Paradoxical absence of M lines and downregulation of creatine kinase in mouse extraocular muscle

Effect of eccentric exercise on markers of muscle damage in patients with chronic obstructive pulmonary disease

Background: Eccentric exercise may be considered as an attractive alternative to conventional exercise in pulmonary rehabilitation (PR) for patients with chronic obstructive pulmonary disease (COPD). However, due to muscle damage associated with eccentric exercise, there has been reluctance in using this exercise form in PR.Objective: The aim of the present study was to investigate the effect of eccentric exercise on markers of muscle damage in patients with COPD.Methods: We analyzed 14 patients with moderate-severe COPD and 14 age-matched healthy controls. Both groups performed submaximal eccentric exercise of the elbow flexors. Muscle soreness (MS), maximum voluntary isometric contraction (MVC) of the elbow flexors, elbow range of motion (ROM), upper arm circumference (CIR), and biochemical markers such as creatine Kinase (CK) and lactate Dehydrogenase (LDH) were measured at pre-exercise, 24 h, 48 h, and 72 h following submaximal eccentric exercise.Results: There was a significant difference in markers of muscle damage, MS (p = .002), MVC (p < .001), ROM (p = .010), CIR (p < .001), and LDH (p = .001). However, no significant differences were observed in the activity of CK (p = .261) between COPD and control group following eccentric exercise which indicates greater degree of muscle damage in COPD as compared with control.Conclusion: Sub-maximal eccentric exercise causes significantly greater muscle damage in elderly COPD patients than healthy controls. Therefore, initial exercise should be progressed with lower intensities to prevent undue muscle damage in these patients.

Genomic profiling reveals Pitx2 controls expression of mature extraocular muscle contraction-related genes

PURPOSE

To assess the influence of the Pitx2 transcription factor on the global gene expression profile of extraocular muscle (EOM) of mice.

METHODS

Mice with a conditional knockout of Pitx2, designated Pitx2(Δflox/Δflox) and their control littermates Pitx2(flox/flox), were used. RNA was isolated from EOM obtained at 3, 6, and 12 weeks of age and processed for microarray-based profiling. Pairwise comparisons were performed between mice of the same age and differentially expressed gene lists were generated. Select genes from the profile were validated using real-time quantitative polymerase chain reaction and protein immunoblot. Ultrastructural analysis was performed to evaluate EOM sarcomeric structure.

RESULTS

The number of differentially expressed genes was relatively small. Eleven upregulated and 23 downregulated transcripts were identified common to all three age groups in the Pitx2-deficient extraocular muscle compared with littermate controls. These fell into a range of categories including muscle-specific structural genes, transcription factors, and ion channels. The differentially expressed genes were primarily related to muscle contraction. We verified by protein and ultrastructural analysis that myomesin 2 was expressed in the Pitx2-deficient mice, and this was associated with development of M lines evident in their orbital region.

CONCLUSIONS

The global transcript expression analysis uncovered that Pitx2 primarily regulates a relatively select number of genes associated with muscle contraction. Pitx2 loss led to the development of M line structures, a feature more typical of other skeletal muscle.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Underdeveloped extraocular muscles in the naked mole-rat (Heterocephalus glaber)

The extraocular muscles (EOM), the effector arm of the ocular motor system, have a unique embryological origin and phenotype. The naked mole-rat (NMR) is a subterranean rodent with an underdeveloped visual system. It has not been established if their ocular motor system is also less developed. The NMR is an ideal model to examine the potential codependence of oculomotor and visual system development and evolution. Our goal was to compare the structural features of NMR EOMs to those of the mouse, a similar sized rodent with a fully developed visual system. Perfusion-fixed whole orbits and EOMs were dissected from adult NMR and C57BL mice and examined by light and electron microscopy. NMR orbital anatomy showed smaller EOMs in roughly the same distribution around the eye as in mouse and surrounded by a very small Harderian gland. The NMR EOMs did not appear to have the two-layer fiber distribution seen in mouse EOMs; fibers were also significantly smaller (112.3 +/- 46.2 vs. 550.7 +/- 226 sq microm in mouse EOMs, *P < 0.05). Myofibrillar density was less in NMR EOMs, and triad and other membranous structures were rudimentary. Finally, mitochondrial volume density was significantly less in NMR EOMs than in mouse EOM (4.5% +/- 1.9 vs. 21.2% +/- 11.6, respectively, *P < 0.05). These results demonstrate that NMR EOMs are smaller and less organized than those in the mouse. The "simpler" EOM organization and structure in NMR may be explained by the poor visual ability of these rodents, initially demonstrated by their primitive visual system.

