Arpp, a new homolog of carp, is preferentially expressed in type 1 skeletal muscle fibers and is markedly induced by denervation

Pilot Study on the Profiling and Functional Analysis of mRNA, miRNA, and lncRNA in the Skeletal Muscle of Mongolian Horses, Xilingol Horses, and Grassland-Thoroughbreds

Muscle fibers, as the fundamental units of muscle tissue, play a crucial role in determining skeletal muscle function through their growth, development, and composition. To investigate changes in muscle fiber types and their regulatory mechanisms in Mongolian horses (MG), Xilingol horses (XL), and Grassland-Thoroughbreds (CY), we conducted histological and bioinformatic analyses on the gluteus medius muscle of these three horse breeds. Immunofluorescence analysis revealed that Grassland-Thoroughbreds had the highest proportion of fast-twitch muscle fibers at 78.63%, while Mongolian horses had the lowest proportion at 57.54%. Whole-transcriptome analysis identified 105 differentially expressed genes (DEGs) in the CY vs. MG comparison and 104 DEGs in the CY vs. XL comparison. Time-series expression profiling grouped the DEGs into eight gene sets, with three sets showing significantly up-regulated or down-regulated expression patterns (p < 0.05). Additionally, 280 differentially expressed long non-coding RNAs (DELs) were identified in CY vs. MG, and 213 DELs were identified in CY vs. XL. A total of 32 differentially expressed microRNAs (DEMIRs) were identified in CY vs. MG, while 44 DEMIRs were found in CY vs. XL. Functional enrichment analysis indicated that the DEGs were significantly enriched in essential biological processes, such as actin filament organization, muscle contraction, and protein phosphorylation. KEGG pathway analysis showed their involvement in key signaling pathways, including the mTOR signaling pathway, FoxO signaling pathway, and HIF-1 signaling pathway. Furthermore, functional variation-based analyses revealed associations between non-coding RNAs and mRNAs, with some non-coding RNAs targeting genes potentially related to muscle function regulation. These findings provide valuable insights into the molecular basis for the environmental adaptability, athletic performance, and muscle characteristics in horses, offering new perspectives for the breeding of Grassland-Thoroughbreds.

Single-nucleus transcriptomics reveals subsets of degenerative myonuclei after rotator cuff tear-induced muscle atrophy

Rotator cuff tear (RCT) is the primary cause of shoulder pain and disability and frequently trigger muscle degeneration characterised by muscle atrophy, fatty infiltration and fibrosis. Single-nucleus RNA sequencing (snRNA-seq) was used to reveal the transcriptional changes in the supraspinatus muscle after RCT. Supraspinatus muscles were obtained from patients with habitual shoulder dislocation (n = 3) and RCT (n = 3). In response to the RCT, trajectory analysis showed progression from normal myonuclei to ANKRD1+ myonuclei, which captured atrophy-and fatty infiltration-related regulons (KLF5, KLF10, FOSL1 and BHLHE40). Transcriptomic alterations in fibro/adipogenic progenitors (FAPs) and muscle satellite cells (MuSCs) have also been studied. By predicting cell-cell interactions, we observed communication alterations between myofibers and muscle-resident cells following RCT. Our findings reveal the plasticity of muscle cells in response to RCT and offer valuable insights into the molecular mechanisms and potential therapeutic targets of RCT.

Fusion of Normoxic- and Hypoxic-Preconditioned Myoblasts Leads to Increased Hypertrophy

Injuries, high altitude, and endurance exercise lead to hypoxic conditions in skeletal muscle and sometimes to hypoxia-induced local tissue damage. Thus, regenerative myoblasts/satellite cells are exposed to different levels and durations of partial oxygen pressure depending on the spatial distance from the blood vessels. To date, it is unclear how hypoxia affects myoblasts proliferation, differentiation, and particularly fusion with normoxic myoblasts. To study this, we investigated how 21% and 2% oxygen affects C2C12 myoblast morphology, proliferation, and myogenic differentiation and evaluated the fusion of normoxic- or hypoxic-preconditioned C2C12 cells in 21% or 2% oxygen in vitro. Out data show that the long-term hypoxic culture condition does not affect the proliferation of C2C12 cells but leads to rounder cells and reduced myotube formation when compared with myoblasts exposed to normoxia. However, when normoxic- and hypoxic-preconditioned myoblasts were differentiated together, the resultant myotubes were significantly larger than the control myotubes. Whole transcriptome sequencing analysis revealed several novel candidate genes that are differentially regulated during the differentiation under normoxia and hypoxia in mixed culture conditions and may thus be involved in the increase in myotube size. Taken together, oxygen-dependent adaption and interaction of myoblasts may represent a novel approach for the development of innovative therapeutic targets.

Muscle ankyrin repeat protein 1 (MARP1) locks titin to the sarcomeric thin filament and is a passive force regulator

Muscle ankyrin repeat protein 1 (MARP1) is frequently up-regulated in stressed muscle, but its effect on skeletal muscle function is poorly understood. Here, we focused on its interaction with the titin–N2A element, found in titin’s molecular spring region. We show that MARP1 binds to F-actin, and that this interaction is stronger when MARP1 forms a complex with titin–N2A. Mechanics and super-resolution microscopy revealed that MARP1 “locks” titin–N2A to the sarcomeric thin filament, causing increased extension of titin’s elastic PEVK element and, importantly, increased passive force. In support of this mechanism, removal of thin filaments abolished the effect of MARP1 on passive force. The clinical relevance of this mechanism was established in diaphragm myofibers of mechanically ventilated rats and of critically ill patients. Thus, MARP1 regulates passive force by locking titin to the thin filament. We propose that in stressed muscle, this mechanism protects the sarcomere from mechanical damage.

Cloning and expression profiling of muscle regulator ANKRD2 in domestic chicken Gallus gallus

Striated muscle signaling protein and transcriptional regulator ANKRD2 participates in myogenesis, myogenic differentiation, muscle adaptation and stress response. It is preferentially expressed in slow, oxidative fibers of mammalian skeletal muscle. In this study, we report on characterization of chicken ANKRD2. The chicken ANKRD2 coding region contains 1002 bp and encodes a 334-amino acid protein which shares approximately 58% identity with human and mouse orthologs, mostly in the conserved region of ankyrin repeats. Comprehensive analysis of the ANKRD2 gene and protein expression in adult chicken demonstrated its predominant expression in red muscles of thigh and drumstick, compared to white muscle. It was not detected in heart and white pectoral muscle. Uneven expression of ANKRD2 in chicken skeletal muscles, observed by immunohistochemistry, was attributed to its selective expression in slow, oxidative, type I and fast, oxidative-glycolytic, type IIA myofibers. Association of chicken ANKRD2 with phenotypic differences between red and white muscles points to its potential role in the process of myofiber-type specification. In addition to expression in slow oxidative myofibers, as demonstrated for mammalian protein, chicken ANKRD2 was also detected in fast fibers with mixed oxidative and glycolytic metabolism. This finding suggests that ANKRD2 is responsive to metabolic differences between types of avian myofibers and orientates future studies towards investigation of its role in molecular mechanisms of myofiber-type-specific gene expression.

