), and is likely to have contributed to the 21 and 18% decline in V(0) in the soleus and gastrocnemius type I fibres. Peak power was depressed in all fibre types with the greatest loss (55%) in the soleus. An obvious conclusion is that the exercise countermeasures employed were incapable of providing the high intensity needed to adequately protect fibre and muscle mass, and that the crew's ability to perform strenuous exercise might be seriously compromised. Our results highlight the need to study new exercise programmes on the ISS that employ high resistance and contractions over a wide range of motion to mimic the range occurring in Earth's 1 g environment.

Syncoilin modulates peripherin filament networks and is necessary for large-calibre motor neurons

Syncoilin is an atypical type III intermediate filament (IF) protein, which is expressed in muscle and is associated with the dystrophin-associated protein complex. Here, we show that syncoilin is expressed in both the central and peripheral nervous systems. Isoform Sync1 is dominant in the brain, but isoform Sync2 is dominant in the spinal cord and sciatic nerve. Peripherin is a type III IF protein that has been shown to colocalise and interact with syncoilin. Our analyses suggest that syncoilin might function to modulate formation of peripherin filament networks through binding to peripherin isoforms. Peripherin is associated with the disease amyotrophic lateral sclerosis (ALS), thus establishing a link between syncoilin and ALS. A neuronal analysis of the syncoilin-null mouse (Sync(-/-)) revealed a reduced ability in accelerating treadmill and rotarod tests. This phenotype might be attributable to the impaired function of extensor digitorum longus muscle and type IIb fibres caused by a shift from large- to small-calibre motor axons in the ventral root.

Physiological Adaptations to Taper in Competitive Distance Runners

Little is known about the physiological response to taper in distance runners. The aim of this study was to examine the effects of a 3-wk taper (progressive reduction in training volume with no change in intensity) on performance and physiology of competitive distance runners. We implemented a 'whole body to gene' approach to study seven collegiate distance runners (20 ± 1 yr, 66 ± 1 kg) before and after taper. The primary measures included 8-km cross country race performance, gastrocnemius single muscle fiber size and function (peak force, shortening velocity, and power), resting and run-induced gene expression 4 hrs after a standardized 8-km indoor track run [pre-30:18 (min:sec), post-30:20], citrate synthase activity, and maximal and submaximal cardiovascular physiology (Vo2, VE, HR, and RER) before and after taper. The taper was successful at enhancing cross country race performance (-3%, P<0.05). MHC IIa fiber diameter (+7%, P<0.05), force (+11%, P=0.06), and absolute power (+9%, P<0.05) were elevated following taper. In addition to the MHC IIa fiber adaptations, taper elicited a distinct post-run gene response. Specifically, the induction of MuRF-1 was attenuated following taper, whereas MRF-4, HSP 72, and MT-2A displayed an exaggerated response to the standardized run (P<0.05). No changes were observed in MHC I size/function, resting gene expression, citrate synthase activity, or cardiovascular function. Our findings show that tapered training in competitive runners promoted MHC IIa fiber remodeling and a differential transcriptional response following the same exercise perturbation, while having no adverse effects on aerobic physiology. These data suggest that myocellular remodeling is a key ingredient for taper induced gains in running performance.

Myocellular basis for tapering in competitive distance runners

The purpose of this study was to examine the effects of a 3-wk taper on the physiology of competitive distance runners. We studied seven collegiate distance runners (20+/-1 yr, 66+/-1 kg) before and after a 3-wk taper. The primary measures included 8-km cross-country race performance, gastrocnemius single muscle fiber size and function (peak force, shortening velocity, and power), baseline and exercise-induced gene expression 4 h after a standardized 8-km run, citrate synthase activity, and maximal and submaximal cardiovascular physiology (oxygen consumption, ventilation, heart rate, and respiratory exchange ratio). Race performance improved by 3% following taper (P<0.05). Myosin heavy chain (MHC) IIa fiber diameter (+7%, P<0.05), peak force (+11%, P=0.06), and absolute power (+9%, P<0.05) increased following taper. In addition to the MHC IIa adaptations, taper elicited a distinct postexercise gene response. Specifically, the induction of MuRF-1 was attenuated following taper, whereas MRF4, HSP 72, and MT-2A displayed an exaggerated response (P<0.05). No changes were observed in MHC I size or function, baseline gene expression, citrate synthase activity, or cardiovascular function. Our findings show that tapered training in competitive runners promoted MHC IIa fiber remodeling and an altered transcriptional response following the same exercise perturbation, with no adverse affects on aerobic fitness. Together, these results provide a myocellular basis for distance runners to taper in preparation for peak performance.

