Slow Muscle Fibers Research Articles

N-cadherin and M-cadherin are sequentially expressed in myoblast populations contributing to the first and second waves of myogenesis in the trout (Oncorhynchus mykiss)

The objective of this study was to investigate the expression of two promyogenic cell surface adhesion receptors, N- and M-cadherin, in developing trout (Oncorhynchus mykiss) somite, taking account of the recent identification of a dermomyotome-like epithelium in teleosts. In situ hybridization showed that N-cadherin was expressed throughout the paraxial mesoderm and nascent somite. As the somite matured, N-cadherin expression disappeared ventrally from the sclerotome, and then mediolaterally from the differentiating slow and fast muscle cells of the embryonic myotome, to become finally restricted to the undifferentiated myogenic precursors forming the dermomyotome-like epithelium that surrounds the embryonic myotome. By contrast, M-cadherin, which was transcribed in the differentiating embryonic myotome, was never expressed in the dermomyotome-like epithelium. In late-stage trout embryos, M-cadherin transcript was only detected at the periphery of the expanding myotome, where muscle cells stemming from the N-cadherin positive dermomyotome-like epithelium differentiate. Collectively, our results support the view that, in trout embryo, N-cadherin is associated with muscle cell immaturity while M-cadherin is associated with muscle cell maturation and differentiation and this during the two successive phases of myogenesis.

PPARδ regulates satellite cell proliferation and skeletal muscle regeneration

Peroxisome proliferator-activated receptors (PPARs) are a class of nuclear receptors that play important roles in development and energy metabolism. Whereas PPARδ has been shown to regulate mitochondrial biosynthesis and slow-muscle fiber types, its function in skeletal muscle progenitors (satellite cells) is unknown. Since constitutive mutation of Pparδ leads to embryonic lethality, we sought to address this question by conditional knockout (cKO) of Pparδ using Myf5-Cre/Pparδflox/flox alleles to ablate PPARδ in myogenic progenitor cells. Although Pparδ-cKO mice were born normally and initially displayed no difference in body weight, muscle size or muscle composition, they later developed metabolic syndrome, which manifested as increased body weight and reduced response to glucose challenge at age nine months. Pparδ-cKO mice had 40% fewer satellite cells than their wild-type littermates, and these satellite cells exhibited reduced growth kinetics and proliferation in vitro. Furthermore, regeneration of Pparδ-cKO muscles was impaired after cardiotoxin-induced injury. Gene expression analysis showed reduced expression of the Forkhead box class O transcription factor 1 (FoxO1) gene in Pparδ-cKO muscles under both quiescent and regenerating conditions, suggesting that PPARδ acts through FoxO1 in regulating muscle progenitor cells. These results support a function of PPARδ in regulating skeletal muscle metabolism and insulin sensitivity, and they establish a novel role of PPARδ in muscle progenitor cells and postnatal muscle regeneration.

The Methyltransferases PRMT4/CARM1 and PRMT5 Control Differentially Myogenesis in Zebrafish

In vertebrates, skeletal myogenesis involves the sequential activation of myogenic factors to lead ultimately to the differentiation into slow and fast muscle fibers. How transcriptional co-regulators such as arginine methyltransferases PRMT4/CARM1 and PRMT5 control myogenesis in vivo remains poorly understood. Loss-of-function experiments using morpholinos against PRMT4/CARM1 and PRMT5 combined with in situ hybridization, quantitative polymerase chain reaction, as well as immunohistochemistry indicate a positive, but differential, role of these enzymes during myogenesis in vivo. While PRMT5 regulates myod, myf5 and myogenin expression and thereby slow and fast fiber formation, PRMT4/CARM1 regulates myogenin expression, fast fiber formation and does not affect slow fiber formation. However, our results show that PRMT4/CARM1 is required for proper slow myosin heavy chain localization. Altogether, our results reveal a combinatorial role of PRMT4/CARM1 and PRMT5 for proper myogenesis in zebrafish.

Anal sphincter fatigue: Is the mechanism peripheral or central?