Effect of dystrophin deficiency on selected intrinsic laryngeal muscles of the mdx mouse

BACKGROUND

Intrinsic laryngeal muscles (ILM) show biological differences from the broader class of skeletal muscles. Yet most research regarding ILM specialization has been completed on a few muscles, most notably the thyroarytenoid and posterior cricoarytenoid. Little information exists regarding the biology of other ILM. Early evidence suggests that the interarytenoid (IA) and cricothyroid (CT) may be more similar to classic skeletal muscle than their associated laryngeal muscles. Knowledge of the IA and CT's similarity or dissimilarity to typical skeletal muscle may hold implications for the treatment of dysphonia.

PURPOSE

The purpose of this study was to further define IA and CT biology by examining their response to the biological challenge of dystrophin deficiency.

METHOD

Control and dystrophin-deficient superior cricoarytenoid (SCA; mouse counterpart of IA) and CT muscles were examined for fiber morphology, sarcolemmal integrity, and immunohistochemical detection of dystrophin.

RESULTS

Despite the absence of dystrophin, experimental muscles did not show disease markers.

CONCLUSIONS

The SCA and the CT appear spared in dystrophin-deficient mouse models. These laryngeal muscles possess specializations that separate them from typical skeletal muscle. Considered in light of previous research, the CT and IA may represent transitional form of muscle, evidencing properties of typical and specialized skeletal muscle.

Nonmuscle myosin IIB, a sarcomeric component in the extraocular muscles

Extraocular muscles (EOMs) are categorized as skeletal muscles; however, emerging evidence indicates that their gene expression profile, metabolic characteristics and functional properties are significantly different from the prototypical members of this muscle class. Gene expression profiling of developing and adult EOM suggest that many myofilament and cytoskeletal proteins have unique expression patterns in EOMs, including the maintained expression of embryonic and fetal isoforms of myosin heavy chains (MyHC), the presence of a unique EOM specific MyHC and mixtures of both cardiac and skeletal muscle isoforms of thick and thin filament accessory proteins. We demonstrate that nonmuscle myosin IIB (nmMyH IIB) is a sarcomeric component in approximately 20% of the global layer fibers in adult rat EOMs. Comparisons of the myofibrillar distribution of nmMyHC IIB with sarcomeric MyHCs indicate that nmMyH IIB co-exists with slow MyHC isoforms. In longitudinal sections of adult rat EOM, nmMyHC IIB appears to be restricted to the A-bands. Although nmMyHC IIB has been previously identified as a component of skeletal and cardiac sarcomeres at the level of the Z-line, the novel distribution of this protein within the A band in EOMs is further evidence of both the EOMs complexity and unconventional phenotype.

Adenylate kinase and AMP signaling networks: metabolic monitoring, signal communication and body energy sensing

