Deubiquitinating enzymes in skeletal muscle atrophy—An essential role for USP19

The ubiquitin proteasome system is well recognized to be involved in mediating muscle atrophy in response to diverse catabolic conditions. To date, almost all of the genes that have been implicated are ubiquitin ligases. Although ubiquitination is modulated also by deubiquitinating enzymes, the roles of these enzymes in muscle wasting remains largely unexplored. In this article, the potential roles of deubiquitinating enzymes in regulating muscle size are discussed. This is followed by a review of the roles described for USP19, the deubiquitinating enzyme that has been most studied in muscle wasting. This enzyme is upregulated in muscle in many catabolic conditions and its inactivation leads to protection from muscle loss induced by stimuli that are common in many illnesses causing cachexia. It can regulate both protein synthesis and protein degradation as well as myogenesis, thereby modulating the key processes that control muscle mass. Roles for other deubiquitinating enzymes remain possible and to be explored.     

Glycogen synthase kinase-3β is required for the induction of skeletal muscle atrophy

Skeletal muscle atrophy commonly occurs in acute and chronic disease. The expression of the muscle-specific E3 ligases atrogin-1 (MAFbx) and muscle RING finger 1 (MuRF1) is induced by atrophy stimuli such as glucocorticoids or absence of IGF-I/insulin and subsequent Akt signaling. We investigated whether glycogen synthase kinase-3β (GSK-3β), a downstream molecule in IGF-I/Akt signaling, is required for basal and atrophy stimulus-induced expression of atrogin-1 and MuRF1, and myofibrillar protein loss in C(2)C(12) skeletal myotubes. Abrogation of basal IGF-I signaling, using LY294002, resulted in a prominent induction of atrogin-1 and MuRF1 mRNA and was accompanied by a loss of myosin heavy chain fast (MyHC-f) and myosin light chains 1 (MyLC-1) and -3 (MyLC-3). The synthetic glucocorticoid dexamethasone (Dex) also induced the expression of both atrogenes and likewise resulted in the loss of myosin protein abundance. Genetic ablation of GSK-3β using small interfering RNA resulted in specific sparing of MyHC-f, MyLC-1, and MyLC-3 protein levels after Dex treatment or impaired IGF-I/Akt signaling. Interestingly, loss of endogenous GSK-3β suppressed both basal and atrophy stimulus-induced atrogin-1 and MuRF1 expression, whereas pharmacological GSK-3β inhibition, using CHIR99021 or LiCl, only reduced atrogin-1 mRNA levels in response to LY294002 or Dex. In conclusion, our data reveal that myotube atrophy and myofibrillar protein loss are GSK-3β dependent, and demonstrate for the first time that basal and atrophy stimulus-induced atrogin-1 mRNA expression requires GSK-3β enzymatic activity, whereas MuRF1 expression depends solely on the physical presence of GSK-3β.     

Housekeeping proteins: How useful are they in skeletal muscle diabetes studies and muscle hypertrophy models?

The use of Western blot analysis is of great importance in research, and the measurement of housekeeping proteins is commonly used for loading controls. However, Ponceau S staining has been shown to be an alternative to analysis of housekeeping protein levels as loading controls in some conditions. In the current study, housekeeping protein levels were measured in skeletal muscle hypertrophy and streptozotocin-induced diabetes experimental models. The following housekeeping proteins were investigated: glyceraldehyde-3-phosphate dehydrogenase (GAPDH), β-actin, α-tubulin, γ-tubulin, and α-actinin. Evidence is presented that Ponceau S is more reliable than housekeeping protein levels for specific protein quantifications in Western blot analysis.     

Are natural microcosms useful model systems for ecology?

Several recent, high-impact ecological studies feature natural microcosms as tools for testing effects of fragmentation, metacommunity theory or links between biodiversity and ecosystem processes. These studies combine the microcosm advantages of small size, short generation times, contained structure and hierarchical spatial arrangement with advantages of field studies: natural environmental variance, 'openness' and realistic species combinations with shared evolutionary histories. This enables tests of theory pertaining to spatial and temporal dynamics, for example, the effects of neighboring communities on local diversity, or the effects of biodiversity on ecosystem function. Using examples, we comment on the position of natural microcosms in the roster of ecological research strategies and tools. We conclude that natural microcosms are as versatile as artificial microcosms, but as complex and biologically realistic as other natural systems. Research to date combined with inherent attributes of natural microcosms make them strong candidate model systems for ecology.     