Striated muscles of continence appear to exhibit marked fatigue during voluntary efforts. This is counterintuitive considering the high proportion of slow twitch muscle fibers. One explanation is that fatigue is due to central, rather than peripheral mechanisms. Here we examined the contribution of reduced voluntary activation (central fatigue) to the decline in anal sphincter (AS) and elbow flexor muscle force during voluntary contractions. Ten healthy subjects participated. Fatigue was induced using 10 maximal voluntary contractions sustained for 20 s. During each fatiguing contraction, transcranial magnetic stimulation (TMS) was delivered over the motor cortex at 5 s intervals. Central fatigue was assessed using the superimposed twitch force elicited by TMS. Peripheral fatigue was measured using brachial plexus (elbow flexors) or sacral plexus (AS) stimulation during contraction and at rest. Ability to maximally activate AS (75.9%) was less than for the elbow flexors at baseline (91.6%). Voluntary activation declined in both muscles, but the decline was greater in AS (AS 28%; elbow flexors 12%). There was no change in the amplitude of the twitch evoked by peripheral nerve stimulation in either muscle. AS exhibits a greater decline in voluntary activation during fatiguing contractions than elbow flexor muscles. This is not accompanied by peripheral changes, which implies central mechanisms are responsible. Thus, we conclude that AS is susceptible to central fatigue during maximal voluntary activations. We propose this may be a protective mechanism to conserve contractile potential of the anal sphincter for function.

Species-specific expression variation of fish MYH14, an ancient vertebrate myosin heavy chain gene orthologue

MYH14 (MYH7b) is the most recently identified sarcomeric myosin heavy chain gene (MYH) from the human genome. Recent observations have revealed that MYH M5 , a torafugu orthologue of MYH14, is one of the major components of the MYH repertoire expressed in torafugu embryos and adults, suggesting its pivotal role in fish muscle formation. We have examined the expression pattern of MYH14 in zebrafish, a model organism for the study of vertebrate development. In situ hybridization studies revealed that zebrafish MYH14 was expressed in the most myotomal region containing fast muscle fibers in the embryonic stage and in superficial slow muscle fibers in the adult stage. Weak signals of MYH14 transcripts were also detected in intermediate muscle fibers located between superficial slow and inner fast muscle fibers in the adult. Reverse transcription-PCR studies showed that MYH14 was not only expressed in skeletal muscles but also in cardiac muscles. These expression patterns are in a marked contrast to that of torafugu MYH M5 , which is solely expressed in slow and cardiac muscle fibers in both the embryonic and adult stages. Our results imply species-specific functions of MYH14 in fish muscle formation.

Non-invasive Estimation Of Muscle Fiber Type Composition In Elite Athletes

It has been established that excellence in sports with short and long exercise duration requires a high proportion of fast and slow twitch muscle fibers, respectively. Until today, the muscle biopsy method is still accepted as gold standard to measure muscle fiber type composition. Because of the invasive nature and high sampling variance of a muscle biopsy, it would be useful to develop a non-invasive alternative, which can be used as a tool for talent identification. PURPOSE: To develop a new and non-invasive estimation method of the fiber type composition in human muscles, based on proton magnetic resonance spectroscopy (1H-MRS) measurement of muscle carnosine content, a typical fast twitch metabolite. METHODS: A total of 160 healthy subjects volunteered to participate in this cross-sectional study. The 4 subgroups consisted of 83 untrained control subjects (47 males and 36 females), 12 talented young male track-and-field athletes, 51 active elite athletes (40 males and 11 females) and 14 male ex-athletes. The carnosine content was measured in the gastrocnemius medialis muscle by 1H-MRS. RESULTS: The muscle carnosine content was ∼30 % higher in explosive athletes (6.58 ± 0.92 mM), such as sprint runners, compared to a reference population (4.94 ± 1.43 mM), whereas it was ∼20 % lower than normal in typical endurance athletes (3.98 ± 0.64 mM), such as 3,000-marathon runners. Decathletes showed intermediate carnosine levels (5.32 ± 0.72 mM). When active elite runners were ranked according to their best running distance, a negative sigmoidal curve (R2=0.9874) was found between the logarithm of running distance and muscle carnosine content. Noteworthy is that the muscle carnosine concentration remained significantly different between ex-sprinters and ex-endurance athletes (5.11 ± 1.07 vs. 3.61 ± 0.81 mM; p=0.012), who had discontinued training for many years already. Moreover, a similar difference was observed in young talents between explosive (7.04 ± 2.49 mM) and endurance (4.90 ± 0.93 mM) athletes. This suggests that the muscle carnosine content is probably genetically determined. CONCLUSION:1H-MRS based muscle carnosine quantification is a new non-invasive method for the estimation of muscle fiber type composition. Supported by Research Foundation - Flanders (FWO 1.5.149.08)

Expression of a Dominant Negative CELF Protein In Vivo Leads to Altered Muscle Organization, Fiber Size, and Subtype

