Src mediates the mechanical activation of myogenesis by activating TNFα-converting enzyme

Covalent Immobilization of Collagen Type I to a Polydimethylsiloxane Surface for Preventing Cell Detachment by Retaining Collagen Molecules under Uniaxial Cyclic Mechanical Stretching Stress

Surface modification of polydimethylsiloxane (PDMS) with an extracellular matrix (ECM) is useful for enhancing stable cell attachment. However, few studies have investigated the correlation between the stability of deposited ECM and cell behavior on the PDMS surfaces in external stretched cell culture systems. Herein, covalent collagen type I (Col)-immobilized PDMS surfaces were fabricated using 3-aminopropyl-trimethoxysilane, glutaraldehyde, and Col molecules. The immobilized collagen molecules on the PDMS surface were more stable and uniform than the physisorbed collagen. The cells stably adhered to the Col-immobilized surface and proliferated even under uniaxial cyclic mechanical stretching stress (UnCyMSt), whereas the cells gradually detached from the Col-physisorbed PDMS surface, accompanied by a decrease in the number of deposited collagen molecules. Moreover, the immobilization of collagen molecules enhanced cell alignment under the UnCyMSt. This study reveals that cell adhesion, proliferation, and alignment under the UnCyMSt can be attributed to the retention of collagen molecules on the PDMS surface.

The Biochemistry and Physiology of A Disintegrin and Metalloproteinases (ADAMs and ADAM-TSs) in Human Pathologies

Metalloproteinases are a group of proteinases that plays a substantial role in extracellular matrix remodeling and its molecular signaling. Among these metalloproteinases, ADAMs (a disintegrin and metalloproteinases) and ADAM-TSs (ADAMs with thrombospondin domains) have emerged as highly efficient contributors mediating proteolytic processing of various signaling molecules. ADAMs are transmembrane metalloenzymes that facilitate the extracellular domain shedding of membrane-anchored proteins, cytokines, growth factors, ligands, and their receptors and therefore modulate their biological functions. ADAM-TSs are secretory, and soluble extracellular proteinases that mediate the cleavage of non-fibrillar extracellular matrix proteins. ADAMs and ADAM-TSs possess pro-domain, metalloproteinase, disintegrin, and cysteine-rich domains in common, but ADAM-TSs have characteristic thrombospondin motifs instead of the transmembrane domain. Most ADAMs and ADAM-TSs are activated by cleavage of pro-domain via pro-protein convertases at their N-terminus, hence directing them to various signaling pathways. In this article, we are discussing not only the structure and regulation of ADAMs and ADAM-TSs, but also the importance of these metalloproteinases in various human pathophysiological conditions like cardiovascular diseases, colorectal cancer, autoinflammatory diseases (sepsis/rheumatoid arthritis), Alzheimer's disease, proliferative retinopathies, and infectious diseases. Therefore, based on the emerging role of ADAMs and ADAM-TSs in various human pathologies, as summarized in this review, these metalloproteases can be considered as critical therapeutic targets and diagnostic biomarkers.

Myostatin/Activin Receptor Ligands in Muscle and the Development Status of Attenuating Drugs

Muscle wasting disease indications are among the most debilitating and often deadly noncommunicable disease states. As a comorbidity, muscle wasting is associated with different neuromuscular diseases and myopathies, cancer, heart failure, chronic pulmonary and renal diseases, peripheral neuropathies, inflammatory disorders, and, of course, musculoskeletal injuries. Current treatment strategies are relatively ineffective and can at best only limit the rate of muscle degeneration. This includes nutritional supplementation and appetite stimulants as well as immunosuppressants capable of exacerbating muscle loss. Arguably, the most promising treatments in development attempt to disrupt myostatin and activin receptor signaling because these circulating factors are potent inhibitors of muscle growth and regulators of muscle progenitor cell differentiation. Indeed, several studies demonstrated the clinical potential of "inhibiting the inhibitors," increasing muscle cell protein synthesis, decreasing degradation, enhancing mitochondrial biogenesis, and preserving muscle function. Such changes can prevent muscle wasting in various disease animal models yet many drugs targeting this pathway failed during clinical trials, some from serious treatment-related adverse events and off-target interactions. More often, however, failures resulted from the inability to improve muscle function despite preserving muscle mass. Drugs still in development include antibodies and gene therapeutics, all with different targets and thus, safety, efficacy, and proposed use profiles. Each is unique in design and, if successful, could revolutionize the treatment of both acute and chronic muscle wasting. They could also be used in combination with other developing therapeutics for related muscle pathologies or even metabolic diseases.

