Muscle LIM protein is upregulated in fast skeletal muscle during transition toward slower phenotypes

Whole transcriptome analyses and comparison reveal the metabolic differences between oxidative and glycolytic skeletal muscles of yak

Mammalian skeletal muscle is composed of various muscle fibers that exhibit different physiological and metabolic features. Muscle fiber type composition has significant influences on the meat quality of livestock. In this study, we comprehensively analyzed the whole transcriptome profiles of the oxidative muscle biceps femoris (BF) and the glycolytic muscle obliquus externus abdominis (OEA) of yak. A total of 1436 mRNAs, 1172 lncRNAs, and 218 circRNAs were differentially expressed in the oxidative muscles compared with the glycolytic muscles. KEGG annotation showed that differentially expressed mRNAs regulated by lncRNA and circRNA were mainly involved in PPAR signaling pathway, citrate cycle (TCA cycle), and PI3K-Akt signaling pathway, which reflect the different metabolic properties between oxidative and glycolytic muscles. In addition, regulatory networks associated with muscle fiber type conversion and mitochondria energy metabolism in muscles were constructed. Our study provides new evidence for a better understanding of the molecular mechanisms underlying skeletal muscle fiber determination and meat quality traits of yak.

Characterization of an MLP Homologue from Haemaphysalis longicornis (Acari: Ixodidae) Ticks

Members of the cysteine-rich protein (CRP) family are known to participate in muscle development in vertebrates. Muscle LIM protein (MLP) belongs to the CRP family and has an important function in the differentiation and proliferation of muscle cells. In this study, the full-length cDNA encoding MLP from Haemaphysalis longicornis (H. longicornis; HLMLP) ticks was obtained by 5' rapid amplification of cDNA ends (RACE). To verify the transcriptional status of MLP in ticks, HLMLP gene expression was assessed during various developmental stages by real-time PCR (RT-PCR). Interestingly, HLMLP expression in the integument was significantly (P < 0.01) higher than that observed in other tested tissues of engorged adult ticks. In addition, HLMLP mRNA levels were significantly downregulated in response to thermal stress at 4 °C for 48 h. Furthermore, recombinant HLMLP was expressed in Escherichia coli, and Western blot analysis showed that rabbit antiserum against H. longicornis adults recognized HLMLP and MLPs from different ticks. Ten 3-month-old rabbits that had never been exposed to ticks were used for the immunization and challenge experiments. The rabbits were divided into two groups of five rabbits each, where rabbits in the first group were immunized with HLMLP, while those in the second group were immunized with phosphate-buffered saline (PBS) diluent as controls. The vaccination of rabbits with the recombinant HLMLP conferred partial protective immunity against ticks, resulting in 20.00% mortality and a 17.44% reduction in the engorgement weight of adult ticks. These results suggest that HLMLP is not ideal as a candidate for use in anti-tick vaccines. However, the results of this study generated novel information on the MLP gene in H. longicornis and provide a basis for further investigation of the function of this gene that could potentially lead to a better understanding of the mechanism of myofiber determination and transformation.

Effects of aging, exercise, and disease on force transfer in skeletal muscle

The loss of muscle strength and increased injury rate in aging skeletal muscle has previously been attributed to loss of muscle protein (cross-sectional area) and/or decreased neural activation. However, it is becoming clear that force transfer within and between fibers plays a significant role in this process as well. Force transfer involves a secondary matrix of proteins that align and transmit the force produced by the thick and thin filaments along muscle fibers and out to the extracellular matrix. These specialized networks of cytoskeletal proteins aid in passing force through the muscle and also serve to protect individual fibers from injury. This review discusses the cytoskeleton proteins that have been identified as playing a role in muscle force transmission, both longitudinally and laterally, and where possible highlights how disease, aging, and exercise influence the expression and function of these proteins.

