Involvement of SPARC in in vitro differentiation of skeletal myoblasts

Expression of intramuscular extracellular matrix proteins in vastus lateralis muscle fibres between atrophic and non-atrophic COPD

Background

Extracellular matrix (ECM) proteins are the major constituents of the muscle cell micro-environment, imparting instructive signalling, steering cell behaviour and controlling muscle regeneration. ECM remodelling is among the most affected signalling pathways in COPD and aged muscle. As a fraction of COPD patients present muscle atrophy, we questioned whether ECM composition would be altered in patients with peripheral muscle wasting (atrophic COPD) compared to those without muscle wasting (non-atrophic COPD).

Methods

A set of ECM molecules with known impact on myogenesis were quantified in vastus lateralis muscle biopsies from 29 COPD patients (forced expiratory volume in 1 s 55±12% predicted) using ELISA and real-time PCR. COPD patients were grouped to atrophic or non-atrophic based on fat-free mass index (<17 or ≥17 kg·m-2).

Results

Atrophic COPD patients presented a lower average vastus lateralis muscle fibre cross-sectional area (3872±258 μm2) compared to non-atrophic COPD (4509±198 μm2). Gene expression of ECM molecules was found significantly lower in atrophic COPD compared to non-atrophic COPD for collagen type I alpha 1 chain (COL1A1), fibronectin (FN1), tenascin C (TNC) and biglycan (BGN). In terms of protein levels, there were no significant differences between the two COPD cohorts for any of the ECM molecules tested.

Conclusions

Although atrophic COPD presented decreased contractile muscle tissue, the differences in ECM mRNA expression between atrophic and non-atrophic COPD were not translated at the protein level, potentially indicating an accumulation of long-lived ECM proteins and dysregulated proteostasis, as is typically observed during deconditioning and ageing.

Proteomics and Machine Learning in the Prediction and Explanation of Low Pectoralis Muscle Area

Low muscle mass is associated with numerous adverse outcomes independent of other associated comorbid diseases. We aimed to predict and understand an individual's risk for developing low muscle mass using proteomics and machine learning. We identified 8 biomarkers associated with low pectoralis muscle area (PMA). We built 3 random forest classification models that used either clinical measures, feature selected biomarkers, or both to predict development of low PMA. The area under the receiver operating characteristic curve for each model was: clinical-only = 0.646, biomarker-only = 0.740, and combined = 0.744. We displayed the heterogenetic nature of an individual's risk for developing low PMA and identified 2 distinct subtypes of participants who developed low PMA. While additional validation is required, our methods for identifying and understanding individual and group risk for low muscle mass could be used to enable developments in the personalized prevention of low muscle mass.

Extraocular muscle stem cells exhibit distinct cellular properties associated with non-muscle molecular signatures

Skeletal muscle stem cells (MuSCs) are recognised as functionally heterogeneous. Cranial MuSCs are reported to have greater proliferative and regenerative capacity when compared with those in the limb. A comprehensive understanding of the mechanisms underlying this functional heterogeneity is lacking. Here, we have used clonal analysis, live imaging and single cell transcriptomic analysis to identify crucial features that distinguish extraocular muscle (EOM) from limb muscle stem cell populations. A MyogeninntdTom reporter showed that the increased proliferation capacity of EOM MuSCs correlates with deferred differentiation and lower expression of the myogenic commitment gene Myod. Unexpectedly, EOM MuSCs activated in vitro expressed a large array of extracellular matrix components typical of mesenchymal non-muscle cells. Computational analysis underscored a distinct co-regulatory module, which is absent in limb MuSCs, as driver of these features. The EOM transcription factor network, with Foxc1 as key player, appears to be hardwired to EOM identity as it persists during growth, disease and in vitro after several passages. Our findings shed light on how high-performing MuSCs regulate myogenic commitment by remodelling their local environment and adopting properties not generally associated with myogenic cells.

Downregulation of Sparc-like protein 1 during cisplatin-induced inhibition of myogenic differentiation of C2C12 myoblasts

Patients with cancer often experience muscle atrophy, which worsens their prognosis. Decreased muscle regenerative capacity plays an important role in the complex processes involved in muscle atrophy. Administration of cisplatin, a cancer chemotherapeutic agent, has been implicated as a cause of muscle atrophy. In this study, we examined whether cisplatin affects the differentiation of myoblasts into myotubes. We treated C2C12 myoblasts with a differentiation medium containing cisplatin and its vehicle during for 8 days and observed the changes in the expression of myosin heavy chain (MyHC) and myogenin in the myoblasts. Cisplatin was injected in mice for 4 consecutive days; on Day 5, the mice quadriceps muscles were sampled and examined. The expression of MyHCs increased and that of myogenin decreased after cisplatin treatment. The secretion of acidic cysteine-rich proteins (e.g., Sparc proteins) reportedly promotes C2C12 myoblast differentiation. Therefore, we investigated the Sparc family gene expression during myogenesis in C2C12 myoblasts after cisplatin treatment. Of all the genes investigated, Sparc-like protein 1 (Sparcl1) expression was significantly suppressed by cisplatin on Days 4-8. Simultaneous treatment with recombinant mouse Sparcl1 almost inhibited the cisplatin-induced suppression of total MyHC and myogenin protein levels. Moreover, Sparcl1 expression decreased in the skeletal muscles of mice, leading to cisplatin-induced muscle atrophy. Our results suggest that cisplatin-induced myogenesis suppression causes muscle atrophy and inhibits the expression of Sparcl1, which promotes C2C12 cell differentiation during myogenesis.

Epithelial and fibroblast SPARC expression patterns in oral leukoplakia and oral squamous cell carcinoma

OBJECTIVE

This study evaluated and compared the expression of secreted protein acidic and rich in cysteine (SPARC) in epithelial cells and fibroblasts of oral leukoplakia (OL) and oral squamous cell carcinoma (OSCC) using normal oral mucosa as a control.

STUDY DESIGN

The expression of SPARC was determined in samples of normal oral mucosa (n = 12), OL without dysplasia (n = 31), OL with dysplasia (n = 54), and OSCC (n = 69) using immunohistochemistry. The percentage of positive cells in epithelial cells and fibroblasts was independently evaluated.