Integrated Analysis Reveals a lncRNA-miRNA-mRNA Network Associated with Pigeon Skeletal Muscle Development

Growing evidence has demonstrated the emerging role of long non-coding RNA as competitive endogenous RNA (ceRNA) in regulating skeletal muscle development. However, the mechanism of ceRNA regulated by lncRNA in pigeon skeletal muscle development remains unclear. To reveal the function and regulatory mechanisms of lncRNA, we first analyzed the expression profiles of lncRNA, microRNA (miRNA), and mRNA during the development of pigeon skeletal muscle using high-throughput sequencing. We then constructed a lncRNA-miRNA-mRNA ceRNA network based on differentially expressed (DE) lncRNAs, miRNAs, and mRNAs according to the ceRNA hypothesis. Functional enrichment and short time-series expression miner (STEM) analysis were performed to explore the function of the ceRNA network. Hub lncRNA-miRNA-mRNA interactions were identified by connectivity degree and validated using dual-luciferase activity assay. The results showed that a total of 1625 DE lncRNAs, 11,311 DE mRNAs, and 573 DE miRNAs were identified. A ceRNA network containing 9120 lncRNA-miRNA-mRNA interactions was constructed. STEM analysis indicated that the function of the lncRNA-associated ceRNA network might be developmental specific. Functional enrichment analysis identified potential pathways regulating pigeon skeletal muscle development, such as cell cycle and MAPK signaling. Based on the connectivity degree, lncRNAs TCONS_00066712, TCONS_00026594, TCONS_00001557, TCONS_00001553, and TCONS_00003307 were identified as hub genes in the ceRNA network. lncRNA TCONS_00026594 might regulate the FSHD region gene 1 (FRG1)/ SRC proto-oncogene, non-receptor tyrosine kinase (SRC) by sponge adsorption of cli-miR-1a-3p to affect the development of pigeon skeletal muscle. Our findings provide a data basis for in-depth elucidation of the lncRNA-associated ceRNA mechanism underlying pigeon skeletal muscle development.

Myrtenal and β-caryophyllene oxide screened from Liquidambaris Fructus suppress NLRP3 inflammasome components in rheumatoid arthritis

Background

Liquidambaris Fructus (LF) is the infructescence of Liquidambar formosana. In Traditional Chinese Medicine, LF has been used to treat joint pain, a common symptom of arthritis and rheumatism; however, a lack of pharmacological evidence has limited its applications in modern clinics. Therefore, this study aims to explore the protective effect of LF on rheumatoid arthritis (RA) and to identify its active ingredients.

Methods

Rats with adjuvant-induced arthritis (AIA) were divided into 4 groups and administered petroleum ether extract of LF (PEL), ethyl acetate extract of LF (EEL), water extract of LF (WEL), or piroxicam (PIR) respectively for 3 weeks. Two additional groups were used as normal control (NC) and model control (MC) and administered distilled water as a placebo. The clinical scores for arthritis, bone surface, synovial inflammation and cartilage erosion were used to evaluate the therapeutic efficacy of each treatment. The serum IL-1β and TNF-α level and the expression of NLRP3, IL-1β and caspase-1 p20 in the synovial tissue of AIA rats were evaluated by ELISA and Western blot. The active ingredients of LF were investigated using network pharmacology and molecular docking methods, and their inhibition of NLRP3 inflammasome activation was verified in the human rheumatoid arthritis fibroblast-like synovial cells (RA-FLS) model.

Results

PEL could alleviate paw swelling, bone and joint destruction, synovial inflammation and cartilage erosion in the AIA rats, with significantly superior efficacy to that of EEL and WEL. PEL reduced IL-1β and TNF-α serum levels, and attenuated the upregulation of NLRP3, IL-1β and caspase-1 p20 expression in the synovial tissue of AIA rats. Network pharmacology and molecular docking results indicated that myrtenal and β-caryophyllene oxide were the main two active ingredients of PEL, and these two compounds showed significant inhibition on TNF-α, NLRP3, IL-1β and caspase-1 p20 expression in RA-FLS.

Conclusions

Myrtenal and β-caryophyllene oxide screened from PEL could suppress the activation of NLRP3 inflammasome, thereby alleviating RA symptoms.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12906-021-03410-2.

Interleukin 13 (IL-13)-regulated expression of the chondroprotective metalloproteinase ADAM15 is reduced in aging cartilage

Objective

The adamalysin metalloproteinase 15 (ADAM15) has been shown to protect against development of osteoarthritis in mice. Here, we have investigated factors that control ADAM15 levels in cartilage.