Survival and migration of human dendritic cells are regulated by an IFN-alpha-inducible Axl/Gas6 pathway

Axl, a prototypic member of the transmembrane tyrosine kinase receptor family, is known to regulate innate immunity. In this study, we show that Axl expression is induced by IFN-alpha during human dendritic cell (DC) differentiation from monocytes (IFN/DC) and that constitutively Axl-negative, IL-4-differentiated DC (IL-4/DC) can be induced to up-regulate Axl by IFN-alpha. This effect is inhibited by TLR-dependent maturation stimuli such as LPS, poly(I:C), TLR7/8 ligand, and CD40L. LPS-induced Axl down-regulation on the surface of human IFN-alpha-treated DC correlates with an increased proteolytic cleavage of Axl and with elevated levels of its soluble form. GM6001 and TAPI-1, general inhibitors of MMP and ADAM family proteases, restored Axl expression on the DC surface and diminished Axl shedding. Furthermore, stimulation of Axl by its ligand, Gas6, induced chemotaxis of human DC and rescued them from growth factor deprivation-induced apoptosis. Our study provides the first evidence that Gas6/Axl-mediated signaling regulates human DC activities, and identifies Gas6/Axl as a new DC chemotaxis pathway. This encourages one to explore whether dysregulation of this novel pathway in human DC biology is involved in autoimmunity characterized by high levels of IFN-alpha.

Measurement of, and Some Reasons for, Differences in Eating Habits Between Night and Day Workers

A questionnaire was designed to assess the following: why working people chose to eat or not to eat at a particular time of day; the factors that influenced the type of food eaten; and subjective responses to the meal (hunger before, enjoyment during, satiety afterward). Self-assessments were done every 3 h during a typical week containing work and rest days, by one group of 50 day workers and another group of 43 night workers. During the night work hours compared to rest days, night workers evidenced a significantly altered food intake, with a greater frequency of cold rather than hot food (p < 0.001). The type and frequency of meals were influenced significantly more (p < 0.05) by habit and time availability and less by appetite. This pattern continued into the hours immediately after the night shift had ended. In day workers food intake during work hours, compared to rest days, was also influenced significantly more often (p < 0.05) by time availability than hunger, but less so than with night workers. Moreover, day workers were less dependent than night workers upon snacks (p = 0.01), and any significant differences from rest days did not continue beyond work hours. Not only did night workers change their eating habits during work days more than did day workers but also they looked forward to their meals significantly less (p < 0.001) and felt more bloated after consuming them (p < 0.05), such effects being present to some extent during their rest days also. These findings have clear implications for measures designed to ease eating problems that are commonly problematic in night workers.

Molecular and cellular insights into a distinct myopathy of Great Dane dogs

A myopathy in the Great Dane dog with characteristic pathological and molecular features is reported. Young adults present with progressive weakness and generalised muscle atrophy. To better define this condition, an investigation using histopathology, confocal microscopy, biochemistry and microarray analysis was undertaken. The skeletal muscles of affected dogs exhibited increased oxidative fibre phenotype and core fibre lesions characterised by the disruption of the sarcomeric architecture and the accumulation of mitochondrial organelles. Affected muscles displayed co-ordinated expression of genes consistent with a slow-oxidative phenotype, which was possibly a compensatory response to chronic muscle damage. There was disruption of Z-lines in affected muscles which, at the molecular level, manifested as transcriptional dysregulation of several Z-line associated genes, including alpha-actinin, myotilin, desmin, vimentin and telethonin. The pathology of this canine myopathy is distinct from that of human central core myopathies that are characterised by cores devoid of mitochondria and by the presence of myofibrillar breakdown products.