Adenylate kinase and downstream AMP signaling is an integrated metabolic monitoring system which reads the cellular energy state in order to tune and report signals to metabolic sensors. A network of adenylate kinase isoforms (AK1-AK7) are distributed throughout intracellular compartments, interstitial space and body fluids to regulate energetic and metabolic signaling circuits, securing efficient cell energy economy, signal communication and stress response. The dynamics of adenylate kinase-catalyzed phosphotransfer regulates multiple intracellular and extracellular energy-dependent and nucleotide signaling processes, including excitation-contraction coupling, hormone secretion, cell and ciliary motility, nuclear transport, energetics of cell cycle, DNA synthesis and repair, and developmental programming. Metabolomic analyses indicate that cellular, interstitial and blood AMP levels are potential metabolic signals associated with vital functions including body energy sensing, sleep, hibernation and food intake. Either low or excess AMP signaling has been linked to human disease such as diabetes, obesity and hypertrophic cardiomyopathy. Recent studies indicate that derangements in adenylate kinase-mediated energetic signaling due to mutations in AK1, AK2 or AK7 isoforms are associated with hemolytic anemia, reticular dysgenesis and ciliary dyskinesia. Moreover, hormonal, food and antidiabetic drug actions are frequently coupled to alterations of cellular AMP levels and associated signaling. Thus, by monitoring energy state and generating and distributing AMP metabolic signals adenylate kinase represents a unique hub within the cellular homeostatic network.

Lower respiratory capacity in extraocular muscle mitochondria: evidence for intrinsic differences in mitochondrial composition and function

PURPOSE

The constant activity of the extraocular muscles is supported by abundant mitochondria. These organelles may enhance energy production by increasing the content of respiratory complexes. The authors tested the hypothesis that extraocular muscle mitochondria respire faster than do mitochondria from limb muscles because of the higher content of respiratory complexes.

METHODS

Inner mitochondrial membrane density was determined by stereological analysis of triceps surae (a limb muscle) and extraocular muscles of adult male Sprague-Dawley rats. The authors measured respiration rates of isolated mitochondria using a Clark-type electrode. The activity of respiratory complexes I, II, and IV was determined by spectrophotometry. The content of respiratory complexes was estimated by Western blot.

RESULTS

States 3, 4, and 5 respiration rates in extraocular muscle mitochondria were 40% to 60% lower than in limb muscle mitochondria. Extraocular muscle inner mitochondrial membrane density was similar to that of other skeletal muscles. Activity of complexes I and IV was lower in extraocular muscle mitochondria (approximately 50% the activity in triceps), but their content was approximately 15% to 30% higher. There was no difference in complex II content or activity or complex III content. Finally, complex V was less abundant in extraocular muscle mitochondria.

CONCLUSIONS

The results demonstrate that extraocular muscle mitochondria respire at slower rates than mitochondria from limb muscles, despite similar mitochondrial ultrastructure. Instead, differences were found in the activity (I, IV) and content (I, IV, V) of electron transport chain complexes. The discrepancy between activity and content of some complexes is suggestive of alternative subunit isoform expression in the extraocular muscles compared with limb muscles.

Progressive decrease of phosphocreatine, creatine and creatine kinase in skeletal muscle upon transformation to sarcoma

In vertebrates, phosphocreatine and ATP are continuously interconverted by the reversible reaction of creatine kinase in accordance with cellular energy needs. Sarcoma tissue and its normal counterpart, creatine-rich skeletal muscle, are good source materials to study the status of creatine and creatine kinase with the progression of malignancy. We experimentally induced sarcoma in mouse leg muscle by injecting either 3-methylcholanthrene or live sarcoma 180 cells into one hind leg. Creatine, phosphocreatine and creatine kinase isoform levels decreased as malignancy progressed and reached very low levels in the final stage of sarcoma development; all these parameters remained unaltered in the unaffected contralateral leg muscle of the same animal. Creatine and creatine kinase levels were also reduced significantly in frank malignant portions of human sarcoma and gastric and colonic adenocarcinoma compared with the distal nonmalignant portions of the same samples. In mice, immunoblotting with antibodies against cytosolic muscle-type creatine kinase and sarcomeric mitochondrial creatine kinase showed that both of these isoforms decreased as malignancy progressed. Expressions of mRNA of muscle-type creatine kinase and sarcomeric mitochondrial creatine kinase were also severely downregulated. In human sarcoma these two isoforms were undetectable also. In human gastric and colonic adenocarcinoma, brain-type creatine kinase was found to be downregulated, whereas ubiquitous mitochondrial creatine kinase was upregulated. These significantly decreased levels of creatine and creatine kinase isoforms in sarcoma suggest that: (a) the genuine muscle phenotype is lost during sarcoma progression, and (b) these parameters may be used as diagnostic marker and prognostic indicator of malignancy in this tissue.