Vitamin D repletion ameliorates adipose tissue browning and muscle wasting in infantile nephropathic cystinosis-associated cachexia

BACKGROUND

Ctns-/- mice, a mouse model of infantile nephropathic cystinosis, exhibit hypermetabolism with adipose tissue browning and profound muscle wasting. Ctns-/- mice are 25(OH)D3 and 1,25(OH)2 D3 insufficient. We investigated whether vitamin D repletion could ameliorate adipose tissue browning and muscle wasting in Ctns-/- mice.

METHODS

Twelve-month-old Ctns-/- mice and wild-type controls were treated with 25(OH)D3 and 1,25(OH)2 D3 (75 μg/kg/day and 60 ng/kg/day, respectively) or an ethylene glycol vehicle for 6 weeks. Serum chemistry and parameters of energy homeostasis were measured. We quantitated total fat mass and studied expression of molecules regulating adipose tissue browning, energy metabolism, and inflammation. We measured lean mass content, skeletal muscle fibre size, in vivo muscle function (grip strength and rotarod activity), and expression of molecules regulating muscle metabolism. We also analysed the transcriptome of skeletal muscle in Ctns-/- mice using RNAseq.

RESULTS

Supplementation of 25(OH)D3 and 1,25(OH)2 D3 normalized serum concentration of 25(OH)D3 and 1,25(OH)2 D3 in Ctns-/- mice, respectively. Repletion of vitamin D partially or fully normalized food intake, weight gain, gain of fat, and lean mass, improved energy homeostasis, and attenuated perturbations of uncoupling proteins and adenosine triphosphate content in adipose tissue and muscle in Ctns-/- mice. Vitamin D repletion attenuated elevated expression of beige adipose cell biomarkers (UCP-1, CD137, Tmem26, and Tbx1) as well as aberrant expression of molecules implicated in adipose tissue browning (Cox2, Pgf2α, and NF-κB pathway) in inguinal white adipose tissue in Ctns-/- mice. Vitamin D repletion normalized skeletal muscle fibre size and improved in vivo muscle function in Ctns-/- mice. This was accompanied by correcting the increased muscle catabolic signalling (increased protein contents of IL-1β, IL-6, and TNF-α as well as an increased gene expression of Murf-2, atrogin-1, and myostatin) and promoting the decreased muscle regeneration and myogenesis process (decreased gene expression of Igf1, Pax7, and MyoD) in skeletal muscles of Ctns-/- mice. Muscle RNAseq analysis revealed aberrant gene expression profiles associated with reduced muscle and neuron regeneration, increased energy metabolism, and fibrosis in Ctns-/- mice. Importantly, repletion of 25(OH)D3 and 1,25(OH)2 D3 normalized the top 20 differentially expressed genes in Ctns-/- mice.

CONCLUSIONS

We report the novel findings that correction of 25(OH)D3 and 1,25(OH)2 D3 insufficiency reverses cachexia and may improve quality of life by restoring muscle function in an animal model of infantile nephropathic cystinosis. Mechanistically, vitamin D repletion attenuates adipose tissue browning and muscle wasting in Ctns-/- mice via multiple cellular and molecular mechanisms.

Ankrd2 in Mechanotransduction and Oxidative Stress Response in Skeletal Muscle: New Cues for the Pathogenesis of Muscular Laminopathies

Ankrd2 (ankyrin repeats containing domain 2) or Arpp (ankyrin repeat, PEST sequence, and proline-rich region) is a member of the muscle ankyrin repeat protein family. Ankrd2 is mostly expressed in skeletal muscle, where it plays an intriguing role in the transcriptional response to stress induced by mechanical stimulation as well as by cellular reactive oxygen species. Our studies in myoblasts from Emery-Dreifuss muscular dystrophy 2, a LMNA-linked disease affecting skeletal and cardiac muscles, demonstrated that Ankrd2 is a lamin A-binding protein and that mutated lamins found in Emery-Dreifuss muscular dystrophy change the dynamics of Ankrd2 nuclear import, thus affecting oxidative stress response. In this review, besides describing the latest advances related to Ankrd2 studies, including novel discoveries on Ankrd2 isoform-specific functions, we report the main findings on the relationship of Ankrd2 with A-type lamins and discuss known and potential mechanisms involving defective Ankrd2-lamin A interplay in the pathogenesis of muscular laminopathies.

Effect of lifelong carnitine supplementation on plasma and tissue carnitine status, hepatic lipid metabolism and stress signalling pathways and skeletal muscle transcriptome in mice at advanced age

While strong evidence from clinical studies suggests beneficial effects of carnitine supplementation on metabolic health, serious safety concerns associated with carnitine supplementation have been raised from studies in mice. Considering that the carnitine doses in these mice studies were up to 100 times higher than those used in clinical studies, the present study aimed to address possible safety concerns associated with long-term supplementation of a carnitine dose used in clinical trials. Two groups of NMRI mice were fed either a control or a carnitine-supplemented diet (1 g/kg diet) from weaning to 19 months of age, and parameters of hepatic lipid metabolism and stress signalling and skeletal muscle gene expression were analysed in the mice at 19 months of age. Concentrations of free carnitine and acetylcarnitine in plasma and tissues were higher in the carnitine than in the control group (P<0·05). Plasma concentrations of free carnitine and acetylcarnitine were higher in mice at adult age (10 and 15 months) than at advanced age (19 months) (P<0·05). Hepatic mRNA and protein levels of genes involved in lipid metabolism and stress signalling and hepatic and plasma lipid concentrations did not differ between the carnitine and the control group. Skeletal muscle transcriptome analysis in 19-month-old mice revealed only a moderate regulation between carnitine and control group. Lifelong carnitine supplementation prevents an age-dependent impairment of plasma carnitine status, but safety concerns associated with long-term supplementation of carnitine at doses used in clinical trials can be considered as unfounded.