The sarcomere and the nucleus: functional links to hypertrophy, atrophy and sarcopenia

Skeletal muscle has a remarkable ability to rapidly adjust to changes in physiological requirements. This includes hypertrophic muscle growth and the atrophic loss of muscle mass, both of which occur in response to hormonal, endocrine and mechanical stimuli. In ageing muscle, sarcopenia (the loss of muscle fibres) can aggravate hormonally and mechanically induced atrophy. Hypertrophy and atrophy are associated with changes in sarcomeric protein composition and metabolic enzymes. The coordinated changes of transcriptional and splice mechanisms, protein turnover and cell fate integrates signalling pathways from hormone and cytokine receptors, as well as the sarcomere itself. This involves a number of proteins that shuttle between sarcomeric and nonsarcomeric localisations and thus convey signals from the contractile machinery to the nucleus. The M-band is emerging as a hub mainly for protein-kinase regulated ubiquitin signalling and protein turnover, whereas the I-band and Z-disk contain stretch-sensitive pathways involving transcriptional modifiers. Disruptions of these pathways can cause hereditary myopathies.

Effect of the electrical currents generated by the intestinal smooth muscle layers on pancreatic enzyme activity: An in vitro study

Gut enzymes in the small intestine are exposed to extremely low electrical currents (ELEC) generated by the smooth muscle. In the present study, the in vitro tests were undertaken to evaluate the effect of these electric currents on the activity of the proteolytic pancreatic digestive enzymes. A simulator generating the typical electrical activity of pig gut was used for these studies. The electric current emitted by the simulator was transmitted to the samples, containing enzyme and its substrate, using platinum plate electrodes. All samples were incubated at 37 degrees C for 1 h. The changes in optical density, corresponding to enzyme activity, in samples stimulated for 1 h with ELEC was compared with that not exposed to ELEC. The obtained results show that the electrical current with the characteristics of the myoelectrical migrating complex (MMC) has an influence on pancreatic enzyme activity. Increased endopeptidase and reduced exopeptidase activity was noticed in samples treated with ELEC. This observation can be of important as analyzed factors which can alter enzymatic activity of the gut, can thus also affect feed/food digestibility.

Structural and regulatory roles of muscle ankyrin repeat protein family in skeletal muscle

The biological response of muscle to eccentric contractions (ECs) results in strengthening and protection from further injury. However, the cellular basis for this response remains unclear. Previous studies identified the muscle ankyrin repeat protein (MARP) family, consisting of cardiac ankyrin repeat protein (CARP), ankyrin repeat domain 2/ankyrin repeat protein with PEST and proline-rich region (Ankrd2/Arpp), and diabetes-associated ankyrin repeat protein (DARP), as rapidly and specifically upregulated in mice after a single bout of EC. To determine the role of these genes in skeletal muscle, a survey of skeletal muscle structural and functional characteristics was performed on mice lacking all three MARP family members (MKO). There was a slight trend toward MKO muscles having a slower fiber type distribution but no differences in muscle fiber size. Single MKO fibers were less stiff, tended to have longer resting sarcomere lengths, and expressed a longer isoform of titin than their wild-type counterparts, indicating that these proteins may play a role in the passive mechanical behavior of muscle. Finally, MKO mice showed a greater degree of torque loss after a bout of ECs compared with wild-type mice, although they recovered from the injury with the same or even improved time course. This recovery was associated with enhanced expression of the muscle regulatory genes MyoD and muscle LIM protein (MLP), suggesting that the MARP family may play both important structural and gene regulatory roles in skeletal muscle.

Effects of streptozotocin-induced diabetes and physical training on gene expression of titin-based stretch-sensing complexes in mouse striated muscle

In striated muscle, a sarcomeric noncontractile protein, titin, is proposed to form the backbone of the stress- and strain-sensing structures. We investigated the effects of diabetes, physical training, and their combination on the gene expression of proteins of putative titin stretch-sensing complexes in skeletal and cardiac muscle. Mice were divided into control (C), training (T), streptozotocin-induced diabetic (D), and diabetic training (DT) groups. Training groups performed for 1, 3, or 5 wk of endurance training on a motor-driven treadmill. Muscle samples from T and DT groups together with respective controls were collected 24 h after the last training session. Gene expression of calf muscles (soleus, gastrocnemius, and plantaris) and cardiac muscle were analyzed using microarray and quantitative PCR. Diabetes induced changes in mRNA expression of the proteins of titin stretch-sensing complexes in Z-disc (MLP, myostatin), I-band (CARP, Ankrd2), and M-line (titin kinase signaling). Training alleviated diabetes-induced changes in most affected mRNA levels in skeletal muscle but only one change in cardiac muscle. In conclusion, we showed diabetes-induced changes in mRNA levels of several fiber-type-biased proteins (MLP, myostatin, Ankrd2) in skeletal muscle. These results are consistent with previous observations of diabetes-induced atrophy leading to slower fiber type composition. The ability of exercise to alleviate diabetes-induced changes may indicate slower transition of fiber type.