Nebulin isoforms of extraocular muscle

The extraocular muscles (EOMs), which are responsible for reflexive and voluntary eye movements, have many unique biochemical, physiological, and ultrastructural features that set them apart from other skeletal muscles. For example, rodent EOMs lack M-lines and express EOM-specific myosin heavy chain (MYH13) and alpha-cardiac myosin heavy chain. Recent gene-expression profiling studies indicate the presence of other cardiac-specific proteins in adult EOMs. This interesting mixture of myofibrillar and cytoskeletal proteins poses the questions as to whether nebulette, as opposed to nebulin, might be expressed in EOM, and what isoforms of titin are expressed in the EOM. We have performed gel electrophoresis and immunological analyses to determine the titin and nebulin isoforms expressed in the EOM. We have found that the mass of the titin isoforms expressed in the EOM most closely resemble those found in the skeletal muscles tested, viz., the soleus and extensor digitorum longus (EDL). We also demonstrate that, although the EOM expresses cardiac isoforms of myosin, it does not express nebulette and contains a nebulin isoform with a mass consistent with that found in the prototypical fast hindlimb muscle EDL.

Biological organization of the extraocular muscles

Extraocular muscle is fundamentally distinct from other skeletal muscles. Here, we review the biological organization of the extraocular muscles with the intent of understanding this novel muscle group in the context of oculomotor system function. The specific objectives of this review are threefold. The first objective is to understand the anatomic arrangement of the extraocular muscles and their compartmental or layered organization in the context of a new concept of orbital mechanics, the active pulley hypothesis. The second objective is to present an integrated view of the morphologic, cellular, and molecular differences between extraocular and the more traditional skeletal muscles. The third objective is to relate recent data from functional and molecular biology studies to the established extraocular muscle fiber types. Developmental mechanisms that may be responsible for the divergence of the eye muscles from a skeletal muscle prototype also are considered. Taken together, a multidisciplinary understanding of extraocular muscle biology in health and disease provides insights into oculomotor system function and malfunction. Moreover, because the eye muscles are selectively involved or spared in a variety of neuromuscular diseases, knowledge of their biology may improve current pathogenic models of and treatments for devastating systemic diseases.

Lactate is a metabolic substrate that sustains extraocular muscle function

Lactic acid is considered the end product of glycolysis and is a major cause of muscle fatigue. However, the lactate dehydrogenase (LDH) reaction is bidirectional: Lactate can be oxidized to pyruvate and used as a substrate for the Krebs cycle. Therefore, our hypothesis was that lactate sustains the contractile function of rat extraocular muscles during periods of increased activity. The study used extraocular and extensor digitorum longus (EDL) muscles from adult Sprague-Dawley rats to determine LDH isoform expression, total LDH activity, and contractile function in vitro. To evaluate the role of lactate on fatigue, we tested the effect of cinnamate, a blocker of lactate transport, and exogenous lactate on fatigue resistance. Cinnamate accelerated fatigue in the extraocular muscles: Endurance and residual force decreased significantly. Conversely, cinnamate did not affect the endurance or residual force of EDL muscles. Replacing glucose with exogenous lactate increased EDL fatigability but had no effect on the extraocular muscles. However, the extraocular muscles fatigued faster when exposed to exogenous lactate combined with cinnamate. The LDH-A and LDH-C isoforms were expressed at lower levels in extraocular muscle; LDH-B was equally abundant in the EDL and extraocular muscles. Total LDH activity in the extraocular muscles was only approximately 32% of the level in EDL. These results support the hypothesis that lactate sustains the contractile performance of the extraocular muscles.