Characterization of zebrafish (Danio rerio) muscle ankyrin repeat proteins reveals their conserved response to endurance exercise

Muscle proteins with ankyrin repeats (MARPs) ANKRD1 and ANKRD2 are titin-associated proteins with a putative role as transcriptional co-regulators in striated muscle, involved in the cellular response to mechanical, oxidative and metabolic stress. Since many aspects of the biology of MARPs, particularly exact mechanisms of their action, in striated muscle are still elusive, research in this field will benefit from novel animal model system. Here we investigated the MARPs found in zebrafish for protein structure, evolutionary conservation, spatiotemporal expression profiles and response to increased muscle activity. Ankrd1 and Ankrd2 show overall moderate conservation at the protein level, more pronounced in the region of ankyrin repeats, motifs indispensable for their function. The two zebrafish genes, ankrd1a and ankrd1b, counterparts of mammalian ANKRD1/Ankrd1, have different expression profiles during first seven days of development. Mild increase of ankrd1a transcript levels was detected at 72 hpf (1.74±0.24 fold increase relative to 24 hpf time point), while ankrd1b expression was markedly upregulated from 24 hpf onward and peaked at 72 hpf (92.18±36.95 fold increase relative to 24 hpf time point). Spatially, they exhibited non-overlapping expression patterns during skeletal muscle development in trunk (ankrd1a) and tail (ankrd1b) somites. Expression of ankrd2 was barely detectable. Zebrafish MARPs, expressed at a relatively low level in adult striated muscle, were found to be responsive to endurance exercise training consisting of two bouts of 3 hours of forced swimming daily, for five consecutive days. Three hours after the last exercise bout, ankrd1a expression increased in cardiac muscle (6.19±5.05 fold change), while ankrd1b and ankrd2 were upregulated in skeletal muscle (1.97±1.05 and 1.84±0.58 fold change, respectively). This study provides the foundation to establish zebrafish as a novel in vivo model for further investigation of MARPs function in striated muscle.

Human skeletal muscle type 1 fibre distribution and response of stress-sensing proteins along the titin molecule after submaximal exhaustive exercise

Early responses of stress-sensing proteins, muscle LIM protein (MLP), ankyrin repeat proteins (Ankrd1/CARP and Ankrd2/Arpp) and muscle-specific RING finger proteins (MuRF1 and MuRF2), along the titin molecule were investigated in the present experiment after submaximal exhaustive exercise. Ten healthy men performed continuous drop jumping unilaterally on a sledge apparatus with a submaximal height until complete exhaustion. Five stress-sensing proteins were analysed by mRNA measurements from biopsies obtained immediately and 3 h after the exercise from exercised vastus lateralis muscle while control biopsies were obtained from non-exercised legs before the exercise. Decreased maximal jump height and increased serum creatine kinase activities as indirect markers for muscle damage and HSP27 immunostainings on muscle biopsies as a direct marker for muscle damage indicated that the current exercised protocol caused muscle damage. mRNA levels for four (MLP, Ankrd1/CARP, MuRF1 and MuRF2) out of the five studied stress sensors significantly (p < 0.05) increased 3 h after fatiguing exercise. The magnitude of MLP and Ankrd2 responses was related to the proportion of type 1 myofibres. Our data showed that the submaximal exhaustive exercise with subject's own physical fitness level activates titin-based stretch-sensing proteins. These results suggest that both degenerative and regenerative pathways are activated in very early phase after the exercise or probably already during the exercise. Activation of these proteins represents an initial step forward adaptive remodelling of the exercised muscle and may also be involved in the initiation of myofibre repair.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Characterization of muscle ankyrin repeat proteins in human skeletal muscle

Muscle ankyrin repeat proteins (MARPs) are a family of titin-associated, stress-response molecules and putative transducers of stretch-induced signaling in skeletal muscle. In cardiac muscle, cardiac ankyrin repeat protein (CARP) and diabetes-related ankyrin repeat protein (DARP) reportedly redistribute from binding sites on titin to the nucleus following a prolonged stretch. However, it is unclear whether ankyrin repeat domain protein 2 (Ankrd 2) shows comparable stretch-induced redistribution to the nucleus. We measured the following in rested human skeletal muscle: 1) the absolute amount of MARPs and 2) the distribution of Ankrd 2 and DARP in both single fibers and whole muscle preparations. In absolute amounts, Ankrd 2 is the most abundant MARP in human skeletal muscle, there being ~3.1 µmol/kg, much greater than DARP and CARP (~0.11 and ~0.02 µmol/kg, respectively). All DARP was found to be tightly bound at cytoskeletal (or possibly nuclear) sites. In contrast, ~70% of the total Ankrd 2 is freely diffusible in the cytosol [including virtually all of the phosphorylated (p)Ankrd 2-Ser99 form], ~15% is bound to non-nuclear membranes, and ~15% is bound at cytoskeletal sites, likely at the N2A region of titin. These data are not consistent with the proposal that Ankrd 2, per se, or pAnkrd 2-Ser99 mediates stretch-induced signaling in skeletal muscle, dissociating from titin and translocating to the nucleus, because the majority of these forms of Ankrd 2 are already free in the cytosol. It will be necessary to show that the titin-associated Ankrd 2 is modified by stretch in some as-yet-unidentified way, distinct from the diffusible pool, if it is to act as a stretch-sensitive signaling molecule.

Multifunctional protein: cardiac ankyrin repeat protein

Cardiac ankyrin repeat protein (CARP) not only serves as an important component of muscle sarcomere in the cytoplasm, but also acts as a transcription co-factor in the nucleus. Previous studies have demonstrated that CARP is up-regulated in some cardiovascular disorders and muscle diseases; however, its role in these diseases remains controversial now. In this review, we will discuss the continued progress in the research related to CARP, including its discovery, structure, and the role it plays in cardiac development and heart diseases.

Differential expression and localization of Ankrd2 isoforms in human skeletal and cardiac muscles

Four human Ankrd2 transcripts, reported in the Ensembl database, code for distinct protein isoforms (360, 333, 327 and 300 aa), and so far, their existence, specific expression and localization patterns have not been studied in detail. Ankrd2 is preferentially expressed in the slow fibers of skeletal muscle. It is found in both the nuclei and the cytoplasm of skeletal muscle cells, and its localization is prone to change during differentiation and upon stress. Ankrd2 has also been detected in the heart, in ventricular cardiomyocytes and in the intercalated disks (ICDs). The main objective of this study was to distinguish between the Ankrd2 isoforms and to determine the contribution of each one to the general profile of Ankrd2 expression in striated muscles. We demonstrated that the known expression and localization pattern of Ankrd2 in striated muscle can be attributed to the isoform of 333 aa which is dominant in both tissues, while the designated cardiac and canonical isoform of 360 aa was less expressed in both tissues. The 360 aa isoform has a distinct nuclear localization in human skeletal muscle, as well as in primary myoblasts and myotubes. In contrast to the isoform of 333 aa, it was not preferentially expressed in slow fibers and not localized to the ICDs of human cardiomyocytes. Regulation of the expression of both isoforms is achieved at the transcriptional level. Our results set the stage for investigation of the specific functions and interactions of the Ankrd2 isoforms in healthy and diseased human striated muscles.