Quantitative proteomics profiling of sarcomere associated proteins in limb and extraocular muscle allotypes

The sarcomere is the major structural and functional unit of striated muscle. Approximately 65 different proteins have been associated with the sarcomere, and their exact composition defines the speed, endurance, and biology of each individual muscle. Past analyses relied heavily on electrophoretic and immunohistochemical techniques, which only allow the analysis of a small fraction of proteins at a time. Here we introduce a quantitative label-free, shotgun proteomics approach to differentially quantitate sarcomeric proteins from microgram quantities of muscle tissue in a fast and reliable manner by liquid chromatography and mass spectrometry. The high sequence similarity of some sarcomeric proteins poses a problem for shotgun proteomics because of limitations in subsequent database search algorithms in the exclusive assignment of peptides to specific isoforms. Therefore multiple sequence alignments were generated to improve the identification of isoform specific peptides. This methodology was used to compare the sarcomeric proteome of the extraocular muscle allotype to limb muscle. Extraocular muscles are a unique group of highly specialized muscles with distinct biochemical, physiological, and pathological properties. We were able to quantitate 40 sarcomeric proteins; although the basic sarcomeric proteins in extraocular muscle are similar to those in limb muscle, key proteins stabilizing the connection of the Z-bands to thin filaments and the costamere are augmented in extraocular muscle and may represent an adaptation to the eccentric contractions known to normally occur during eye movements. Furthermore, a number of changes are seen that closely relate to the unique nature of extraocular muscle.

Denervation in murine fast-twitch muscle: short-term physiological changes and temporal expression profiling

Denervation deeply affects muscle structure and function, the alterations being different in slow and fast muscles. Because the effects of denervation on fast muscles are still controversial, and high-throughput studies on gene expression in denervated muscles are lacking, we studied gene expression during atrophy progression following denervation in mouse tibialis anterior (TA). The sciatic nerve was cut close to trochanter in adult CD1 mice. One, three, seven, and fourteen days after denervation, animals were killed and TA muscles were dissected out and utilized for physiological experiments and gene expression studies. Target cDNAs from TA muscles were hybridized on a dedicated cDNA microarray of muscle genes. Seventy-one genes were found differentially expressed. Microarray results were validated, and the expression of relevant genes not probed on our array was monitored by real-time quantitative PCR (RQ-PCR). Nuclear- and mitochondrial-encoded genes implicated in energy metabolism were consistently downregulated. Among genes implicated in muscle contraction (myofibrillar and sarcoplasmic reticulum), genes typical of fast fibers were downregulated, whereas those typical of slow fibers were upregulated. Electrophoresis and Western blot showed less pronounced changes in myofibrillar protein expression, partially confirming changes in gene expression. Isometric tension of skinned fibers was little affected by denervation, whereas calcium sensitivity decreased. Functional studies in mouse extensor digitorum longus muscle showed prolongation in twitch time parameters and shift to the left in force-frequency curves after denervation. We conclude that, if studied at the mRNA level, fast muscles appear not less responsive than slow muscles to the interruption of neural stimulation.