Expression and carbonylation of creatine kinase in the quadriceps femoris muscles of patients with chronic obstructive pulmonary disease

Oxidative protein modification involving carbonylation has recently been identified as an important factor in skeletal muscle dysfunction in patients with chronic obstructive pulmonary disease (COPD). However, the exact identity of modified proteins inside limb muscles of patients with COPD remains unknown. We used 2D electrophoresis, immunoblotting, and mass spectrometry to identify carbonylated proteins in the vastus lateralis muscle of 12 patients with COPD and 6 control subjects. Both creatine kinase (CK) and carbonic anhydrase III (CAIII) were identified as being strongly carbonylated in this muscle in both groups of subjects. Total CK activity, CK protein expression, and the intensity of CK carbonylation were significantly greater in the muscles of patients with COPD as compared with control subjects, whereas CAIII protein expression and intensity of carbonylation were similar in the two groups. In patients with COPD, CK activity and protein expression correlated positively with FEV(1) and V O(2)max, whereas the intensity of CK carbonylation correlated negatively with the same parameters. These results indicate that oxygen radicals selectively target CK and CAIII inside limb muscles of humans. The observation that the intensity of CK carbonylation correlates negatively with CK activity in limb muscles of patients with COPD suggests that carbonylation may have a deleterious effect on CK activity, and may contribute to impaired CK function in the limb muscles of these patients.

Fatigue resistance of rat extraocular muscles does not depend on creatine kinase activity

Background

Creatine kinase (CK) links phosphocreatine, an energy storage system, to cellular ATPases. CK activity serves as a temporal and spatial buffer for ATP content, particularly in fast-twitch skeletal muscles. The extraocular muscles are notoriously fast and active, suggesting the need for efficient ATP buffering. This study tested the hypotheses that (1) CK isoform expression and activity in rat extraocular muscles would be higher, and (2) the resistance of these muscles to fatigue would depend on CK activity.

Results

We found that mRNA and protein levels for cytosolic and mitochondrial CK isoforms were lower in the extraocular muscles than in extensor digitorum longus (EDL). Total CK activity was correspondingly decreased in the extraocular muscles. Moreover, cytoskeletal components of the sarcomeric M line, where a fraction of CK activity is found, were downregulated in the extraocular muscles as was shown by immunocytochemistry and western blotting. CK inhibition significantly accelerated the development of fatigue in EDL muscle bundles, but had no major effect on the extraocular muscles. Searching for alternative ATP buffers that could compensate for the relative lack of CK in extraocular muscles, we determined that mRNAs for two adenylate kinase (AK) isoforms were expressed at higher levels in these muscles. Total AK activity was similar in EDL and extraocular muscles.

Conclusion

These data indicate that the characteristic fatigue resistance of the extraocular muscles does not depend on CK activity.

Dimerisation of Myomesin: Implications for the Structure of the Sarcomeric M-band

The sarcomeric M-band is thought to provide a link between the thick and the elastic filament systems. So far, relatively little is known about its structural components and their three-dimensional organisation. Myomesin seems to be an essential component of the M-band, since it is expressed in all types of vertebrate striated muscle fibres investigated and can be found in its mature localisation pattern as soon as the first myofibrils are assembled. Previous work has shown that the N-terminal and central part of myomesin harbour binding sites for myosin, titin and muscle creatine kinase. Intrigued by the highly conserved domain layout of the C-terminal half, we screened for new interaction partners by yeast two-hybrid analysis. This revealed a strong interaction of myomesin with itself. This finding was confirmed by several biochemical assays. Our data suggest that myomesin can form antiparallel dimers via a binding site residing in its C-terminal domain 13. We suggest that, similar to alpha-actinin in the Z-disc, the myomesin dimers cross-link the contractile filaments in the M-band. The new and the already previously identified myomesin interaction sites are integrated into the first three-dimensional model of the sarcomeric M-band on a molecular basis.