RESULTS

Epithelial SPARC was found in 33.3%, 35.5%, 25.9%, and 66.7% of normal oral mucosa, OL without dysplasia, OL with dysplasia, and OSCC, respectively. Fibroblast SPARC was found in 50.0%, 29.0%, 46.3%, and 84.1% of normal oral mucosa, OL without dysplasia, OL with dysplasia, and OSCC, respectively. OSCC had higher epithelial and fibroblast SPARC expression than normal oral mucosa, OL without dysplasia, and OL with dysplasia (P < .05). No significant differences were observed in epithelial and fibroblast SPARC among normal oral mucosa or OL with and without dysplasia.

CONCLUSION

Overexpression of epithelial and fibroblast SPARC was observed in OSCC but not in OL, suggesting that SPARC is involved in the late stage of oral carcinogenesis.

Secreted Protein Acidic and Rich in Cysteine as an Exercise-Induced Gene: Towards Novel Molecular Therapies for Immobilization-Related Muscle Atrophy in Elderly Patients

Long periods of immobilization, among other etiologies, would result is muscle atrophy. Exercise is the best approach to reverse this atrophy. However, the limited or the non-ability to perform the required physical activity for such patients and the limited pharmacological options make developing novel therapeutic approaches a necessity. Within this context, secreted protein acidic and rich in cysteine (SPARC) has been characterized as an exercise-induced gene. Whereas the knock-out of this gene leads to a phenotype that mimics number of the ageing-induced and sarcopenia-related changes including muscle atrophy, overexpressing SPARC in mice or adding it to muscular cell culture produces similar effects as exercise including enhanced muscle mass, strength and metabolism. Therefore, this piece of writing aims to provide evidence supporting the potential use of SPARC/SPARC as a molecular therapy for muscle atrophy in the context of immobilization especially for elderly patients.

Mechanisms by Which Skeletal Muscle Myokines Ameliorate Insulin Resistance

The skeletal muscle is the largest organ in the body and secretes circulating factors, including myokines, which are involved in various cellular signaling processes. Skeletal muscle is vital for metabolism and physiology and plays a crucial role in insulin-mediated glucose disposal. Myokines have autocrine, paracrine, and endocrine functions, serving as critical regulators of myogenic differentiation, fiber-type switching, and maintaining muscle mass. Myokines have profound effects on energy metabolism and inflammation, contributing to the pathophysiology of type 2 diabetes (T2D) and other metabolic diseases. Myokines have been shown to increase insulin sensitivity, thereby improving glucose disposal and regulating glucose and lipid metabolism. Many myokines have now been identified, and research on myokine signaling mechanisms and functions is rapidly emerging. This review summarizes the current state of the field regarding the role of myokines in tissue cross-talk, including their molecular mechanisms, and their potential as therapeutic targets for T2D.

Roles of the synaptic molecules Hevin and SPARC in mouse neuromuscular junction development and repair

Hevin and secreted protein acidic and rich in cysteine (SPARC) are highly homologous matricellular proteins that function in concert to guide the formation of brain synapses. Here, we investigated the role of these glycoproteins in neuromuscular junction (NMJ) maturation, stability, and repair following injury. Hevin and SPARC mRNA levels in developing (postnatal day 9), adult (postnatal days 90 and 120), and injured (fibular nerve crush) skeletal muscles were assessed with qPCR. Muscle fiber size was analyzed in developing (P9) mice lacking SPARC, Hevin, and both SPARC and Hevin. NMJ morphology was assessed in developing (P9), adult (P90) and injured (fibular nerve crush) mice lacking SPARC, Hevin, and both SPARC and Hevin skeletal muscle. Hevin and SPARC are expressed in skeletal muscles and are upregulated following nerve injury. Hevin^-/- mice exhibited delayed NMJ and muscle fiber development but displayed normal NMJ morphology in adulthood and accelerated NMJ reinnervation following nerve injury. Mice lacking SPARC displayed normal NMJ and muscle fiber development but exhibited smaller NMJs with fewer acetylcholine receptor islands in adulthood. Further, SPARC deletion did not result in overt changes in NMJ reformation following nerve injury. The combined deletion of Hevin and SPARC had little effect on NMJ phenotypes observed in single knockouts, however deletion of SPARC in combination with Hevin reversed deficiencies in muscle fiber maturation observed in Hevin^-/- muscle. These results identify SPARC and Hevin as extracellular matrix proteins with roles in NMJ development and repair.

IL-1β and TNF-α Modulation of Proliferated and Committed Myoblasts: IL-6 and COX-2-Derived Prostaglandins as Key Actors in the Mechanisms Involved

In this study, we investigated the effects and mechanisms of the pro-inflammatory cytokines IL-1β and TNF-α on the proliferation and commitment phases of myoblast differentiation. C2C12 mouse myoblast cells were cultured to reach a proliferated or committed status and were incubated with these cytokines for the evaluation of cell proliferation, cyclooxygenase 2 (COX-2) expression, release of prostaglandins (PGs) and myokines, and activation of myogenic regulatory factors (MRFs). We found that inhibition of the IL-6 receptor reduced IL-1β- and TNF-α-induced cell proliferation, and that the IL-1β effect also involved COX-2-derived PGs. Both cytokines modulated the release of the myokines myostatin, irisin, osteonectin, and IL-15. TNF-α and IL-6 reduced the activity of Pax7 in proliferated cells and reduced MyoD and myogenin activity at both proliferative and commitment stages. Otherwise, IL-1β increased myogenin activity only in committed cells. Our data reveal a key role of IL-6 and COX-2-derived PGs in IL-1β and TNF-α-induced myoblast proliferation and support the link between TNF-α and IL-6 and the activation of MRFs. We concluded that IL-1β and TNF-α induce similar effects at the initial stages of muscle regeneration but found critical differences between their effects with the progression of the process, bringing new insights into inflammatory signalling in skeletal muscle regeneration.

SPARCL1 Influences Bovine Skeletal Muscle-Derived Satellite Cell Migration and Differentiation through an ITGB1-Mediated Signaling Pathway

Simple Summary

It is known that cell migration and differentiation have a very important yet simple basis for muscle development and muscle disease treatment. Secreted protein acidic and rich in cysteine like 1 (SPARCL1), one of the components of extracellular matrix, has been proved to regulate bovine skeletal muscle-derived satellite cell differentiation. However, the exact mechanism is not yet clear. This study reveals that SPARCL1 promotes muscle-derived satellite cell early differentiation through integrin β1, thereby providing a new insight into the role of SPARCL1 in muscle development.