Muscle LIM protein plays both structural and functional roles in skeletal muscle

Muscle LIM protein (MLP) has been suggested to be an important mediator of mechanical stress in cardiac tissue, but the role that it plays in skeletal muscle remains unclear. Previous studies have shown that it is dramatically upregulated in fast-to-slow fiber-type transformation and also after eccentric contraction (EC)-induced muscle injury. The functional consequences of this upregulation, if any, are unclear. In the present study, we have examined the skeletal muscle phenotype of MLP-knockout (MLPKO) mice in terms of their response to EC-induced muscle injuries. The data suggest that while the MLPKO mice recover completely after EC-induced injury, their torque production lags behind that of heterozygous littermates in the early stages of the recovery process. This lag is accompanied by decreased expression of the muscle regulatory factor MyoD, suggesting that MLP may influence gene expression. In addition, there is evidence of type I fiber atrophy and a shorter resting sarcomere length in the MLPKO mice, but no significant differences in fiber type distribution. In summary, MLP appears to play a subtle role in the maintenance of normal muscle characteristics and in the early events of the recovery process of skeletal muscle to injury, serving both structural and gene-regulatory roles.

Activity-unrelated neural control of myogenic factors in a slow muscle

The properties of skeletal muscles are modulated by neural and nonneural factors, and the neural factors can be modulated by activity-independent as well as activity-dependent mechanisms. Given that daily activation of fast muscles is considerably less than of slow muscles, we hypothesized that the myogenic properties of the rat soleus (a slow muscle) would be more dependent on activity-dependent than activity-independent factors. Muscle mass, MyoD, and myogenin mRNA and protein levels, and satellite cell proliferation and differentiation rates (bromodeoxyuridine incorporation) were examined at 3, 14, and 28 days after either spinal cord isolation (SI, neuromuscular connectivity intact with minimal activation) or denervation (no neural influence). Soleus atrophy was similar in the SI and denervated groups at each time point, although increases in whole-muscle expression of myogenin and, to a lesser degree, MyoD were lower (P < 0.05) in SI than denervated soleus muscles. Proliferation and differentiation of satellite cells, as well as mitotic activity of connective tissue cells, were lower (P < 0.05) in SI than denervated soleus muscles. In some instances, these changes were not observed until the later time points, i.e., 14 or 28 days. These results demonstrate that the motoneurons that innervate the slow soleus muscle have a significant modulatory influence on some muscle properties via mechanisms that are independent of activation. These activity-independent modulatory influences, however, are less in the slow soleus than previously observed in fast muscles.

Difference in skeletal muscle function in males vs. females: role of estrogen receptor-beta

Male skeletal muscles are generally faster and have higher maximum power output than female muscles. Conversely, during repeated contractions, female muscles are generally more fatigue resistant and recover faster. We studied the role of estrogen receptor-beta (ERbeta) in this gender difference by comparing contractile function of soleus (mainly slow-twitch) and extensor digitorum longus (fast-twitch) muscles isolated from ERbeta-deficient (ERbeta(-/-)) and wild-type mice of both sexes. Results showed generally shorter contraction and relaxation times in male compared with female muscles, and ERbeta deficiency had no effect on this. Fatigue (induced by repeated tetanic contractions) and recovery of female muscles were not affected by ERbeta deficiency. However, male ERbeta(-/-) muscles were slightly more fatigue resistant and produced higher forces during the recovery period than wild-type male muscles. In fact, female muscles and male ERbeta(-/-) muscles displayed markedly better recovery than male wild-type muscles. Gene screening of male soleus muscles showed 25 genes that were differently expressed in ERbeta(-/-) and wild-type mice. Five of these genes were selected for further analysis: muscle ankyrin repeat protein-2, muscle LIM protein, calsequestrin, parvalbumin, and aquaporin-1. Expression of these genes showed a similar general pattern: increased expression in male and decreased expression in female ERbeta(-/-) muscles. In conclusion, ERbeta deficiency results in increased performance during fatigue and recovery of male muscles, whereas female muscles are not affected. Improved contractile performance of male ERbeta(-/-) mouse muscles was associated with increased expression of mRNAs encoding important muscle proteins.