Overload-induced skeletal muscle hypertrophy is not impaired in STZ-diabetic rats

The aim of this study was to evaluate the effect of overload-induced hypertrophy on extensor digitorum longus (EDL) and soleus muscles of streptozotocin-induced diabetic rats. The overload-induced hypertrophy and absolute tetanic and twitch forces increases in EDL and soleus muscles were not different between diabetic and control rats. Phospho-Akt and rpS6 contents were increased in EDL muscle after 7 days of overload and returned to the pre-overload values after 30 days. In the soleus muscle, the contents of total and phospho-Akt and total rpS6 were increased in both groups after 7 days. The contents of total Akt in controls and total rpS6 and phospho-Akt in the diabetic rats remained increased after 30 days. mRNA expression after 7 days of overload in the EDL muscle of control and diabetic animals showed an increase in MGF and follistatin and a decrease in myostatin and Axin2. The expression of FAK was increased and of MuRF-1 and atrogin-1 decreased only in the control group, whereas Ankrd2 expression was enhanced only in diabetic rats. In the soleus muscle caused similar changes in both groups: increase in FAK and MGF and decrease in Wnt7a, MuRF-1, atrogin-1, and myostatin. Differences between groups were observed only in the increased expression of follistatin in diabetic animals and decreased Ankrd2 expression in the control group. So, insulin deficiency does not impair the overload-induced hypertrophic response in soleus and EDL muscles. However, different mechanisms seem to be involved in the comparable hypertrophic responses of skeletal muscle in control and diabetic animals.

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.

Profiling of skeletal muscle Ankrd2 protein in human cardiac tissue and neonatal rat cardiomyocytes

Muscle-specific mechanosensors Ankrd2/Arpp (ankyrin repeat protein 2) and Ankrd1/CARP (cardiac ankyrin repeat protein) have an important role in transcriptional regulation, myofibrillar assembly, cardiogenesis and myogenesis. In skeletal muscle myofibrils, Ankrd2 has a structural role as a component of a titin associated stretch-sensing complex, while in the nucleus it exerts regulatory function as transcriptional co-factor. It is also involved in myogenic differentiation and coordination of myoblast proliferation. Although expressed in the heart, the role of Ankrd2 in the cardiac muscle is completely unknown. Recently, we have shown that hypertrophic and dilated cardiomyopathy pathways are altered upon Ankrd2 silencing suggesting the importance of this protein in cardiac tissue. Here we provide the underlying basis for the functional investigation of Ankrd2 in the heart. We confirmed reduced Ankrd2 expression levels in human heart in comparison with Ankrd1 using RNAseq and Western blot. For the first time we demonstrated that, apart from the sarcomere and nucleus, both proteins are localized to the intercalated disks of human cardiomyocytes. We further tested the expression and localization of endogenous Ankrd2 in rat neonatal cardiomyocytes, a well-established model for studying cardiac-specific proteins. Ankrd2 was found to be expressed in both the cytoplasm and nucleus, independently from maturation status of cardiomyocytes. In contrast to Ankrd1, it is not responsive to the cardiotoxic drug Doxorubicin, suggesting that different mechanisms govern their expression in cardiac cells.

Proteomics of muscle chronological ageing in post-menopausal women

Background

Muscle ageing contributes to both loss of functional autonomy and increased morbidity. Muscle atrophy accelerates after 50 years of age, but the mechanisms involved are complex and likely result from the alteration of a variety of interrelated functions. In order to better understand the molecular mechanisms underlying muscle chronological ageing in human, we have undertaken a top-down differential proteomic approach to identify novel biomarkers after the fifth decade of age.

Results

Muscle samples were compared between adult (56 years) and old (78 years) post-menopausal women. In addition to total muscle extracts, low-ionic strength extracts were investigated to remove high abundance myofibrillar proteins and improve the detection of low abundance proteins. Two-dimensional gel electrophoreses with overlapping IPGs were used to improve the separation of muscle proteins. Overall, 1919 protein spots were matched between all individuals, 95 were differentially expressed and identified by mass spectrometry, and they corresponded to 67 different proteins. Our results suggested important modifications in cytosolic, mitochondrial and lipid energy metabolism, which may relate to dysfunctions in old muscle force generation. A fraction of the differentially expressed proteins were linked to the sarcomere and cytoskeleton (myosin light-chains, troponin T, ankyrin repeat domain-containing protein-2, vinculin, four and a half LIM domain protein-3), which may account for alterations in contractile properties. In line with muscle contraction, we also identified proteins related to calcium signal transduction (calsequestrin-1, sarcalumenin, myozenin-1, annexins). Muscle ageing was further characterized by the differential regulation of several proteins implicated in cytoprotection (catalase, peroxiredoxins), ion homeostasis (carbonic anhydrases, selenium-binding protein 1) and detoxification (aldo-keto reductases, aldehyde dehydrogenases). Notably, many of the differentially expressed proteins were central for proteostasis, including heat shock proteins and proteins involved in proteolysis (valosin-containing protein, proteasome subunit beta type-4, mitochondrial elongation factor-Tu).

Conclusions

This study describes the most extensive proteomic analysis of muscle ageing in humans, and identified 34 new potential biomarkers. None of them were previously recognized as differentially expressed in old muscles, and each may represent a novel starting point to elucidate the mechanisms of muscle chronological ageing in humans.

The muscle ankyrin repeat proteins CARP, Ankrd2, and DARP are not essential for normal cardiac development and function at basal conditions and in response to pressure overload

Ankrd1/CARP, Ankrd2/Arpp, and Ankrd23/DARP belong to a family of stress inducible ankyrin repeat proteins expressed in striated muscle (MARPs). The MARPs are homologous in structure and localized in the nucleus where they negatively regulate gene expression as well as in the sarcomeric I-band, where they are thought to be involved in mechanosensing. Together with their strong induction during cardiac disease and the identification of causative Ankrd1 gene mutations in cardiomyopathy patients, this suggests their important roles in cardiac development, function, and disease. To determine the functional role of MARPs in vivo, we studied knockout (KO) mice of each of the three family members. Single KO mice were viable and had no apparent cardiac phenotype. We therefore hypothesized that the three highly homologous MARP proteins may have redundant functions in the heart and studied double and triple MARP KO mice. Unexpectedly, MARP triple KO mice were viable and had normal cardiac function both at basal levels and in response to mechanical pressure overload induced by transverse aortic constriction as assessed by echocardiography and hemodynamic studies. Thus, CARP, Ankrd2, and DARP are not essential for normal cardiac development and function at basal conditions and in response to mechanical pressure overload.

CARP, a myostatin-downregulated gene in CFM Cells, is a novel essential positive regulator of myogenesis

Myostatin, a member of the TGF-β superfamily, has been shown to act as a negative regulator of myogenesis. Although its role in myogenesis has been clearly documented through genetic analysis, few gene cascades that respond to myostatin signaling and regulate myogenesis have been characterized, especially in avian species. In a previous study, we screened for such genes in chicken fetal myoblasts (CFMs) using the differential display PCR method and found that cardiac ankyrin repeat protein (CARP) was downregulated by myostatin and specifically expressed in chicken skeletal muscle. However, little is known about the potential functions of CARP in chicken skeletal myogenesis. In this study, the expression patterns of chicken CARP and the possible function of this gene in skeletal muscle growth were characterized. Our data showed that CARP was predominantly expressed in postnatal skeletal muscle, and its expression increased during myogenic differentiation in CFM cells. When CARP was overexpressed, CFM cell growth was enhanced by accelerating the cell cycle at the G1 to S phase transition and increasing cyclin D1 expression. CARP knockdown had the opposite effect: while myoblasts underwent differentiation, knockdown of CARP expression induced extensive cell death, suppressed the formation of myotubes, and markedly decreased the expression of differentiation-related genes such as myosin heavy chain (MHC), myoD, and caveolin-3. Our findings indicate that CARP may play a key role in the myostatin signaling cascade that governs chicken skeletal myogenesis through promoting proliferation and avoiding apoptosis during CFM cell differentiation.