BackgroundCUG-BP and ETR-3-like factor (CELF) proteins regulate tissue- and developmental stage-specific alternative splicing in striated muscle. We previously demonstrated that heart muscle-specific expression of a nuclear dominant negative CELF protein in transgenic mice (MHC-CELFΔ) effectively disrupts endogenous CELF activity in the heart in vivo, resulting in impaired cardiac function. In this study, transgenic mice that express the dominant negative protein under a skeletal muscle-specific promoter (Myo-CELFΔ) were generated to investigate the role of CELF-mediated alternative splicing programs in normal skeletal muscle.Methodology/Principal FindingsMyo-CELFΔ mice exhibit modest changes in CELF-mediated alternative splicing in skeletal muscle, accompanied by a reduction of endomysial and perimysial spaces, an increase in fiber size variability, and an increase in slow twitch muscle fibers. Weight gain and mean body weight, total number of muscle fibers, and overall muscle strength were not affected.Conclusions/SignificanceAlthough these findings demonstrate that CELF activity contributes to the normal alternative splicing of a subset of muscle transcripts in vivo, the mildness of the effects in Myo-CELFΔ muscles compared to those in MHC-CELFΔ hearts suggests CELF activity may be less determinative for alternative splicing in skeletal muscle than in heart muscle. Nonetheless, even these small changes in CELF-mediated splicing regulation were sufficient to alter muscle organization and muscle fiber properties affected in myotonic dystrophy. This lends further evidence to the hypothesis that dysregulation of CELF-mediated alternative splicing programs may be responsible for the disruption of these properties during muscle pathogenesis.

Skeletal muscle myosin light chain composition of highly‐trained endurance runners

Individual slow (MHC I) and fast (MHC IIa) twitch muscle fibers from highly-trained competitive runners display a greater contractile velocity (Vo) compared to moderately-active individuals. Myosin light chains (MLC) have been implicated in altering myofiber Vo, but this relationship is ambiguous in human skeletal muscle. Therefore, the purpose of this study was to examine MLC composition and its association with single muscle fiber contractile function in competitive (RUN; 71±1 ml·kg−1·min−1, 95±2 km/wk running) and recreational runners (REC; 47±2 ml·kg−1·min−1, <25 km/wk). SDS-PAGE, silver staining, and densitometry were used to assess MLC composition in mixed muscle homogenates and individual myofibers (MHC I and IIa) obtained from skeletal muscle biopsy samples. At the mixed muscle level, MLC 2f was lower (p<0.05) in RUN compared to REC with trends (p<0.08) for greater 1s, 2s and lower 3f ratios. The MLC profile of MHC I fibers was similar between RUN and REC. However, the MLC profile of the MHC IIa fibers from RUN contained less MLC 1s and greater amounts of 1f and 2s (p<0.05) compared to REC. These data suggest that only subtle differences exist in the MLC profile between RUN and REC. Therefore, it is likely that MLC composition contributes only a minor, if any, role in the robust differences in contractile function between competitive and recreational runners. Supported by NIH Grant AG032127

BFR Exercise Increases S6K1 Phosphorylation in Type‐I and Type‐II Skeletal Muscle Fibers

Restriction of blood flow to a contracting muscle during low-intensity resistance exercise (BFR exercise) stimulates mTORC1 signaling and muscle protein synthesis during post-exercise recovery. However, it is unknown if mTORC1 activation is occurring in all muscle fiber types. The aim of our study was to determine whether ribosomal S6 kinase 1 (S6K1), a downstream effector of mTORC1, is activated in primarily fast twitch muscle fibers. Young men performed 4 sets of leg extensions at a 20% 1-RM while a pressure cuff was inflated restricting blood flow to the quadriceps, then released immediately following exercise. Muscle biopsies were collected from the vastus lateralis at baseline, 1 and 3 hr post-exercise. Western blotting confirmed an increase in mTOR, S6K1, and S6 phosphorylation in whole muscle homogenates post-exercise. Epifluorescence microscopy was used to assess mTORC1 activation in individual fiber types of muscle samples. We show that phospho-S6K1 is localized to the perinuclear space prior to exercise and following exercise there is a progressive increase in S6K1 phosphorylation and translocation to the sarcoplasm in type-I and type-II muscle fibers. Our preliminary results suggest that BFR exercise increases mTORC1 activation in both slow and fast-twitch muscle fiber types. Supported by NIH/NIAMS RO1 AR049877, P30AG024832, NIH 1UL1RR029876-01 and T32HD07539.