Abstract

As an extracellular matrix protein, secreted protein acidic and rich in cysteine (SPARC)-like 1 (SPARCL1) is involved in various cell functions. It was previously implicated in bovine skeletal muscle-derived satellite cell (MDSC) differentiation; however, the underlying mechanism remains unknown. In this study, immunoprecipitation and mass spectrometry revealed that integrin β1 (ITGB1) combines with SPARCL1. Further, co-immunoprecipitation demonstrated that SPARCL1 interacts with ITGB1. Cell scratch assays explored the influence of SPARCL1 on MDSC migration through ITGB1. In addition, desmin staining for myotube fusion rate and MyoD protein expression results showed that SPARCL1 promotes MDSC early differentiation through ITGB1. Furthermore, Western blotting results demonstrated that SPARCL1 regulates the expression of p-FAK, p-paxillin, vinculin, Cdc42, and Arp2/3 through ITGB1. These findings indicate that SPARCL1 may influence bovine MDSC migration and differentiation through an ITGB1-mediated cell signaling pathway. Herein, we elucidated the mechanism through which SPARCL1 affects MDSC differentiation. Our results provide insight into the molecular mechanism of muscle development and may in the future facilitate skeletal muscle regeneration and treatment.

Exercise and High-Fat Diet in Obesity: Functional Genomics Perspectives of Two Energy Homeostasis Pillars

The heavy impact of obesity on both the population general health and the economy makes clarifying the underlying mechanisms, identifying pharmacological targets, and developing efficient therapies for obesity of high importance. The main struggle facing obesity research is that the underlying mechanistic pathways are yet to be fully revealed. This limits both our understanding of pathogenesis and therapeutic progress toward treating the obesity epidemic. The current anti-obesity approaches are mainly a controlled diet and exercise which could have limitations. For instance, the “classical” anti-obesity approach of exercise might not be practical for patients suffering from disabilities that prevent them from routine exercise. Therefore, therapeutic alternatives are urgently required. Within this context, pharmacological agents could be relatively efficient in association to an adequate diet that remains the most efficient approach in such situation. Herein, we put a spotlight on potential therapeutic targets for obesity identified following differential genes expression-based studies aiming to find genes that are differentially expressed under diverse conditions depending on physical activity and diet (mainly high-fat), two key factors influencing obesity development and prognosis. Such functional genomics approaches contribute to elucidate the molecular mechanisms that both control obesity development and switch the genetic, biochemical, and metabolic pathways toward a specific energy balance phenotype. It is important to clarify that by “gene-related pathways”, we refer to genes, the corresponding proteins and their potential receptors, the enzymes and molecules within both the cells in the intercellular space, that are related to the activation, the regulation, or the inactivation of the gene or its corresponding protein or pathways. We believe that this emerging area of functional genomics-related exploration will not only lead to novel mechanisms but also new applications and implications along with a new generation of treatments for obesity and the related metabolic disorders especially with the modern advances in pharmacological drug targeting and functional genomics techniques.

SPARCL1 promotes C2C12 cell differentiation via BMP7-mediated BMP/TGF-β cell signaling pathway

The extracellular matrix (ECM) is known to regulate tissue development and cell morphology, movement, and differentiation. SPARCL1 is an ECM protein, but its role in mouse cell differentiation has not been widely investigated. The results of western blotting and immunofluorescence showed that SPARCL1 is associated with the repair of muscle damage in mice and that SPARCL1 binds to bone morphogenetic protein 7 (BMP7) by regulating BMP/transforming growth factor (TGF)-β cell signaling. This pathway promotes the differentiation of C2C12 cells. Using CRISPR/Cas9 technology, we also showed that SPARCL1 activates BMP/TGF-β to promote the differentiation of C2C12 cells. BMP7 molecules were found to interact with SPARCL1 by immunoprecipitation analysis. Western blotting and immunofluorescence were performed to verify the effect of BMP7 on C2C12 cell differentiation. Furthermore, SPARCL1 was shown to influence the expression of BMP7 and activity of the BMP/TGF-β signaling pathway. Finally, SPARCL1 activation was accompanied by BMP7 inhibition in C2C12 cells, which confirmed that SPARCL1 affects BMP7 expression and can promote C2C12 cell differentiation through the BMP/TGF-β pathway. The ECM is essential for muscle regeneration and damage repair. This study intends to improve the understanding of the molecular mechanisms of muscle development and provide new treatment ideas for muscle injury diseases.

Secreted protein acidic and rich in cysteine (SPARC) improves glucose tolerance via AMP-activated protein kinase activation

During exercise, skeletal muscles release cytokines, peptides, and metabolites that exert autocrine, paracrine, or endocrine effects on glucose homeostasis. In this study, we investigated the effects of secreted protein acidic and rich in cysteine (SPARC), an exercise-responsive myokine, on glucose metabolism in human and mouse skeletal muscle. SPARC-knockout mice showed impaired systemic metabolism and reduced phosphorylation of AMPK and protein kinase B in skeletal muscle. Treatment of SPARC-knockout mice with recombinant SPARC improved glucose tolerance and concomitantly activated AMPK in skeletal muscle. These effects were dependent on AMPK-γ3 because SPARC treatment enhanced skeletal muscle glucose uptake in wild-type mice but not in AMPK-γ3-knockout mice. SPARC strongly interacted with the voltage-dependent calcium channel, and inhibition of calcium-dependent signaling prevented SPARC-induced AMPK phosphorylation in human and mouse myotubes. Finally, chronic SPARC treatment improved systemic glucose tolerance and AMPK signaling in skeletal muscle of high-fat diet-induced obese mice, highlighting the efficacy of SPARC treatment in the management of metabolic diseases. Thus, our findings suggest that SPARC treatment mimics the effects of exercise on glucose tolerance by enhancing AMPK-dependent glucose uptake in skeletal muscle.-Aoi, W., Hirano, N., Lassiter, D. G., Björnholm, M., Chibalin, A. V., Sakuma, K., Tanimura, Y., Mizushima, K., Takagi, T., Naito, Y., Zierath, J. R., Krook, A. Secreted protein acidic and rich in cysteine (SPARC) improves glucose tolerance via AMP-activated protein kinase activation.