Effect of short-term gravitational unloading on rat and mongolian gerbil muscles

Gravitational unloading leads to destructive changes in the structure and function of muscle fibers. However, the role of the EMG activity level is still unclear. We measured changes caused by one- and three-day hypogravity in the following muscles: Soleus (Sol), Tibialis anterior (TA) and Gastrocnemius c.m. (MG). We used Wistar rats and Mongolian gerbils. The following parameters were assessed: the specific force of contraction of isolated fibers by tensometry, the transverse stiffness of the contractile apparatus by atomic force microscopy, and the calcium content by Fluo-4. We detected the accumulation of calcium ions in all muscles even after one-day unloading. In Sol this effect was more significant than in other muscles. After one-day of hypogravity we detected an increase in the specific force in all muscle types and species. Meanwhile, the transverse stiffness of the contractile apparatus, M-band and Z-disc increased only in fast muscles but not in Sol. After three-days of unloading, the specific force in Sol decreased, and the transverse stiffness of the contractile apparatus behaved in the same way as the force. The specific tension of fast muscle fibers decreased significantly in comparison with one-day unloading. In addition, the transverse stiffness of some areas of MG had a tendency to decrease in comparison to "one-day" unloading, although there was no such a tendency in the fibers of TA. In Mongolian gerbils the tendencies were the same as in the rats, but showed less dramatic changes. The reduction in the magnitude of changes in the Sol-MG-TA series correlates with EMG activity.

Changed mitochondrial function by pre- and/or postpartum diet alterations in sheep

In a sheep model, we investigated diet effects on skeletal muscle mitochondria to look for fetal programming. During pregnancy, ewes were fed normally (N) or were 50% food restricted (L) during the last trimester, and lambs born to these ewes received a normal (N) or a high-fat diet (H) for the first 6 mo of life. We examined mitochondrial function in permeabilized muscle fibers from the lambs at 6 mo of age (adolescence) and after 24 mo of age (adulthood). The postpartum H diet for the lambs induced an approximately 30% increase (P < 0.05) of mitochondrial VO(2max) and an approximately 50% increase (P < 0.05) of the respiratory coupling ratio (RCR) combined with lower levels of UCP3 and PGC-1alpha mRNA levels (P < 0.05). These effects proved to be reversible by a normal diet from 6 to 24 mo of age. However, at 24 mo, a long-term effect of the maternal gestational diet restriction (fetal programming) became evident as a lower VO(2max) (approximately 40%, P < 0.05), a lower state 4 respiration (approximately 40%, P < 0.05), and lower RCR ( approximately 15%, P < 0.05). Both PGC-1alpha and UCP3 mRNA levels were increased (P < 0.05). Two analyzed muscles were affected differently, and muscle rich in type I fibers was more susceptible to fetal programming. We conclude that fetal programming, seen as a reduced VO(2max) in adulthood, results from gestational undernutrition. Postnatal high-fat diet results in a pronounced RCR and VO(2max) increase in adolescence. However, these effects are reversible by diet correction and are not maintained in adulthood.

The Brazilian Consensus on the Management of Pompe Disease

T he Brazilian Consensus on the Management of Pompe Disease was established through collaboration of all Brazilian physicians known to be treating patients with Pompe disease (PD) in Brazil. It was based on the clinical presentation of 7 cases of early-onset PD (EOPD) and 18 cases of late-onset PD (LOPD) presented to a working group in the city of Rio de Janeiro in 2007. Coordinated by the Brazilian Network for Studies in PD (ReBrPOM) with key objectives of enhancing understanding of the clinical heterogeneity, progression, and natural history of this severe and lethal genetic disease, as well as providing an overview of the Brazilian experience with enzyme replacement therapy with recombinant human acid alpha glucosidase (rhGAA; Myozyme; Genzyme, Cambridge, Massachusetts). This is not an exhaustive investigation of the issue, but rather preliminary guidelines to optimize the management of these patients under the care of the Brazilian public health care system (SUS-Brasil). PD (MIM 232300), also known as glycogen storage disease type II or acid maltase deficiency, is rare, progressive, and, in its early form, often fatal. The disease is inherited in an autosomal recessive fashion and is caused by a deficiency of the enzyme acid alpha-glucosidase (GAA; 3.2.1.20), which has an intralysosomal action and is responsible for releasing glucose units from glycogen. Glycogen buildup occurs within lysosomes of several tissues, particularly in skeletal and cardiac striated muscle cells in early-onset PD (EOPD), destroying the cells and compromising muscle fiber function. The disease may present in the first year of life, characterized mainly by hypertrophic cardiomyopathy and generalized muscular hypotonia, or after the first year through the sixth decade of life, with progressive proximal muscular weakness and respiratory complaints as the major symptoms.

Overexpression of MHC Class I Heavy Chain Protein in Young Skeletal Muscle Leads to Severe Myositis: Implications for Juvenile Myositis

Folding and transport of proteins, such as major histocompatibility complex (MHC) class I, through the endoplasmic reticulum (ER) is tightly regulated in all cells, including muscle tissue, where the specialized ER sarcoplasmic reticulum is also critical to muscle fiber function. Overexpression of MHC class I protein is a common feature of many muscle pathologies including idiopathic myositis and can induce ER stress. However, there has been no comparison of the consequences of MHC overexpression in muscle at different ages. We have adapted a transgenic model of myositis induced by overexpression of MHC class I protein in skeletal muscle to investigate the effects of this protein overload on young muscle fibers, as compared with adult tissue. We find a markedly more severe disease phenotype in young mice, with rapid onset of muscle weakness and pathology. Gene expression profiling to compare the two models indicates rapid onset of ER stress in young muscle tissue but also that gene expression of key muscle structural proteins is affected more rapidly in young mice than adults after this insult. This novel model has important implications for our understanding of muscle pathology in dermatomyositis of both adults and children.