Fine-tuning of Hh signaling by the RNA-binding protein Quaking to control muscle development

The development of the different muscles within the somite is a complex process that involves the Hedgehog (Hh) signaling pathway. To specify the proper number of muscle cells and organize them spatially and temporally, the Hh signaling pathway needs to be precisely regulated at different levels, but only a few factors external to the pathway have been described. Here, we report for the first time the role of the STAR family RNA-binding protein Quaking A (QkA) in somite muscle development. We show in zebrafish that the loss of QkA function affects fast muscle fiber maturation as well as Hh-induced muscle derivative specification and/or morphogenesis. Mosaic analysis reveals that fast fiber maturation depends on the activity of QkA in the environment of fast fiber progenitors. We further show that Hh signaling requires QkA activity for muscle development. By an in silico approach, we screened the 3'UTRs of known Hh signaling component mRNAs for the Quaking response element and found the transcription factor Gli2a, a known regulator of muscle fate development. Using destabilized GFP as a reporter, we show that the gli2a mRNA 3'UTR is a functional QkA target. Consistent with this notion, the loss of QkA function rescued slow muscle fibers in yot mutant embryos, which express a dominant-negative Gli2a isoform. Thus, our results reveal a new mechanism to ensure muscle cell fate diversity by fine-tuning of the Hh signaling pathway via RNA-binding proteins.

Zebrafish deficient for Muscleblind-like 2 exhibit features of myotonic dystrophy.

SUMMARYMyotonic dystrophy (DM; also known as dystrophia myotonica) is an autosomal dominant disorder that affects the heart, eyes, brain and endocrine system, but the predominant symptoms are neuromuscular, with progressive muscle weakness and wasting. DM presents in two forms, DM1 and DM2, both of which are caused by nucleotide repeat expansions: CTG in the DMPK gene for DM1 and CCTG in ZNF9 (CNBP) for DM2. Previous studies have shown that the mutant mRNAs containing the transcribed CUG or CCUG repeats are retained within the nuclei of cells from individuals with DM, where they bind and sequester the muscleblind-like proteins MBNL1, MBNL2 and MBNL3. It has been proposed that the sequestration of these proteins plays a key role in determining the classic features of DM. However, the functions of each of the three MBNL genes are not completely understood. We have generated a zebrafish knockdown model in which we demonstrate that a lack of mbnl2 function causes morphological abnormalities at the eye, heart, brain and muscle levels, supporting an essential role for mbnl2 during embryonic development. Major features of DM are replicated in our model, including muscle defects and splicing abnormalities. We found that the absence of mbnl2 causes disruption to the organization of myofibrils in skeletal and heart muscle of zebrafish embryos, and a reduction in the amount of both slow and fast muscle fibres. Notably, our findings included altered splicing patterns of two transcripts whose expression is also altered in DM patients: clcn1 and tnnt2. The studies described herein provide broader insight into the functions of MBNL2. They also lend support to the hypothesis that the sequestration of this protein is an important determinant in DM pathophysiology, and imply a direct role of MBNL2 in splicing regulation of specific transcripts, which, when altered, contributes to the DM phenotype.

The SR Calcium Content of Fast Muscle Fibres Lacking Calsequestrin is Reduced and not Sufficient for Sustained Contractions

Calsequestrin (CASQ) is an acidic, high capacity Ca2+-binding protein located within the terminal cisternae of the sarcoplasmic reticulum (SR). CASQ1 is the major SR Ca2+- buffer in fast muscle fibers, whereas CASQ2 predominates in cardiomyocytes and slow muscle fibers. Mice devoid of CASQ were generated by crossing mice lacking CASQ1 (Paolini et al J Physiol 2007, 583, 767) and mice lacking CASQ2 (Knollman et al J Clin Inv 2006, 116, 2510). When studied ex vivo, fast muscles (EDL) showed an earlier decline in tension during tetanic stimulation than WT fibers. This was not the case in slow muscle, soleus. Such decline of the developed tension was compatible with the decay of free cytosolic calcium during repeated stimulation observed using Fura-2 in single intact fast FDB muscle fibers. Single permeabilized fibers from EDL and tibialis anterior showed a reduction of the amount of calcium released by 30 mM caffeine, which is supposed to release about 80% of SR calcium (Murphy et al. J Physiol 2009, 587, 443), thus suggesting a depletion of SR. The depletion was confirmed by measurements of SR fee [Ca2+] using a FRET-based indicator (D1ER) genetically targeted to the SR. During contraction a massive/marked reduction in intraluminal free [Ca2+] was observed to a level close to full depletion.The results are consistent with the function of CASQ as a intraluminal SR buffer. CASQ1/2 null mice lack sufficient compensation for the loss in buffering power by other SR proteins. The diversity between slow and fast muscles might be attributed to the presence of parvalbumin as a relevant sink of calcium in cytosol of fast but not slow muscles (Celio et al Nature 1982, 297, 504).