Loss of miR-451a enhances SPARC production during myogenesis

MicroRNAs (miRNAs) are small noncoding RNAs that critically regulate gene expression. Their abundance and function have been linked to a range of physiologic and pathologic processes. In aged monkey muscle, miR-451a and miR-144-3p were far more abundant than in young monkey muscle. This observation led us to hypothesize that miR-451a and miR-144-3p may influence muscle homeostasis. To test if these conserved microRNAs were implicated in myogenesis, we investigated their function in the mouse myoblast line C2C12. The levels of both microRNAs declined with myogenesis; however, only overexpression of miR-451a, but not miR-144-3p, robustly impeded C2C12 differentiation, suggesting an inhibitory role for miR-451a in myogenesis. Further investigation of the regulatory influence of miR-451a identified as one of the major targets Sparc mRNA, which encodes a secreted protein acidic and rich in cysteine (SPARC) that functions in wound healing and cellular differentiation. In mouse myoblasts, miR-451a suppressed Sparc mRNA translation. Together, our findings indicate that miR-451a is downregulated in differentiated myoblasts and suggest that it decreases C2C12 differentiation at least in part by suppressing SPARC biosynthesis.

Effect of ECM2 expression on bovine skeletal muscle-derived satellite cell differentiation

Extracellular matrix components have important regulatory functions during cell proliferation and differentiation. In recent study, extracellular matrix were shown to have a strong effect on skeletal muscle differentiation. Here, we aimed to elucidate the effects of extracellular matrix protein 2 (ECM2), an extracellular matrix component, on the differentiation of bovine skeletal muscle-derived satellite cells (MDSCs). Western blot and immunofluorescence analyses were used to elucidate the ECM2 expression pattern in bovine MDSCs during differentiation in vitro. CRISPR/Cas9 technology was used to activate or inhibit ECM2 expression to study its effects on the in vitro differentiation of bovine MDSCs. ECM2 expression was shown to increase gradually during bovine MDSC differentiation, and the levels of this protein were higher in more highly differentiated myotubes. ECM2 activation promoted MDSC differentiation, whereas its suppression inhibited the differentiation of these cells. Here, for the first time, we demonstrated the importance of ECM2 expression during bovine MDSC differentiation; these results could lead to treatments that help to increase beef cattle muscularity.

Implication of SPARC in the modulation of the extracellular matrix and mitochondrial function in muscle cells

Secreted protein, acidic and rich in cysteine (SPARC) is differentially associated with cell proliferation and extracellular matrix (ECM) assembly. We show here the effect of exogenous SPARC inhibition/induction on ECM and mitochondrial proteins expression and on the differentiation of C2C12 cells. The cells were cultured in growth medium (GM) supplemented with different experimental conditions. The differentiation of myoblasts was studied for 5 days, the expressions of ECM and mitochondrial proteins were measured and the formation of the myotubes was quantified after exogenous induction/inhibition of SPARC. The results indicate that the addition of recombinant SPARC protein (rSPARC) in cell culture medium increased the differentiation of C2C12 myoblasts and myogenin expression during the myotube formation. However, the treatment with antibody specific for SPARC (anti-SPARC) prevented the differentiation and decreased myogenin expression. The induction of SPARC in the proliferating and differentiating C2C12 cells increased collagen 1a1 protein expression, whereas the inhibition decreased it. The effects on fibronectin protein expression were opposite. Furthermore, the addition of rSPARC in C2C12 myoblast increased the expression of mitochondrial proteins, ubiquinol-cytochrome c reductase core protein II (UQCRC2) and succinate dehydrogenase iron-sulfur subunit (SDHB), whereas the anti-SPARC decreased them. During the differentiation, only the anti-SPARC had the effects on mitochondrial proteins, NADH dehydrogenase ubiquinone 1 beta subcomplex subunit 8 (NADHB8), SDHB and cytochrome c oxidase 1 (MTCO1). Thus, SPARC plays a crucial role in the proliferation and differentiation of C2C12 and may be involved in the link between the ECM remodeling and mitochondrial function.

Differential gene expression analysis in ageing muscle and drug discovery perspectives

Identifying therapeutic target genes represents the key step in functional genomics-based therapies. Within this context, the disease heterogeneity, the exogenous factors and the complexity of genomic structure and function represent important challenges. The functional genomics aims to overcome such obstacles via identifying the gene functions and therefore highlight disease-causing genes as therapeutic targets. Genomic technologies promise to reshape the research on ageing muscle, exercise response and drug discovery. Herein, we describe the functional genomics strategies, mainly differential gene expression methods microarray, serial analysis of gene expression (SAGE), massively parallel signature sequence (MPSS), RNA sequencing (RNA seq), representational difference analysis (RDA), and suppression subtractive hybridization (SSH). Furthermore, we review these illustrative approaches that have been used to discover new therapeutic targets for some complex diseases along with the application of these tools to study the modulation of the skeletal muscle transcriptome.

SPARC overexpression alters microRNA expression profiles involved in tumor progression

Medulloblastoma is the most common malignant brain tumor in children. SPARC (secreted protein acidic and rich in cysteine), a multicellular non-structural glycoprotein is known to be involved in multiple processes in various cancers. Previously, we reported that SPARC expression significantly impairs medulloblastoma tumor growth in vitro and in vivo and also alters chemo sensitivity. MicroRNAs are a class of post-transcriptional gene regulators with critical functions in tumor progression. In addition, microRNA (miRNA) expression changes are also involved in chemo-resistance. Herein, we assessed microRNA (miRNA) profiling to identify the functional network and biological pathways altered in SPARC-overexpressed medulloblastoma cells. A total of 27 differentially expressed miRNAs were identified between the control and SPARC-overexpressed samples. Potential messenger RNA (mRNA) targets of the differentially expressed miRNA were identified using Ingenuity Pathway Analysis (IPA). Network-based functional analyses were performed on the available human protein interaction and miRNA-gene association data to highlight versatile miRNAs among the significantly deregulated miRNAs using the IPA, and the biological pathway analysis using the PANTHER web-based tool. We have identified six miRNAs (miR-125b1*, miR-146a-5p, miR-181a-5p, miR-204-5p, miR-219-5p and miR-509-3p) that are associated with SPARC sensitivity by comparison of miRNA expression patterns from the SPARC treated cells with the control cells. Furthermore, pathway enrichment analysis outline that these six microRNAs mainly belong to biological processes related to cancer related signaling pathways. Collectively, these studies have the potential to indicate novel biomarkers for treatment response and can also be applied to develop novel therapeutic treatment for medulloblastoma.