Ankrd2 is a modulator of NF-κB-mediated inflammatory responses during muscle differentiation

Adaptive responses of skeletal muscle regulate the nuclear shuttling of the sarcomeric protein Ankrd2 that can transduce different stimuli into specific adaptations by interacting with both structural and regulatory proteins. In a genome-wide expression study on Ankrd2-knockout or -overexpressing primary proliferating or differentiating myoblasts, we found an inverse correlation between Ankrd2 levels and the expression of proinflammatory genes and identified Ankrd2 as a potent repressor of inflammatory responses through direct interaction with the NF-κB repressor subunit p50. In particular, we identified Gsk3β as a novel direct target of the p50/Ankrd2 repressosome dimer and found that the recruitment of p50 by Ankrd2 is dependent on Akt2-mediated phosphorylation of Ankrd2 upon oxidative stress during myogenic differentiation. Surprisingly, the absence of Ankrd2 in slow muscle negatively affected the expression of cytokines and key calcineurin-dependent genes associated with the slow-twitch muscle program. Thus, our findings support a model in which alterations in Ankrd2 protein and phosphorylation levels modulate the balance between physiological and pathological inflammatory responses in muscle.

Label-free quantitative protein profiling of vastus lateralis muscle during human aging

Sarcopenia corresponds to the loss of muscle mass occurring during aging, and is associated with a loss of muscle functionality. Proteomic links the muscle functional changes with protein expression pattern. To better understand the mechanisms involved in muscle aging, we performed a proteomic analysis of Vastus lateralis muscle in mature and older women. For this, a shotgun proteomic method was applied to identify soluble proteins in muscle, using a combination of high performance liquid chromatography and mass spectrometry. A label-free protein profiling was then conducted to quantify proteins and compare profiles from mature and older women. This analysis showed that 35 of the 366 identified proteins were linked to aging in muscle. Most of the proteins were under-represented in older compared with mature women. We built a functional interaction network linking the proteins differentially expressed between mature and older women. The results revealed that the main differences between mature and older women were defined by proteins involved in energy metabolism and proteins from the myofilament and cytoskeleton. This is the first time that label-free quantitative proteomics has been applied to study of aging mechanisms in human skeletal muscle. This approach highlights new elements for elucidating the alterations observed during aging and may lead to novel sarcopenia biomarkers.

Ankrd1 is a transcriptional repressor for the androgen receptor that is downregulated by testosterone

The ankryn repeat domain proteins, Ankrd1 and Ankrd2, are expressed at the highest levels in skeletal muscle and heart where they are localized to the I band of the sarcomere through binding to titin and myopaladin. Ankrd1 and Ankrd2 migrate from the sarcomere to the nucleus when muscle is stressed, and act as coregulators for a growing number of transcription factors. Expression of Ankrd1 is altered by castration suggesting a link to androgen action. This investigation explored the effects of testosterone on Ankrd1 and Ankrd2 expression and determined whether Ankrd1 or Ankrd2 binds to or regulates the transcriptional activity of the androgen receptor (AR). Incubation of rat L6 myoblasts expressing the human AR (L6.AR) with testosterone reduced mRNA levels for Ankrd1 by approximately 50% and increased those for Ankrd2 by 20-fold. In reporter gene assays conducted with CHO cells co-transfected with an ARE-Luc reporter gene, Ankrd1 blocked the ability of testosterone to increase reporter gene activity while Ankrd2 had no effect. The effect of Ankrd1 and Ankrd2 on repression of the MAFbx promoter by testosterone was also tested in C2C12 cells using an MAFbx-Luc reporter gene (pMAF400-Luc); Ankrd1 blocked repression of pMAF400-Luc by testosterone while Ankrd2 did not. Co-immunoprecipitation studies revealed that Ankrd1 bound to the AR whereas Ankrd2 did not. The effect of Ankrd1 or Ankrd2 on changes in gene expression induced by testosterone in L6.AR cells was also evaluated. Incubation of L6.AR cells with testosterone modestly reduced myogenin mRNA levels but did not significantly alter those for mdm2, MEF2d, TnnI1, TnnI2, or p21. When cells were transfected with Ankrd1, testosterone markedly reduced mRNA levels for MEF2d, myogenin, p21 and TnnI1, increased those for TnnI2, but did not alter those for mdm2. When cells were transfected with Ankrd2, testosterone increased MEF2d and myogenin mRNA levels, having the opposite effect to cells transfected with Ankrd1; Ankrd2 did not change the effects of testosterone on TnnI1, TnnI2, p21, or mdm2 mRNA levels. In conclusion, testosterone regulates the expression of Ankrd1 and Ankrd2; Ankrd1 binds to and directly regulates the transcriptional activity of the AR whereas Ankrd2 does not; expression levels of both Ankrd1 and Ankrd2 modulate effects of testosterone on gene expression in cultured myoblasts.

Cloning, expression, and bioinformatics analysis of the sheep CARP gene

The cardiac ankyrin repeat protein (CARP) is a multifunctional protein that is expressed specifically in mammalian cardiac muscle and plays important roles in stress responses, transcriptional regulation, myofibrillar assembly, and the development of cardiac and skeletal muscle. In this study, the sheep homolog of the CARP gene was cloned and characterized. The coding region of the gene consists of 960 bp and encodes 319 amino acids with molecular weight 36.2 KD. Bioinformatics analysis demonstrated that the 3' untranslated region (3'-UTR) of the gene contains many AU-rich elements that are associated with mRNA stability and a potential regulatory site for miRNA binding. The protein was predicted to contain 14 potential phosphorylation sites and an O-GlcNAc glycosylation site and to be expressed in both the nucleus and cytoplasm. The evolutionary analysis revealed that the sheep CARP exhibited a high level of homology with the mammalian counterparts; however, the protein exhibited an increased evolutionary distance from the chicken, frog, and fish homologs. RT-PCR revealed that in addition to its high mRNA expression level in cardiac muscle, trace amounts of the sheep CARP mRNA were expressed in the skeletal muscle, stomach, and small intestine. However, western blot analysis demonstrated that the CARP protein was expressed only in cardiac muscle. The coding sequence was cloned into the pET30a-TEV-LIC vector, and the soluble CARP-MBP (maltose-binding protein) fusion protein was expressed in a prokaryotic host and purified by affinity chromatography. Our data provide the basis for future studies of the structure and function of sheep CARP.