Uncovering Effects of the Familial Hypertrophic Cardiomyopathy (fHCM) Related ß-Myosin Mutation Arg723gly in Human Cardiomyocytes

Mutations in sarcomeric proteins have been identified as the major cause of fHCM. The primary functional effects of these mutations at the sarcomere level of cardiomyocytes, however, are still largely unknown. To address this point we studied cardiomyocytes of the left ventricle (LV) from explanted hearts of two patients with ß-myosin (ß-MHC) mutation R723G (Enjuto et al., JMCC 2000). Our measurements revealed reduced maximum force generation of myocytes but unchanged calcium-sensitivity. In contrast, previous studies on slow skeletal muscle fibers expressing ß-MHC with the same mutation showed reduced calcium-sensitivity and increased maximum force. To address this discrepancy we first determined the expression of mutated and wildtype ß-MHC-mRNA in LV tissue. 62±2% of total ß-MHC-mRNA was found to contain the mutation, just as seen in M. soleus. Secondly, similar to previous findings for failing human heart, gel electrophoresis of the fHCM cardiac tissue showed reduced phosphorylation of troponins I (TnI) and T, myosin binding protein C (MyBP-C), and myosin light chain 2 compared to donor tissue. Adjustment of phosphorylation of TnI and MyBP-C in donor and HCM myocytes by treatment with protein kinase A (PKA), however, uncovered an originally masked reduction in calcium-sensitivity while maximum force was not affected by PKA treatment. Electron microscopy showed reduced myofibrillar density in cardiac tissue samples of the patients which may account for the reduced force. In conclusion, our study reveals that secondary, adaptational processes, triggered by the fHCM-related mutation, can obscure primary effects.

Static Stiffness in Slow and Fast Mouse Muscle Fibers Expressing Different Titin Isoforms

We showed previously (Bagni et al., 2002) that most of the increase of muscle fiber stiffness during the early phases of a tetanic contraction is due to a non-crossbridge sarcomere component whose stiffness (called static stiffness) increases after stimulation with a time course very similar to the internal Ca2+ concentration. This led us to speculate that Ca2+ concentration, in addition to promote crossbridge formation, could also leads to a stiffening of a sarcomere structure, identified with the titin filament, either directly or through a titin-actin interaction leading to the observed sarcomere stiffness increase. According to this hypothesis, it is expected that static stiffness has different properties in muscles expressing titin with different mechanical properties. Therefore we compared the static stiffness values in soleus and EDL adult mouse muscles, which express titin isoforms with long and short PEVK segment, respectively. Considering that Ca2+ binding to E-rich motifs in the PEVK segment increases its bending rigidity, the higher proportion of these motifs in EDL compared to soleus is expected to lead to a greater static stiffness in EDL. Our results showed that in agreement with the titin hypothesis, the static stiffness measured in single fibers at 25°C was more than five times greater in EDL than in soleus and about two times greater than previously reported on FDB muscle. The static stiffness time course in EDL was about the same as in FDB but slightly faster than in soleus, and it became much faster at 35°C in both EDL and soleus similarly to tension time course. These results are in agreement with the idea that static stiffness depends on the increment of titin stiffness due to the interaction between Ca2+ and E-rich motifs in PEVK segment.

Cav1.1 Acts as a Voltage Sensor for Two Separate Processes in Skeletal Muscle with Different Voltage Dependence

In adult muscle fibers, tetanic electrical stimulation induces two separate calcium signals, a fast one related to contraction and dependent on Cav1.1 (DHPR) and RyRs and a slow signal, dependent on DHPR and IP3Rs. We have recently shown that the amplitude of this slow signal is dependent on frequency of stimulation, having a maximum at 20 Hz. Importantly, this signal participates in the activation of genes related to slow muscle fiber type phenotype. This signal is inhibited by nifedipine, supporting a role for the DHPR in its onset. We have stimulated adult muscle fibers with different concentrations of extracellular K+, and found calcium signals at more negative membrane potential than those described for triggering calcium release associated with RyR. Moreover, in the same conditions, we observe an increase in mRNA of the slow isoform of TroponinI, as seen in fibers stimulated with a 20Hz train of pulses. These signals have an intracellular origin, because they are still present when working in solutions with no extracellular calcium and 0.1 mM EGTA. Interestingly, they are inhibited by incubation of fibers with 20 uM XestosponginB, a specific inhibitor of IP3R. These results strongly suggest that the slow signal fires at membrane potential values more negative (Nernst potential estimated −50 mV) than the fast signal (E-C coupling related), where the fast signal threshold (−30 to −20 mV) has not yet been reached. This opens the possibility for a different part of the DHPR molecule being involved in the signal transmission of depolarization to IP3-production machinery, independently of the process of E-C coupling.FONDECYT 1080120, FONDAP 15010006.