Conserved and muscle-group-specific gene expression patterns shape postnatal development of the novel extraocular muscle phenotype

Current models in skeletal muscle biology do not fully account for the breadth, causes, and consequences of phenotypic variation among skeletal muscle groups. The muscle allotype concept arose to explain frank differences between limb, masticatory, and extraocular (EOM) muscles, but there is little understanding of the developmental regulation of the skeletal muscle phenotypic range. Here, we used morphological and DNA microarray analyses to generate a comprehensive temporal profile for rat EOM development. Based upon coordinate regulation of morphologic/gene expression traits with key events in visual, vestibular, and oculomotor system development, we propose a model that the EOM phenotype is a consequence of extrinsic factors that are unique to its local environment and sensory-motor control system, acting upon a novel myoblast lineage. We identified a broad spectrum of differences between the postnatal transcriptional patterns of EOM and limb muscle allotypes, including numerous transcripts not traditionally associated with muscle fiber/group differences. Several transcription factors were differentially regulated and may be responsible for signaling muscle allotype specificity. Significant differences in cellular energetic mechanisms defined the EOM and limb allotypes. The allotypes were divergent in many other functional transcript classes that remain to be further explored. Taken together, we suggest that the EOM allotype is the consequence of tissue-specific mechanisms that direct expression of a limited number of EOM-specific transcripts and broader, incremental differences in transcripts that are conserved by the two allotypes. This represents an important first step in dissecting allotype-specific regulatory mechanisms that may, in turn, explain differential muscle group sensitivity to a variety of metabolic and neuromuscular diseases.

The molecular composition of the sarcomeric M-band correlates with muscle fiber type

The M-band is the transverse structure that cross-links the thick filaments in the center and provides a perfect alignment of the A-band in the activated sarcomere. The molecular composition of the M-bands in adult mouse skeletal muscle is fiber-type dependent. All M-bands in fast fibers contain M-protein while M-bands in slow fibers contain a significant proportion of the EH-myomesin isoform, previously detected only in embryonic heart muscle. This fiber-type specificity develops during the first postnatal weeks. However, the ratio between the amounts of myosin and of myomesin, taken as sum of both isoforms, remains nearly constant in all studied muscles. Ultrastructural analysis demonstrates that some of the soleus fibers show a diffuse appearance of the M-band, resembling the situation in the embryonic heart. A model is proposed to explain the functional consequence of differential M-band composition for the physiological and morphological properties of sarcomeres in different muscle types.

Temporal gene expression profiling of dystrophin-deficient (mdx) mouse diaphragm identifies conserved and muscle group-specific mechanisms in the pathogenesis of muscular dystrophy

Mutations in dystrophin are the proximate cause of Duchenne muscular dystrophy (DMD), but pathogenic mechanisms linking the absence of dystrophin from the sarcolemma to myofiber necrosis are not fully known. The muscular dystrophies also have properties not accounted for by current disease models, including the temporal delay to disease onset, broad species differences in severity, and diversity of skeletal muscle responses. To address the mechanisms underlying the differential targeting of muscular dystrophy, we characterized temporal expression profiles of the diaphragm in dystrophin-deficient (mdx) mice between postnatal days 7 and 112 using oligonucleotide microarrays and contrasted these data with published hindlimb muscle data. Although the diaphragm and hindlimb muscle groups differ in severity of response to dystrophin deficiency, and exhibited substantial divergence in some transcript categories including inflammation and muscle-specific genes, our data show that the general mechanisms operative in muscular dystrophy are highly conserved. The two muscle groups principally differed in expression levels of differentially regulated genes, as opposed to the non-conserved induced/repressed transcripts defining fundamentally distinct mechanisms. We also identified a postnatal divergence of the two wild-type muscle group expression profiles that temporally correlated with the onset and progression of the dystrophic process. These findings support the hypothesis that conserved disease mechanisms interacting with baseline differences in muscle group-specific transcriptomes underlie their differential responses to DMD. We further suggest that muscle group-specific transcriptional profiles contribute toward the muscle targeting and sparing patterns observed for a variety of metabolic and neuromuscular diseases.