The delayed recovery of the remobilized rat tibialis anterior muscle reflects a defect in proliferative and terminal differentiation that impairs early regenerative processes

BACKGROUND

The immobilization-induced tibialis anterior (TA) muscle atrophy worsens after cast removal and is associated with altered extracellular matrix (ECM) composition. The secreted protein acidic and rich in cysteine (Sparc) is an ECM component involved in Akt activation and in β-catenin stabilization, which controls protein turnover and induces muscle regulatory factors (MRFs), respectively. We hypothesized that ECM alterations may influence these intracellular signalling pathways controlling TA muscle mass.

METHODS

Six-month-old Wistar rats were subjected to hindlimb cast immobilization for 8 days (I8) or not (I0) and allowed to recover for 1 to 10 days (R1-10).

RESULTS

The TA atrophy during remobilization correlated with reduced fibre cross-sectional area and thickening of endomysium. mRNA levels for Sparc increased during remobilization until R10 and for integrin-α7 and -β1 at I8 and R1. Integrin-linked kinase protein levels increased during immobilization and remobilization until R10. This was inversely correlated with changes in Akt phosphorylation. β-Catenin protein levels increased in the remobilized TA at R1 and R10. mRNA levels of the proliferative MRFs (Myf5 and MyoD) increased at I8 and R1, respectively, without changes in Myf5 protein levels. In contrast, myogenin mRNA levels (a terminal differentiation MRF) decreased at R1, but only increased at R10 in remobilized muscles, as for protein levels.

CONCLUSIONS

Altogether, this suggests that the TA inefficiently attempted to preserve regeneration during immobilization by increasing transcription of proliferative MRFs, and that the TA could engage recovery during remobilization only when the terminal differentiation step of regeneration is enhanced.

Secreted protein acidic and rich in cysteine internalization and its age-related alterations in skeletal muscle progenitor cells

Aging causes phenotypic changes in skeletal muscle progenitor cells (Skm-PCs), such as reduced myogenesis and increased adipogenesis due to alterations in their environment or niche. Secreted protein acidic and rich in cysteine (SPARC), which is secreted into the niche of Skm-PCs, inhibits adipogenesis and promotes myogenesis. We have previously reported that Skm-PC responsiveness to SPARC declines with age, although the mechanism underlying this decline is unknown. In this study, we found that SPARC is internalized by Skm-PCs and that this uptake increases with age. Internalization is dependent on integrin-α5, a cell surface SPARC-binding molecule, and clathrin-mediated endocytosis. We also demonstrated that internalized SPARC is transported to Rab7-positive endosomes. Skm-PCs from old rats exhibited increased clathrin expression and decreased Rab7 expression exclusively in MyoD-negative cells. In loss-of-function analyses, clathrin knockdown increased the anti-adipogenic effect of SPARC, whereas Rab7 knockdown reduced it, indicating that alterations in SPARC internalization may mediate the age-related decline in its anti-adipogenic effect. These results provide insights into age-related SPARC resistance in Skm-PCs, which may lead to sarcopenia.

Loss of SPARC in mouse skeletal muscle causes myofiber atrophy

INTRODUCTION

The expression of secreted protein acidic and rich in cysteine (SPARC) in skeletal muscle decreases with age. Here, we examined the role of SPARC in skeletal muscle by reducing its expression.

METHODS

SPARC expression was suppressed by introducing short interfering RNA (siRNA) into mouse tibialis anterior muscle. Myofiber diameter, atrogin1, and muscle RING-finger protein 1 (MuRF1) expression, and tumor necrosis factor-α (TNFα) and transforming growth factor-β (TGFβ) signaling were then analyzed.

RESULTS

Reduced SPARC expression caused decreases in the diameter of myofibers, especially fast-type ones, accompanied by upregulation of atrogin1, but not MuRF1, at 10 days after siRNA transfection. The expression of TNFα and TGFβ and the phosphorylation status of p38 were not affected by SPARC knockdown, whereas Smad3 phosphorylation was increased at 2 days after siRNA transfection.

CONCLUSIONS

The loss of SPARC not only upregulates atrogin1 expression but also enhances TGFβ signaling, which may in turn cause muscle atrophy.

Age-related resistance of skeletal muscle-derived progenitor cells to SPARC may explain a shift from myogenesis to adipogenesis

Aging causes phenotypic changes in skeletal muscle progenitor cells (SMPCs) that lead to the loss of myogenicity and adipogenesis. Secreted protein acidic and rich in cysteine (SPARC), which is secreted from SMPCs, stimulates myogenesis and inhibits adipogenesis. The present study aimed to examine whether changes in SPARC expression, its signaling pathway, or both are involved in age-related phenotypic changes in SMPCs. SPARC expression levels were comparable in SMPCs derived from young and old rats. However, when SPARC expression was reduced by a SPARC-specific siRNA, SMPCs from young rats showed reduced myogenesis and increased adipogenesis. In striking contrast, old rats showed little changes in these functions. Recombinant SPARC was effective in inhibiting adipogenesis and promoting myogenesis of SMPCs from young rats but had no effect on SMPCs from old rats when endogenous SPARC levels were reduced by the SPARC-siRNA. Further, the level of integrin α5, a subunit of the putative SPARC receptor, was decreased in SMPCs from old rats, and its inhibition in SMPCs from young rats by siRNA reduced adipogenesis in response to SPARC. These results suggest that, although SPARC plays a role in regulating SMPC function, SMPCs become refractory to the action of SPARC with age. Our data may explain an age-related shift from myogenesis to adipogenesis, associated with sarcopenia.