Visualization, characterization and modulation of calcium signaling during the development of slow muscle cells in intact zebrafish embryos

Intact zebrafish embryos were used as an in vivo animal model to investigate the role of Ca2+ signaling during the differentiation of slow muscle cells (SMCs) within forming skeletal muscle. Transgenic zebrafish were generated using an a-actin promoter that targeted apoaequorin expression specifically to muscle cells. Two distinct Ca2+ signaling periods (CSPs) were visualized in the developing SMCs: between ~17.5-19.5 hours post-fertilization (hpf) and after ~23 hpf, separated by a ~3.5 h Ca2+ signaling quiet period. Further spatial characterization of these Ca2+ signals using confocal fluorescent microscopy and calcium green-1 dextran as a reporter, indicated that the earlier CSP displayed distinct nuclear and cytoplasmic components, whereas the later CSP was predominantly cytoplasmic. Both CSPs consisted of a series of oscillating Ca2+ waves generated at distinct frequencies, while the earlier CSP also displayed a slow rise then fall in the Ca2+ baseline-level. Imaging of cyclopamine- and forskolin-treated wild-type, or smo-/- mutant embryos, where SMCs do not form, confirmed the specific cell population generating the signals. Treating embryos with antagonists indicated that both IP3Rs and RyRs are responsible for generating the temporal characteristics of the Ca2+ signaling signature, and that the latter plays a necessary role in SMC differentiation and subsequent myotome patterning. Together, these data support and extend the proposition that specific spatiotemporal patterns of spontaneous Ca2+ signals might be used for different as well as combinatorial regulation of both nuclear and cytosolic signal transduction cascades, resulting in myofibrillogenesis in SMCs as well as myotome patterning.

Cytoskeletal heart-enriched actin-associated protein (CHAP) is expressed in striated and smooth muscle cells in chick and mouse during embryonic and adult stages

We recently identified a new Z-disc protein, CHAP (Cytoskeletal Heart-enriched Actin-associated Protein), which is expressed in striated muscle and plays an important role during embryonic muscle development in mouse and zebrafish. Here, we confirm and further extend these findings by (i) the identification and characterization of the CHAP orthologue in chick and (ii) providing a detailed analysis of CHAP expression in mouse during embryonic and adult stages. Chick CHAP contains a PDZ domain and a nuclear localization signal, resembling the human and mouse CHAPa. CHAP is expressed in the developing heart and somites, as well as muscle precursors of the limb buds in mouse and chick embryos. CHAP expression in heart and skeletal muscle is maintained in adult mice, both in slow and fast muscle fibers. Moreover, besides expression in striated muscle, we demonstrate that CHAP is expressed in smooth muscle cells of aorta, carotid and coronary arteries in adult mice, but not during embryonic development.

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 Effect of Training on Motoneuron Survival in Amyotrophic Lateral Sclerosis: Which Motoneuron Type is Saved?

The Effect of Training on Motoneuron Survival in Amyotrophic Lateral Sclerosis: Which Motoneuron Type is Saved?

Up-regulation of the vitamin C transporter SVCT2 upon differentiation and depolarization of myotubes

In addition to its role as a strong antioxidant, vitamin C regulates the differentiation of several cell lineages. In vertebrate skeletal muscle, the vitamin C transporter SVCT2 is preferentially expressed in slow muscle fibers. To gain insights into the possible involvement of intracellular vitamin C on early myogenesis, we investigated the regulation of SVCT2 expression in cultures of chick fetal myoblasts. SVCT2 expression increases in cultures of both, slow and fast muscle-derived myoblasts, as they fuse to form mainly fast myotubes. Interestingly, we found that SVCT2 could be positively modulated by potassium-induced depolarization of myotubes. These findings suggest that SVCT2-mediated uptake of vitamin C could play diverse roles on skeletal muscle development and physiology.