Are skeletal muscles independent actuators? Force transmission from soleus muscle in the cat

It is unclear if skeletal muscles act mechanically as independent actuators. The purpose of the present study was to investigate force transmission from soleus (SO) muscle for physiological lengths as well as relative positions in the intact cat hindlimb. We hypothesized that force transmission from SO fibers will be affected by length changes of its two-joint synergists. Ankle plantar flexor moment on excitation of the SO was measured for various knee angles (70-140 degrees ). This involved substantial length changes of gastrocnemius and plantaris muscles. Ankle angle was kept constant (80 degrees -90 degrees ). However, SO ankle moment was not significantly affected by changes in knee angle; neither were half-relaxation time and the maximal rate of relaxation (P > 0.05). Following tenotomy, SO ankle moment decreased substantially (55 +/- 16%) but did not reach zero, indicating force transmission via connective tissues to the Achilles tendon (i.e., epimuscular myofascial force transmission). During contraction SO muscle shortened to a much greater extent than in the intact case (16.0 +/- 0.6 vs. 1.0 +/- 0.1 mm), which resulted in a major position shift relative to its synergists. If the SO was moved back to its position corresponding to the intact condition, SO ankle moment approached zero, and most muscle force was exerted at the distal SO tendon. Our results also suggested that in vivo the lumped intact tissues linking SO to its synergists are slack or are operating on the toe region of the stress-strain curve. Thus, within the experimental conditions of the present study, the intact cat soleus muscle appears to act mechanically as an independent actuator.     

Are mapped markers more useful for assessing genetic diversity?

Genetic diversity within populations of organisms and species is commonly measured using molecular-marker data. It has been claimed that more reliable diversity measurements can be obtained using selected genetically mapped markers to ensure that all regions of the genome are represented in the data sets employed. However, this has not been tested. In the present study, using rice (Oryza sativa L.) as a model species, we have shown that the use of unmapped AFLP markers reveals a pattern of diversity that is very similar to that obtained using a range of other marker types and which reflects the known crossability groups within this species. In contrast, we show that use of mapped-marker data can, in some cases, result in highly misleading patterns of diversity; the results obtained are critically related to the choice ofparents used in the cross from which the mapping population was produced. For diversity analyses, we propose that it is appropriate to use unmapped markers provided that the marker-type has been shown to have a wide distribution over the genome. Received: 13 November 1998 / Accepted: 17 June 1999     

Are comparative studies of extinction risk useful for conservation?

Large-scale, comparative studies of species extinction risk have become common in conservation science, but their influence on conservation practice appears limited. The link between such studies and the practice of conservation breaks down in two key places. First, results of comparative studies are often ambiguous, inconsistent and difficult to translate into policy. Second, conservation as currently practiced emphasizes the rescue and protection of currently threatened biodiversity, whereas comparative studies are often better suited to a proactive approach that anticipates and prevents future species declines. Scientists should make their research more accessible by addressing the first issue. Policymakers and managers, in turn, could make better use of comparative studies by moving towards more preventative approaches to conservation planning.     

Does exercise-induced muscle damage play a role in skeletal muscle hypertrophy?