SPARC stimulates neuronal differentiation of medulloblastoma cells via the Notch1/STAT3 pathway

Secreted protein acidic and rich in cysteine (SPARC) participates in the regulation of morphogenesis and cellular differentiation through its modulation of cell-matrix interactions. We previously reported that SPARC expression significantly impairs medulloblastoma tumor growth in vivo. In this study, we show that adenoviral-mediated overexpression of SPARC cDNA (Ad-DsRed-SP) elevated the expression of the neuronal markers NeuN, nestin, neurofilament, and MAP-2 in medulloblastoma cells and induced neuron-like differentiation. SPARC overexpression decreased STAT3 phosphorylation; constitutive expression of STAT3 reversed SPARC-mediated expression of neuronal markers. We also show that Notch signaling is suppressed in the presence of SPARC, as well as the Notch effector basic helix-loop-helix (bHLH) transcription factor hairy and enhancer of split 1 (HES1). Notch signaling was found to be responsible for the decreased STAT3 phosphorylation in response to SPARC expression. Furthermore, expression of SPARC decreased the production of interleukin 6 (IL-6) and supplemented IL-6-abrogated, SPARC-mediated suppression of Notch signaling and expression of neuronal markers. Immunohistochemical analysis of tumor sections from mice treated with Ad-DsRed-SP showed increased immunoreactivity for the neuronal markers and a decrease in Notch1 expression and phosphorylation of STAT3. Taken together, our results suggest that SPARC induces expression of neuronal markers in medulloblastoma cells through its inhibitory effect on IL-6-regulated suppression of Notch pathway-mediated STAT3 signaling, thus giving further support to the potential use of SPARC as a therapeutic candidate for medulloblastoma treatment. Findings show that SPARC-induced neuronal differentiation can sensitize medulloblastoma cells for therapy.

Secretome Analysis of Skeletal Myogenesis Using SILAC and Shotgun Proteomics

Myogenesis, the formation of skeletal muscle, is a multistep event that commences with myoblast proliferation, followed by cell-cycle arrest, and finally the formation of multinucleated myotubes via fusion of mononucleated myoblasts. Each step is orchestrated by well-documented intracellular factors, such as cytoplasmic signalling molecules and nuclear transcription factors. Regardless, the key step in getting a more comprehensive understanding of the regulation of myogenesis is to explore the extracellular factors that are capable of eliciting the downstream intracellular factors. This could further provide valuable insight into the acute cellular response to extrinsic cues in maintaining normal muscle development. In this paper, we survey the intracellular factors that respond to extracellular cues that are responsible for the cascades of events during myogenesis: myoblast proliferation, cell-cycle arrest of myoblasts, and differentiation of myoblasts into myotubes. This focus on extracellular perspective of muscle development illustrates our mass spectrometry-based proteomic approaches to identify differentially expressed secreted factors during skeletal myogenesis.

Identification of differentially regulated secretome components during skeletal myogenesis

Myogenesis is a well-characterized program of cellular differentiation that is exquisitely sensitive to the extracellular milieu. Systematic characterization of the myogenic secretome (i.e. the ensemble of secreted proteins) is, therefore, warranted for the identification of novel secretome components that regulate both the pluripotency of these progenitor mesenchymal cells, and also their commitment and passage through the differentiation program. Previously, we have successfully identified 26 secreted proteins in the mouse skeletal muscle cell line C2C12 (1). In an effort to attain a more comprehensive picture of the regulation of myogenesis by its extracellular milieu, quantitative profiling employing stable isotope labeling by amino acids in cell culture was implemented in conjunction with two parallel high throughput online reverse phase liquid chromatography-tandem mass spectrometry systems. In summary, 34 secreted proteins were quantified, 30 of which were shown to be differentially expressed during muscle development. Intriguingly, our analysis has revealed several novel up- and down-regulated secretome components that may have critical biological relevance for both the maintenance of pluripotency and the passage of cells through the differentiation program. In particular, the altered regulation of secretome components, including follistatin-like protein-1, osteoglycin, spondin-2, and cytokine-induced apoptosis inhibitor-1, along with constitutively expressed factors, such as fibulin-2, illustrate dynamic changes in the secretome that take place when differentiation to a specific lineage occurs.

Transcriptional profiling of skeletal muscle reveals factors that are necessary to maintain satellite cell integrity during ageing

Skeletal muscle ageing is characterized by faulty degenerative/regenerative processes that promote the decline of its mass, strength, and endurance. In this study, we used a transcriptional profiling method to better understand the molecular pathways and factors that contribute to these processes. To more appropriately contrast the differences in regenerative capacity of old muscle, we compared it with young muscle, where robust growth and efficient myogenic differentiation is ongoing. Notably, in old mice, we found a severe deficit in satellite cells activation. We performed expression analyses on RNA from the gastrocnemius muscle of young (3-week-old) and old (24-month-old) mice. The differential expression highlighted genes that are involved in the efficient functioning of satellite cells. Indeed, the greatest number of up-regulated genes in young mice encoded components of the extracellular matrix required for the maintenance of the satellite cell niche. Moreover, other genes included Wnt inhibitors (Wif1 and Sfrp2) and Notch activator (Dner), which are putatively involved in the interconnected signalling networks that control satellite cell function. The widespread expression differences for inhibitors of TGFbeta signalling further emphasize the shortcomings in satellite cell performance. Therefore, we draw attention to the breakdown of features required to maintain satellite cell integrity during the ageing process.

Secreted protein acidic and rich in cysteine (SPARC) in human skeletal muscle

Secreted protein acidic and rich in cysteine (SPARC)/osteonectin is expressed in different tissues during remodeling and repair, suggesting a function in regeneration. Several gene expression studies indicated that SPARC was expressed in response to muscle damage. Studies on myoblasts further indicated a function of SPARC in skeletal muscle. We therefore found it of interest to study SPARC expression in human skeletal muscle during development and in biopsies from Duchenne and Becker muscular dystrophy and congenital muscular dystrophy, congenital myopathy, inclusion body myositis, and polymyositis patients to analyze SPARC expression in a selected range of inherited and idiopathic muscle wasting diseases. SPARC-positive cells were observed both in fetal and neonatal muscle, and in addition, fetal myofibers were observed to express SPARC at the age of 15-16 weeks. SPARC protein was detected in the majority of analyzed muscle biopsies (23 of 24), mainly in mononuclear cells of which few were pax7 positive. Myotubes and regenerating myofibers also expressed SPARC. The expression-degree seemed to reflect the severity of the lesion. In accordance with these in vivo findings, primary human-derived satellite cells were found to express SPARC both during proliferation and differentiation in vitro. In conclusion, this study shows SPARC expression both during muscle development and in regenerating muscle. The expression is detected both in satellite cells/myoblasts and in myotubes and muscle fibers, indicating a role for SPARC in the skeletal muscle compartment.