Dystrophin- and MLP-deficient mouse hearts: marked differences in morphology and function, but similar accumulation of cytoskeletal proteins

In humans, cytoskeletal dystrophin and muscle LIM protein (MLP) gene mutations can cause dilated cardiomyopathy, yet these mutations may have different effects in mice, owing to increased accumulation of other, compensatory cytoskeletal proteins. Consequently, we characterized left-ventricular (LV) morphology and function in vivo using high-resolution cine-magnetic resonance imaging (MRI) in 2- to 3-month old dystrophin-deficient (mdx) and MLP-null mice, and their respective controls. LV passive stiffness was assessed in isolated, perfused hearts, and cytoskeletal protein levels were determined using Western blot analyses. In mdx mouse hearts, LV-to-body weight ratio, cavity volume, ejection fraction, stroke volume, and cardiac output were normal. However, MLP-null mouse hearts had 1.2-fold higher LV-to-body weight ratios (P<0.01), 1.5-fold higher end-diastolic volumes (P<0.01), and decreased ejection fraction compared with controls (25% vs. 66%, respectively, P<0.01), indicating dilated cardiomyopathy and heart failure. In both models, isolated, perfused heart end-diastolic pressure-volume relationships and passive left-ventricular stiffness were normal. Hearts from both models accumulated desmin and beta-tubulin, mdx mouse hearts accumulated utrophin and MLP, and MLP-null mouse hearts accumulated dystrophin and syncoilin. Although the increase in MLP and utrophin in the mdx mouse heart was able to compensate for the loss of dystrophin, accumulation of desmin, syncoilin and dystrophin were unable to compensate for the loss of MLP, resulting in heart failure.

Rapid muscle-specific gene expression changes after a single bout of eccentric contractions in the mouse

Eccentric contractions (ECs), in which a muscle is forced to lengthen while activated, result in muscle injury and, eventually, muscle strengthening and prevention of further injury. Although the mechanical basis of EC-induced injury has been studied in detail, the biological response of muscle is less well characterized. This study presents the development of a minimally invasive model of EC injury in the mouse, follows the time course of torque recovery after an injurious bout of ECs, and uses Affymetrix microarrays to compare the gene expression profile 48 h after ECs to both isometrically stimulated muscles and contralateral muscles. Torque dropped by ∼55% immediately after the exercise bout and recovered to initial levels 7 days later. Thirty-six known genes were upregulated after ECs compared with contralateral and isometrically stimulated muscles, including five muscle-specific genes: muscle LIM protein (MLP), muscle ankyrin repeat proteins (MARP1 and -2; also known as cardiac ankyrin repeat protein and Arpp/Ankrd2, respectively), Xin, and myosin binding protein H. The time courses of MLP and MARP expression after the injury bout (determined by quantitative real-time polymerase chain reaction) indicate that these genes are rapidly induced, reaching a peak expression level of 6–11 times contralateral values 12–24 h after the EC bout and returning to baseline within 72 h. Very little gene induction was seen after either isometric activation or passive stretch, indicating that the MLP and MARP genes may play an important and specific role in the biological response of muscle to EC-induced injury.

Combined cDNA array/RT‐PCR analysis of gene expression profile in rat gastrocnemius muscle: relation to its adaptive function in energy metabolism during fasting