Exercise-induced muscle damage (EIMD) occurs primarily from the performance of unaccustomed exercise, and its severity is modulated by the type, intensity, and duration of training. Although concentric and isometric actions contribute to EIMD, the greatest damage to muscle tissue is seen with eccentric exercise, where muscles are forcibly lengthened. Damage can be specific to just a few macromolecules of tissue or result in large tears in the sarcolemma, basal lamina, and supportive connective tissue, and inducing injury to contractile elements and the cytoskeleton. Although EIMD can have detrimental short-term effects on markers of performance and pain, it has been hypothesized that the associated skeletal muscle inflammation and increased protein turnover are necessary for long-term hypertrophic adaptations. A theoretical basis for this belief has been proposed, whereby the structural changes associated with EIMD influence gene expression, resulting in a strengthening of the tissue and thus protection of the muscle against further injury. Other researchers, however, have questioned this hypothesis, noting that hypertrophy can occur in the relative absence of muscle damage. Therefore, the purpose of this article will be twofold: (a) to extensively review the literature and attempt to determine what, if any, role EIMD plays in promoting skeletal muscle hypertrophy and (b) to make applicable recommendations for resistance training program design.     

Is rat skeletal muscle hexokinase an allosteric enzyme?

A recent report by M. Gregoriou, I. P. Trayer, and A. Cornish-Bowden (1986, Eur. J. Biochem. 161, 171-176) showed that the mechanism for rat skeletal muscle hexokinase contains two allosteric sites: one for ATP and one for glucose 6-phosphate. In this report, we show that the allosteric mechanism is at variance with a large amount of kinetic data for the skeletal muscle hexokinase reaction in the literature. In addition, the allosteric mechanism conflicts with isotope exchange at chemical equilibrium data reported by M. Gregoriou, I. P. Trayer, and A. Cornish-Bowden (1983, Eur. J. Biochem. 134, 283-288).     

Brown fat and skeletal muscle: unlikely cousins?

Although the functions of white fat and brown fat are increasingly well understood, their developmental origins remain unclear. A recent study published in Nature (Seale et al., 2008) identifies a population of progenitor cells that gives rise to brown fat and skeletal muscle but not white fat.     

Chronic hypobaric hypoxia mediated skeletal muscle atrophy: role of ubiquitin–proteasome pathway and calpains

The most frequently reported symptom of exposure to high altitude is loss of body mass and decreased performance which has been attributed to altered protein metabolism affecting skeletal muscles mass. The present study explores the mechanism of chronic hypobaric hypoxia mediated skeletal muscle wasting by evaluating changes in protein turnover and various proteolytic pathways. Male Sprague-Dawley rats weighing about 200 g were exposed to hypobaric hypoxia (7,620 m) for different durations of exposure. Physical performance of rats was measured by treadmill running experiments. Protein synthesis, protein degradation rates were determined by (14)C-Leucine incorporation and tyrosine release, respectively. Chymotrypsin-like enzyme activity of the ubiquitin-proteasome pathway and calpains were studied fluorimetrically as well as using western blots. Declined physical performance by more than 20%, in terms of time taken in exhaustion on treadmill, following chronic hypobaric hypoxia was observed. Compared to 1.5-fold increase in protein synthesis, the increase in protein degradation was much higher (five-folds), which consequently resulted in skeletal muscle mass loss. Myofibrillar protein level declined from 46.79 ± 1.49 mg/g tissue at sea level to 37.36 ± 1.153 (P < 0.05) at high altitude. However, the reduction in sarcoplasmic proteins was less as compared to myofibrillar protein. Upregulation of Ub-proteasome pathway (five-fold over control) and calpains (three-fold) has been found to be important factors for the enhanced protein degradation rate. The study provided strong evidences suggesting that elevated protein turnover rate lead to skeletal muscle atrophy under chronic hypobaric hypoxia via ubiquitin-proteasome pathway and calpains.     

Mitochondria and endoplasmic reticulum: Targets for a better insulin sensitivity in skeletal muscle?

Obesity and its associated metabolic disorders represent a major health burden, with economic and social consequences. Although adapted lifestyle and bariatric surgery are effective in reducing body weight, obesity prevalence is still rising. Obese individuals often become insulin-resistant. Obesity impacts on insulin responsive organs, such as the liver, adipose tissue and skeletal muscle, and increases the risk of cardiovascular diseases, type 2 diabetes and cancer. In this review, we discuss the effects of obesity and insulin resistance on skeletal muscle, an important organ for the control of postprandial glucose. The roles of mitochondria and the endoplasmic reticulum in insulin signaling are highlighted and potential innovative research and treatment perspectives are proposed.