Multi-tasking role of the mechanosensing protein Ankrd2 in the signaling network of striated muscle

BACKGROUND

Ankrd2 (also known as Arpp) together with Ankrd1/CARP and DARP are members of the MARP mechanosensing proteins that form a complex with titin (N2A)/calpain 3 protease/myopalladin. In muscle, Ankrd2 is located in the I-band of the sarcomere and moves to the nucleus of adjacent myofibers on muscle injury. In myoblasts it is predominantly in the nucleus and on differentiation shifts from the nucleus to the cytoplasm. In agreement with its role as a sensor it interacts both with sarcomeric proteins and transcription factors.

METHODOLOGY/PRINCIPAL FINDINGS

Expression profiling of endogenous Ankrd2 silenced in human myotubes was undertaken to elucidate its role as an intermediary in cell signaling pathways. Silencing Ankrd2 expression altered the expression of genes involved in both intercellular communication (cytokine-cytokine receptor interaction, endocytosis, focal adhesion, tight junction, gap junction and regulation of the actin cytoskeleton) and intracellular communication (calcium, insulin, MAPK, p53, TGF-β and Wnt signaling). The significance of Ankrd2 in cell signaling was strengthened by the fact that we were able to show for the first time that Nkx2.5 and p53 are upstream effectors of the Ankrd2 gene and that Ankrd1/CARP, another MARP member, can modulate the transcriptional ability of MyoD on the Ankrd2 promoter. Another novel finding was the interaction between Ankrd2 and proteins with PDZ and SH3 domains, further supporting its role in signaling. It is noteworthy that we demonstrated that transcription factors PAX6, LHX2, NFIL3 and MECP2, were able to bind both the Ankrd2 protein and its promoter indicating the presence of a regulatory feedback loop mechanism.

CONCLUSIONS/SIGNIFICANCE

In conclusion we demonstrate that Ankrd2 is a potent regulator in muscle cells affecting a multitude of pathways and processes.

Molecular characterization and different expression patterns of the muscle ankyrin repeat protein (MARP) family during porcine skeletal muscle development in vitro and in vivo

CARP, ANKRD2, and DARP belong to the ankyrin repeat protein (MARP) family and play a critical role in the integration of cytoskeletal architecture, stress response, and transcriptional regulation. In this study, we cloned the cDNA and promoter sequences of porcine ankyrin repeat protein (MARP) gene family. RT-PCR analysis revealed that porcine CARP gene was predominantly expressed in heart. ANKRD2 was widely expressed in many tissues, a high expression level was observed in the skeletal muscle and heart. DARP gene was expressed specifically in skeletal muscle and heart. Moreover, the expression of CARP and ANKRD2 was significantly different in porcine skeletal muscle among different developmental stages and between the two breeds. Expression analysis in porcine satellite cells showed that CARP and ANKRD2 were induced in differentiated porcine satellite cells, suggesting a role of them in myogenic differentiation. This result suggests that the MARP gene family may be important genes for skeletal muscle growth and provides useful information for further studies on their roles in porcine skeletal muscle.

Anchoring skeletal muscle development and disease: the role of ankyrin repeat domain containing proteins in muscle physiology

The ankyrin repeat is a protein module with high affinity for other ankyrin repeats based on strong Van der Waals forces. The resulting dimerization is unusually resistant to both mechanical forces and alkanization, making this module exceedingly useful for meeting the extraordinary demands of muscle physiology. Many aspects of muscle function are controlled by the superfamily ankyrin repeat domain containing proteins, including structural fixation of the contractile apparatus to the muscle membrane by ankyrins, the archetypical member of the family. Additionally, other ankyrin repeat domain containing proteins critically control the various differentiation steps during muscle development, with Notch and developmental stage-specific expression of the members of the Ankyrin repeat and SOCS box (ASB) containing family of proteins controlling compartment size and guiding the various steps of muscle specification. Also, adaptive responses in fully formed muscle require ankyrin repeat containing proteins, with Myotrophin/V-1 ankyrin repeat containing proteins controlling the induction of hypertrophic responses following excessive mechanical load, and muscle ankyrin repeat proteins (MARPs) acting as protective mechanisms of last resort following extreme demands on muscle tissue. Knowledge on mechanisms governing the ordered expression of the various members of superfamily of ankyrin repeat domain containing proteins may prove exceedingly useful for developing novel rational therapy for cardiac disease and muscle dystrophies.

MARP Protein Family: A Possible Role in Molecular Mechanisms of Tumorigenesis

The MARP (muscle ankyrin repeat protein) family comprises three structurally similar proteins: CARP/Ankrd1, Ankrd2/Arpp and DARP/Ankrd23. They share four conserved copies of 33-residue ankyrin repeats and contain a nuclear localization signal, allowing the sorting of MARPs to the nucleus. They are found both in the nucleus and in the cytoplasm of skeletal and cardiac muscle cells, suggesting that MARPs shuttle within the cell enabling them to play a role in signal transduction in striated muscle. Expression of MARPs is altered under different pathological conditions. In skeletal muscle, CARP/Ankrd1 and Ankrd2/Arpp are up-regulated in muscle in patients suffering from Duchene muscular dystrophy, congenital myopathy and spinal muscular atrophy. Mutations inAnkrd1gene (coding CARP/Ankrd1) were identified in dilated and hypertrophic cardiomyopathies. Altered expression of MARPs is also observed in rhabdomyosarcoma, renal oncocytoma and ovarian cancer. In order to functionally characterize MARP family members CARP/Ankrd1 and Ankrd2/Arpp, we have found that both proteins interact with the tumor suppressor p53 bothin vivoandin vitroand that p53 up-regulates their expression. Our results implicate the potential role of MARPs in molecular mechanisms relevant to tumor response and progression.

Anisotropic regulation of Ankrd2 gene expression in skeletal muscle by mechanical stretch

The diaphragm muscles in vivo are subjected to mechanical forces both in the direction of the muscle fibers and in the direction transverse to the fibers. However, the effect of directional mechanical forces in skeletal muscle gene regulation is completely unknown. Here, we identified that stretch in the longitudinal and transverse directions to the diaphragm muscle fibers up-regulated Ankrd2 gene expression by two distinct signaling pathways in wild-type (WT) and mdm, a mouse model of muscular dystrophy with early-onset of progressive muscle-wasting. Stretch in the longitudinal direction activated both NF-kappaB and AP-1 transcription factors, whereas stretch in the transverse direction activated only AP-1 transcription factor. Interestingly, longitudinal stretch activated Ankrd2 promoter only by NF-kappaB, whereas transverse stretch activated Ankrd2 promoter by AP-1. Moreover, we found that longitudinal stretch activated Akt, which up-regulated Ankrd2 expression through NF-kappaB. However, transverse stretch activated Ras-GTP, Raf-1, and Erk1/2 proteins, which up-regulated Ankrd2 expression through AP-1. Surprisingly, the stretch-activated NF-kappaB and AP-1 signaling pathways was not involved in Ankrd2 regulation at the basal level, which was high in the mdm mouse diaphragm. Taken together, our data show the anisotropic regulation of Ankrd2 gene expression in the diaphragm muscles of WT and mdm mice via two distinct mechanosensitive signaling pathways.