Apoptosis and differentiation commitment: novel insights revealed by gene profiling studies in mouse embryonic stem cells

Mouse embryonic stem (ES) cells remain pluripotent in vitro when grown in the presence of leukemia inhibitory factor (LIF). LIF starvation leads to apoptosis of some of the ES-derived differentiated cells, together with p38alpha mitogen-activated protein kinase (MAPK) activation. Apoptosis, but not morphological cell differentiation, is blocked by a p38 inhibitor, PD169316. To further understand the mechanism of action of this compound, we have identified its specific targets by microarray studies. We report on the global expression profiles of genes expressed at 3 days upon LIF withdrawal (d3) compared to pluripotent cells and of genes whose expression is modulated at d3 under anti-apoptotic conditions. We showed that at d3 without LIF cells express, earlier than anticipated, specialized cell markers and that when the apoptotic process was impaired, expression of differentiation markers was altered. In addition, functional tests revealed properties of anti-apoptotic proteins not to alter cell pluripotency and a novel role for metallothionein 1 gene, which prevents apoptosis of early differentiated cells.

Parietal endoderm secreted SPARC promotes early cardiomyogenesis in vitro

Cardiomyogenesis proceeds in the presence of signals emanating from extra-embryonic lineages emerging before and during early eutherian gastrulation. In embryonic stem cell derived embryoid bodies, primitive endoderm gives rise to visceral and parietal endoderm. Parietal endoderm undergoes an epithelial to mesenchymal transition shortly before first cardiomyocytes start to contract rhythmically. Here, we demonstrate that Secreted Protein, Acidic, Rich in Cysteine, SPARC, predominantly secreted by mesenchymal parietal endoderm specifically promotes early myocardial cell differentiation in embryoid bodies. SPARC enhanced the expression of bmp2 and nkx2.5 in embryoid bodies and fetal cardiomyocytes. Inhibition of either SPARC or Bmp2 attenuated in both cases cardiomyogenesis and downregulated nkx2.5 expression. Thus, SPARC directly affects cardiomyogenesis, modulates Bmp2 signaling, and contributes to a positive autoregulatory loop of Bmp2 and Nkx2.5 in cardiomyocytes.

Changes in cell shape and desmin intermediate filament distribution are associated with down-regulation of desmin expression in C2C12 myoblasts grown in the absence of extracellular Ca2+

Desmin is the main intermediate filament (IF) protein of muscle cells. In skeletal muscle, desmin IFs form a scaffold that interconnects the entire contractile apparatus with the subsarcolemmal cytoskeleton and cytoplasmic organelles. The interaction between desmin and the sarcolemma is mediated by a number of membrane proteins, many of which are Ca2+-sensitive. In the present study, we analyzed the effects of the Ca2+ chelator EGTA (1.75 mM) on the expression and distribution of desmin in C2C12 myoblasts grown in culture. We used indirect immunofluorescence microscopy and reverse transcription polymerase chain reaction (RT-PCR) to analyze desmin distribution and expression in C2C12 cells grown in the presence or absence of EGTA. Control C2C12 myoblasts showed a well-spread morphology after a few hours in culture and became bipolar when grown for 24 h in the presence of EGTA. Control C2C12 cells showed a dense network of desmin from the perinuclear region to the cell periphery, whereas EGTA-treated cells showed desmin aggregates in the cytoplasm. RT-PCR analysis revealed a down-regulation of desmin expression in EGTA-treated C2C12 cells compared to untreated cells. The present results suggest that extracellular Ca2+ availability plays a role in the regulation of desmin expression and in the spatial distribution of desmin IFs in myoblasts, and is involved in the generation and maintenance of myoblast cell shape.

The matricellular protein SPARC is expressed in human trabecular meshwork

PURPOSE

This investigation was undertaken to determine whether the matricellular protein SPARC is expressed in the human trabecular meshwork (TM) and cultured human trabecular meshwork cells.

METHODS

Human donor trabecular meshwork and cultured cells obtained from trabecular meshwork were used in this study. Total RNA was obtained from TM and cultured TM endothelial cells, and RT-PCR was done with primers specific for SPARC. Western blotting was performed on donor TMs using an anti-SPARC monoclonal antibody prepared against rHuSPARC. Confocal microscopy was used to determine the distribution of SPARC in human anterior segments, and immunofluorescence on cultured TM cells was performed with the anti-SPARC antibody.

RESULTS

SPARC mRNA was expressed both in TM and in cultured TM cells. Immunoblotting for SPARC showed a doublet with a molecular mass approximately 43 kDa. The ratio of the doublet bands varied with each of the samples; some of the cultured cells and the tissue samples exhibited more of the upper band, and other cultured cells contained almost equal amounts of the two bands. The upper band was shown to be a glycosylated form of SPARC. Immunofluorescence showed that SPARC was expressed in the cultured TM, and confocal microscopy with the anti-SPARC antibody demonstrated the presence of this protein in the TM and in other tissues in the anterior segment.

CONCLUSIONS

Our data conclusively show that SPARC mRNA and protein are present in non-glaucomatous TM tissue and in cultured TM cells. Because of its effect on matrix metalloproteinases, SPARC may play a role in the regulation of intraocular pressure.