We evaluated the effects of fasting on the gene expression profile in rat gastrocnemius muscle using a combined cDNA array and RT-PCR approach. Of the 1176 distinct rat genes analyzed on the cDNA array, 114 were up-regulated more than twofold in response to fasting, including all 17 genes related to lipid metabolism present on the membranes and all 10 analyzed components of the proteasome machinery. Only 7 genes were down-regulated more than twofold. On the basis of our analysis of genes on the cDNA array plus the data from our RT-PCR assays, the metabolic adaptations shown by rat gastrocnemius muscle during fasting are reflected by i) increased transcription both of myosin heavy chain (MHC) Ib (associated with type I fibers) and of at least three factors involved in the shift toward type I fibers [p27kip1, muscle LIM protein (MLP), cystein rich protein-2], of which one (MLP) has been shown to enhance the activity of MyoD, which would explain the known increase in the expression of skeletal muscle uncoupling protein-3 (UCP3); ii) increased lipoprotein lipase (LPL) expression, known to trigger UCP3 transcription, which tends, together with the first point, to underline the suggested role of UCP3 in mitochondrial lipid handling (the variations under the first point and this one have not been observed in mice, indicating a species-specific regulation of these mechanisms); iii) reduced expression of the muscle-specific coenzyme Q (CoQ)7 gene, which is necessary for mitochondrial CoQ synthesis, together with an increased expression of mitochondrial adenylate kinase 3, which inactivates the resident key enzyme for CoQ synthesis, 3-hydroxy-3-methylglutaryl CoA reductase (HMGR), the mRNA level for which fell during fasting; and iv) increased transcription of components of the proteasomal pathways involved in protein degradation/turnover.

Expression profiling identifies dysregulation of myosin heavy chains IIb and IIx during limb immobilization in the soleus muscles of old rats

Aged individuals suffer from multiple dysfunctions during skeletal muscle atrophy. The purpose of this study was to determine differential changes in gene expression in atrophied soleus muscle induced by hindlimb immobilization in young (3-4 months) and old (30-31 months) rats. The hypothesis was that differentially expressed mRNAs with age-atrophy interactions would reveal candidates that induce loss of function responses in aged animals. Each muscle was applied to an independent set of Affymetrix micoarrays, with 385 differentially expressed mRNAs with atrophy and 354 age-atrophy interactions detected by two-factor ANOVA (alpha of 0.05 with a Bonferroni adjustment). Functional trends were observed for 23 and 15 probe sets involved in electron transport and the extracellular matrix, respectively, decreasing more in the young than in the old. Other functional categories with atrophy in both ages included chaperones, glutathione-S-transferases, the tricarboxylic acid cycle, reductions in Z-line-associated proteins and increases in probe sets for protein degradation. Surprisingly, myosin heavy chain IIb and IIx mRNAs were suppressed in the atrophied soleus muscle of old rats as opposed to the large increases in the young animals (16- and 25-fold, respectively, with microarrays, and 61- and 68-fold, respectively, with real-time PCR). No significant changes were observed in myosin heavy chain IIb and IIx mRNA with micoarrays in the atrophied soleus muscles of old rats, but they were found to increase six- and fivefold, respectively, with real-time PCR. Therefore, deficiencies in pre-translational signals that normally upregulate myosin heavy chain IIb and IIx mRNAs during atrophy may exist in the soleus muscle of old animals.

Prolonged unloading of rat soleus muscle causes distinct adaptations of the gene profile

Using commercially available microarray technology, we investigated a series of transcriptional adaptations caused by atrophy of rat m. soleus due to 35 days of hindlimb suspension. We detected 395 out of 1,200 tested transcripts, which reflected 1%-5% of totally expressed genes. From various cellular functional pathways, we detected multiple genes that spanned a 200-fold range of gene expression levels. Statistical analysis combining L1 regression with the sign test based on the conservative Bonferroni correction identified 105 genes that underwent transcriptional adaptations with atrophy. Generally, expressional changes were discrete (<50%) and pointed in the same direction for genes belonging to the same cellular functional units. In particular, a distinct expressional adaptation of genes involved in fiber transformation; that is, metabolism, protein turnover, and cell regulation were noted and matched to corresponding transcriptional changes in nutrient trafficking. Expressional changes of extracellular proteases, and of genes involved in nerve-muscle interaction and excitation-contraction coupling identify previously not recognized adaptations that occur in atrophic m. soleus. Considerations related to technical and statistical aspects of the array approach for profiling the skeletal muscle genome and the impact of observed novel adaptations of the m. soleus transcriptome are put into perspective of the physiological adaptations occurring with muscular atrophy.