Differential expression of sarcoplasmic and myofibrillar proteins of rat soleus muscle during denervation atrophy

Denervation is known to induce skeletal muscle atrophy and fiber-type transitions, the molecular mechanisms of which are poorly understood. To investigate the effect of denervation on skeletal muscle, proteomic analysis was performed to compare denervated soleus muscle with normal soleus muscle. The muscles were fractionated to myofibrillar and sarcoplasmic fractions, which were analysed using two-dimensional gel electrophoresis (2-DE), followed by MALDI-TOF-MS. At least 30 differentially regulated proteins were identified in the sarcoplasmic fractions of normal and denervated soleus muscles. This group included metabolic enzymes, signaling molecules, chaperones, and contractile proteins. We also found two proteins, APOBEC-2 (RNA-editing enzyme) and Gamma-synuclein (breast cancer related protein), which have not been recognized as denervation-induced proteins to date. Our results might prove to be beneficial in elucidating the molecular mechanisms of denervation-induced muscle atrophy.

Characterization of antibodies directed against the Ankrd2 human muscle protein

In order to study the function of the Ankrd2 protein, for which commercial antibodies are not available, we report the production and analysis of polyclonal antibodies to full-length Ankrd2 and its C-terminal and N-terminal regions, as well as a monoclonal antibody to the C-terminus of the protein. Epitope mapping making use of recombinant deletion mutants showed that an epitope located in region 323-333 aa of Ankrd2 is detected by the monoclonal antibody. The high specificity of all four anti-Ankrd2 antibodies for recombinant and endogenous Ankrd2 protein is also demon?strated.

Adaptive features of skeletal muscles of mole rats (Spalax ehrenbergi) to intensive activity under subterranean hypoxic conditions

Mole rats of the Spalax ehrenbergi superspecies are blind subterranean rodents that live under fluctuating oxygen supply, reduced to a measured 6% O(2), and mostly probably lower, during the rainy season. Fiber typing of muscles of the neck (trapezius) and leg (gastrocnemius, quadriceps) using standard histochemical techniques (succinic dehydrogenase, myosin ATPase) showed that the muscle fibers of mole rats in natural settings, as well as after extended captivity, were predominantly type IIa. The same muscles in laboratory rats showed the full range of fiber types. In contrast, the hearts of the mole rats and the laboratory rats were very similar. Our results indicate that skeletal muscle in the mole rats appears to have evolved in response to specific environmental demands to permit intensive endurance burrowing activities under conditions of severe or chronic hypoxia.

Expression of cardiac ankyrin repeat protein, CARP, in malignant tumors: diagnostic use of CARP protein immunostaining in rhabdomyosarcoma

Cardiac ankyrin repeat protein (CARP) is highly expressed in cardiac muscles and detectable in normal skeletal muscles. Arpp, a close homolog of CARP, has been demonstrated to be useful for distinguishing rhabdomyosarcoma from other malignant tumors. However, the CARP distributions among malignant tumors have been poorly investigated. Here, we analyzed the comprehensive expression of CARP in malignant tumors and evaluated its potential use for rhabdomyosarcoma diagnosis. A total of 159 malignant tumors, including 34 rhabdomyosarcomas, 85 non-rhabdomyosarcomas, and 40 carcinomas, were immunohistochemically analyzed for CARP expression. Cytoplasmic expression of CARP was detected in 29 (85%) of 34 rhabdomyosarcomas. The immunoreactivity was observed in both small cells with little differentiation and differentiated tumor cells with abundant eosinophilic cytoplasm. In contrast, focal immunoreactivity for CARP was only observed in 5 (4%) of 125 non-rhabdomyosarcomas, comprising 2 malignant fibrous histiocytomas, 1 angiosarcoma, 1 epithelioid sarcoma, and 1 squamous cell carcinoma of the lung. Comparative analysis of the CARP expression profiles with those of myogenic markers in rhabdomyosarcomas revealed that myogenin (88%) and desmin (88%) exhibited the best sensitivity, followed by CARP (85%), MyoD (82%), muscle-specific actin (79%), and myoglobin (65%). MyoD (96%) and myoglobin (96%) had the best specificity, followed by CARP (95%), myogenin (95%), desmin (89%), and muscle-specific actin (86%). Our results indicate that CARP is a sensitive and specific marker for rhabdomyosarcoma and that it will be useful for the differential diagnosis of rhabdomyosarcoma.

The effects of Ankrd2 alteration indicate its involvement in cell cycle regulation during muscle differentiation

Ankrd2 is a member of the Muscle Ankyrin Repeat Protein family (MARPs), consisting of sarcomere-associated proteins that can also localize in the nucleus. There are indications that MARPs might function as shuttle proteins between the cytoplasm and nucleus, likely sending information to the nucleus concerning the changes in the structure or function of the contractile machinery. Even though recent findings suggest that the MARP gene family is not essential for the basal functioning of skeletal muscle, its influence on the gene expression program of skeletal muscle cells was highlighted. To investigate this regulatory role we produced and examined both morphological and functional features of myocytes stable overexpressing or silencing the Ankrd2 protein. The transcriptional profiles of the myocytes revealed that the molecular pathways perturbed by changes in Ankrd2 protein level are congruent with the morpho-physiological and biochemical data obtained in Ankrd2-modified myoblasts induced to differentiate. Our results suggest that Ankrd2 gives an important contribution to the coordination of proliferation and apoptosis during myogenic differentiation in vitro, mainly through the p53 network.

Arpp/Ankrd2, a member of the muscle ankyrin repeat proteins (MARPs), translocates from the I-band to the nucleus after muscle injury

Ankyrin-repeat protein with a PEST motif and a proline-rich region (Arpp), also designated as Ankrd2, is a member of the muscle ankyrin repeat proteins (MARPs), which have been proposed to be involved in muscle stress response pathways. Arpp/Ankrd2 is localized mainly in the I-band of striated muscle. However, it has recently been reported that Arpp/Ankrd2 can interact with nuclear proteins, such as premyelocytic leukemia protein (PML), p53 and YB-1 in vitro. In this study, to determine whether nuclear accumulation of Arpp/Ankrd2 actually occurs, we performed an immunohistochemical investigation of gastrocnemius muscles that had been injured by injection of cardiotoxin or contact with dry ice. We found that Arpp/Ankrd2 accumulated in the nuclei of myofibers located adjacent to severely damaged myofibers after muscle injury. Double-labeled immunohistochemistry revealed that Arpp/Ankrd2 accumulated in the nuclei of sarcomere-damaged myofibers. Furthermore, we found that Arpp/Ankrd2 tended to be localized in euchromatin where genes are transcriptionally activated. Based on these findings, we suggest that Arpp/Ankrd2 may translocate from the I-band to the nucleus in response to muscle damage and may participate in the regulation of gene expression.