Molecular identification of the skin transformation center of anuran larval skin using genes of Rana adult keratin (RAK) and SPARC as probes

Anuran larval skin undergoes a process of metamorphosis into pre-adult and adult skin. Basal skein, larval basal and adult basal cells are basement membrane-attaching cells in the larval, pre-adult and adult epidermis, respectively, and are identified as cells expressing genes of RLK (Rana larval keratin), both RLK and RAK (Rana adult keratin), and RAK. Larval to pre-adult skin conversion takes place in the histological entity called the skin transformation center (STC). The present study performed a cDNA subtractive gene screening on cDNA of the larval and the pre-adult skin, and cloned the secreted protein acidic and rich in cysteine (SPARC) gene as an upregulated gene in the larva to pre-adult skin conversion. RAK gene-positive basal skein cells and fibroblasts in and around the STC were weakly and strongly sparc-positive, respectively. Using sparc and rak, we redefined the STC and visualized it on a histological section as an approximately 150 microm-long region that contained about 20 rak-negative and weakly sparc-positive basal cells. Intense sparc expression was observed in basal skein cells, but not in larval basal cells, suggesting that SPARC acts as a suppressor of rak during epidermal differentiation. This suggestion was tested by investigating the effect of SPARC on cultured larval basal cells. We observed that SPARC suppressed the expression of rak, but not rlk.

Changes in cell shape, cytoskeletal proteins and adhesion sites of cultured cells after extracellular Ca2+ chelation

Although much is known about the molecules involved in extracellular Ca2+ regulation, the relationship of the ion with overall cell morphology is not understood. The objective of the present study was to determine the effect of the Ca2+ chelator EGTA on the major cytoskeleton components, at integrin-containing adhesion sites, and their consequences on cell shape. Control mouse cell line C2C12 has a well-spread morphology with long stress fibers running in many different directions, as detected by fluorescence microscopy using rhodamine-phalloidin. In contrast, cells treated with EGTA (1.75 mM in culture medium) for 24 h became bipolar and showed less stress fibers running in one major direction. The adhesion plaque protein alpha 5-integrin was detected by immunofluorescence microscopy at fibrillar adhesion sites in both control and treated cells, whereas a dense labeling was seen only inside treated cells. Microtubules shifted from a radial arrangement in control cells to a longitudinal distribution in EGTA-treated cells, as analyzed by immunofluorescence microscopy. Desmin intermediate filaments were detected by immunofluorescence microscopy in a fragmented network dispersed within the entire cytoplasm in EGTA-treated cells, whereas a dense network was seen in the whole cytoplasm of control cells. The present results suggest that the role of extracellular Ca2+ in the regulation of C2C12 cell shape can be mediated by actin-containing stress fibers and microtubules and by intermediate filament reorganization, which may involve integrin adhesion sites.

Gene expression comparison of biopsies from Duchenne muscular dystrophy (DMD) and normal skeletal muscle

The primary cause of Duchenne muscular dystrophy (DMD) is a mutation in the dystrophin gene leading to the absence of the corresponding RNA transcript and protein. Absence of dystrophin leads to disruption of the dystrophin-associated protein complex and substantial changes in skeletal muscle pathology. Although the histological pathology of dystrophic tissue has been well documented, the underlying molecular pathways remain poorly understood. To examine the pathogenic pathways and identify new or modifying factors involved in muscular dystrophy, expression microarrays were used to compare individual gene expression profiles of skeletal muscle biopsies from 12 DMD patients and 12 unaffected control patients. Two separate statistical analysis methods were used to interpret the resulting data: t test analysis to determine the statistical significance of differential expression and geometric fold change analysis to determine the extent of differential expression. These analyses identified 105 genes that differ significantly in expression level between unaffected and DMD muscle. Many of the differentially expressed genes reflect changes in histological pathology. For instance, immune response signals and extracellular matrix genes are overexpressed in DMD muscle, an indication of the infiltration of inflammatory cells and connective tissue. Significantly more genes are overexpressed than are underexpressed in dystrophic muscle, with dystrophin underexpressed, whereas other genes encoding muscle structure and regeneration processes are overexpressed, reflecting the regenerative nature of the disease.

Doxycycline-inducible expression of SPARC/Osteonectin/BM40 in MDA-MB-231 human breast cancer cells results in growth inhibition

SPARC (secreted protein acidic and rich in cysteine)/BM40/Osteonectin is a matricellular protein with multiple effects on cell behaviour. In vitro, its major known functions are anti-adhesive and anti-proliferative, and it is associated with tissue remodelling and cancer in vivo. SPARC is overexpressed in many cancers, including breast cancer, and the effects of SPARC seem to be cell type-specific. To study the effects of SPARC on breast cancer, we transfected SPARC into the MDA-MB-231 BAG, human breast cancer cell line using the Tet-On inducible system. By western analysis, we found low background levels in the MDA-MB-231 BAG and clone X parental cells, and prominent induction of SPARC protein expression after doxycycline treatment in SPARC transfected clones X5, X21, X24 and X75. Induction of SPARC expression did not affect cell morphology or adhesiveness to collagens type I and IV, but it slowed the rate of proliferation in adherent cultures. Cell cycle analysis showed that SPARC slowed the progression to S phase. Doxycycline induction of SPARC also slowed the rate of monolayer wound closure in the cultured wound healing assay. Thymidine inhibition of proliferation abrogated this effect, confirming that it was due to anti-proliferation rather than inhibition of migration. Consistent with this, we were unable to detect any differences in migration and Matrigel outgrowth analysis of doxycycline-stimulated cells. We conclude that SPARC is inhibitory to human breast cancer cell proliferation, and does not stimulate migration, in contrast to its stimulatory effects reported for melanoma (proliferation and migration) and glioma (migration) cells. Similar growth repression by SPARC has been reported for ovarian cancer cells, and this may be a common feature among carcinomas.

Characterization of the calcium-dependent proteolytic system in a mouse muscle cell line

Many studies have demonstrated that the calcium-dependent proteolytic system (calpains and calpastatin) is involved in myoblast differentiation. It is also known that myogenic differentiation can be studied in vitro. In the present experiments, using a mouse muscle cell line (C2C12) we have analyzed both the sequences of appearance and the expression profiles of calpains 1, 2, 3 and calpastatin during the course of myoblast differentiation. Our results mainly show that the expression of ubiquitous calpains (calpain 1 and 2) and muscle-specific calpain (calpain 3) at the mRNAs level as well as at the protein level do not change significantly all along this biological process. In the same time, the specific inhibitor of ubiquitous calpains, calpastatin, presents a stable expression at mRNAs level as well as protein level, all along myoblast to myotube transition. A comparison with other myogenic cells is presented.