Iron load and redox stress in skeletal muscle of aged rats

Loss of skeletal muscle mass (sarcopenia) is a major contributor to disability in old age. We used two-dimensional gel electrophoresis and mass spectrometry to screen for changes in proteins, and cDNA profiling to assess transcriptional regulations in the gastrocnemius muscle of adult (4 months) and aged (30 months) male Sprague-Dawley rats. Thirty-five proteins were differentially expressed in aged muscle. Proteins and mRNA transcripts involved in redox homeostasis and iron load were increased, representing novel components that were previously not associated with sarcopenia. Tissue iron levels were elevated in senescence, paralleling an increase in transferrin. Proteins involved in redox homeostasis showed a complex pattern of changes with increased SOD1 and decreased SOD2. These results suggest that an elevated iron load is a significant component of sarcopenia with the potential to be exploited clinically, and that mitochondria of aged striated muscle may be more vulnerable to radicals produced in cell respiration.

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.

ARPP protein is selectively expressed in renal oncocytoma, but rarely in renal cell carcinomas

We have recently isolated a gene, Ankyrin-repeated protein with a proline-rich region (ARPP), that is highly expressed in the skeletal and cardiac muscle. Our previous immunohistochemical analysis revealed that ARPP expression was augmented in rhabdomyosarcoma but scarcely detectable in leiomyosarcoma, showing that ARPP is a useful marker for rhabdomyosarcoma. In the present study, we generated the anti-ARPP monoclonal antibody, YAS11, immunoreactive with the N-terminal region (amino-acids residues 1-145) of the ARPP protein. Further, we immunohistochemically analyzed 100 renal tumors including 14 oncocytomas, and 86 renal cell carcinomas (RCCs). We found that ARPP was highly expressed in 12 of the 14 (85.7%) oncocytomas, but was detectable in only four of the 86 (4.7%) RCCs. Interestingly, ARPP was not detected in any of 11 chromophobe RCCs, suggesting that ARPP may be useful for differential diagnosis between oncocytoma and chromophobe RCC. Furthermore, we found that ARPP was selectively expressed in part of the distal renal tubule in normal kidney. Immunoelectron microscopy with anti-ARPP antibody revealed that ARPP was localized in mitochondria and nuclei in both the normal distal renal tubule and oncocytoma, suggesting that oncocytoma may be derived from the distal nephron, and probably from part of the distal renal tubule.

Upregulated expression of cardiac ankyrin-repeated protein in renal podocytes is associated with proteinuria severity in lupus nephritis

Cardiac ankyrin-repeated protein (CARP) was originally identified as a protein specifically expressed in cardiomyocytes, but has recently been found to be upregulated in some muscle diseases including muscular dystrophy and myopathy, suggesting that CARP may be induced in some pathologic conditions. In this study, we immunohistochemically analyzed 69 renal biopsy samples from patients with glomerular diseases and 2 individuals with normal kidney. We found that CARP was expressed in renal podocytes at a high level in 10 of 13 cases of crescentic glomerulonephritis, 7 of 19 cases of diabetic nephropathy, and 12 of 20 cases of lupus nephritis, although it was not expressed in endocapillary glomerulonephritis, minimal change disease, thin basement membrane disease, membranous glomerulonephritis, and normal kidney. Interestingly, in lupus nephritis, CARP expression tended to be induced in cases exhibiting nephrotic syndrome, but less so in cases without nephrotic syndrome, suggesting that CARP expression is correlated with the severity of proteinuria. Furthermore, we found that CARP was not expressed in membranous glomerulonephritis but evidently expressed in most cases of membranous lupus nephritis. Although membranous glomerulonephritis and membranous lupus nephritis are sometimes morphologically indistinguishable, it is suggested that their pathologic mechanisms differ. Therefore, we propose that examination of CARP expression is useful for precise differential diagnosis of membranous glomerulonephritis and membranous lupus nephritis.

Differential gene expression in skeletal muscle of rats with vitamin E deficiency

Vitamin E (VE) deficiency is accompanied by myopathy in various animal species including man. Although gene expression profiles related to degenerative and regenerative processes in different kinds of myopathies have been studied, no global expression profile for skeletal muscle subject to VE deficiency has previously been reported. In the present study, Affymetrix GeneChip technology was used to obtain such a profile. Two groups of male rats were fed with either a diet deficient in VE or a control diet. Differential gene expression was monitored at five time-points over 430 days, with all animals individually profiled. Out of approximately 7000 genes represented on the Genechip, 56 were found to be up-regulated in response to VE deficiency in at least four consecutive time-points from as early as 91 days of deficiency. Up-regulated genes included muscle structure and extra cellular matrix genes, as well as anti-oxidative, anti-inflammatory and anti-fibrotic genes. Our data show that molecular transcription might provide a very early marker to detect oncoming degenerative conditions in VE deficiency. They provide further insight into possible molecular mechanisms underlying VE deficiency in skeletal muscle, and reveal the activation of an intensive protection program that can explain the long maintenance of muscle structure during deficiency.

Reinnervation-induced alterations in rat skeletal muscle

Denervation-induced myofiber atrophy can be reversed by reinnervation. Growing reinnervated myofibers upregulate numerous molecules, many of which determine the muscle fiber type. In the present study we aimed at identifying factors that might contribute specifically to myofiber growth after reinnervation. The common peroneal nerve of 15 male Wistar rats was cut and resutured without delay (9 animals) or with a delay of 4 weeks (6 animals). We studied the transcriptional repertoire of intact reinnervated tibialis anterior muscle by microarray gene analysis. We assessed SC activation by immunolabeling using anti-MyoD and -myogenin antibodies. The percentage of SC expressing MyoD reached up to 50% of M-cadherin+ cells whereas the percentage of SC expressing myogenin was normal (<10%) in all muscles examined. The values of ipsi- and contralateral muscles did not differ significantly from one another between right and left leg (p<0.05). Thirteen known genes were differentially regulated after reinnervation compared with contralateral muscles. Five of them determine the slow-twitch fiber type (four and a half LIM domains 3, cardiac beta-myosin heavy chain, calsequestrin 2, troponin C (slow), and heart myosin light chain), and three of them are neurally regulated (thrombospondin 4, transferrin receptor, cardiac ankyrin repeat protein). The results strengthen the notion that reinnervaton affects the molecular repertoire of the myofibers directly, leading to fiber type transformation and partial reversal of the denervation phenotype. By contrast, SC do not appear to be affected by reinnervation directly. They can be activated both in reinnervated and contralateral muscles, and they do not fully differentiate. This makes them unlikely to contribute to myofiber growth.