Design

Secretomes from wild-type and Adam15^−/− chondrocytes were compared by label-free quantitative mass spectrometry. mRNA was isolated from murine knee joints, either with or without surgical induction of osteoarthritis on male C57BL/6 mice, and the expression of Adam15 and other related genes quantified by RT-qPCR. ADAM15 in human normal and osteoarthritic cartilage was investigated similarly and by fluorescent immunohistochemistry. Cultured HTB94 chondrosarcoma cells were treated with various anabolic and catabolic stimuli, and ADAM15 mRNA and protein levels evaluated.

Results

There were no significant differences in the secretomes of chondrocytes from WT and Adam15^−/− cartilage. Expression of ADAM15 was not altered in either human or murine osteoarthritic cartilage relative to disease-free controls. However, expression of ADAM15 was markedly reduced upon aging in both species, to the extent that expression in joints of 18-month-old mice was 45-fold lower than in that 4.5-month-old animals. IL-13 increased expression of ADAM15 in HTB94 ​cells by 2.5-fold, while modulators of senescence and autophagy pathways had no effect. Expression of Il13 in the joint was reduced with aging, suggesting this cytokine may control ADAM15 levels in the joint.

Conclusion

Expression of the chondroprotective metalloproteinase ADAM15 is reduced in aging human and murine joints, possibly due to a concomitant reduction in IL-13 expression. We thus propose IL-13 as a novel factor contributing to increased osteoarthritis risk upon aging.

Aspartate β-hydroxylase promotes pancreatic ductal adenocarcinoma metastasis through activation of SRC signaling pathway

BACKGROUND

Signaling pathways critical for embryonic development re-emerge in adult pancreas during tumorigenesis. Aspartate β-hydroxylase (ASPH) drives embryonic cell motility/invasion in pancreatic development/differentiation. We explored if dysregulated ASPH is critically involved in pancreatic cancer pathogenesis.

METHODS

To demonstrate if/how ASPH mediates malignant phenotypes, proliferation, migration, 2-D/3-D invasion, pancreatosphere formation, immunofluorescence, Western blot, co-immunoprecipitation, invadopodia formation/maturation/function, qRT-PCR, immunohistochemistry (IHC), and self-developed in vitro metastasis assays were performed. Patient-derived xenograft (PDX) models of human pancreatic ductal adenocarcinoma (PDAC) were established to illustrate in vivo antitumor effects of the third-generation small molecule inhibitor specifically against ASPH's β-hydroxylase activity. Prognostic values of ASPH network components were evaluated with Kaplan-Meier plots, log-rank tests, and Cox proportional hazards regression models.

RESULTS

ASPH renders pancreatic cancer cells more aggressive phenotypes characterized by epithelial-mesenchymal transition (EMT), 2-D/3-D invasion, invadopodia formation/function as demonstrated by extracellular matrix (ECM) degradation, stemness (cancer stem cell marker upregulation and pancreatosphere formation), transendothelial migration (mimicking intravasation/extravasation), and sphere formation (mimicking metastatic colonization/outgrowth at distant sites). Mechanistically, ASPH activates SRC cascade through direct physical interaction with ADAM12/ADAM15 independent of FAK. The ASPH-SRC axis enables invadopodia construction and initiates MMP-mediated ECM degradation/remodeling as executors for invasiveness. Pharmacologic inhibition of invadopodia attenuates in vitro metastasis. ASPH fosters primary tumor development and pulmonary metastasis in PDX models of PDAC, which is blocked by a leading compound specifically against ASPH enzymatic activity. ASPH is silenced in normal pancreas, progressively upregulated from pre-malignant lesions to invasive/advanced stages of PDAC. Expression profiling of ASPH-SRC network components independently/jointly predicts clinical outcome of PDAC patients. Compared to a negative-low level, a moderate-very high level of ASPH, ADAM12, activated SRC, and MMPs correlated with curtailed overall survival (OS) of pancreatic cancer patients (log-rank test, ps < 0.001). The more unfavorable molecules patients carry, the more deleterious prognosis is destinated. Patients with 0-2 (n = 4), 3-5 (n = 8), 6-8 (n = 24), and 9-12 (n = 73) unfavorable expression scores of the 5 molecules had median survival time of 55.4, 15.9, 9.7, and 5.0 months, respectively (p < 0.001).

CONCLUSION

Targeting the ASPH-SRC axis, which is essential for propagating multi-step PDAC metastasis, may specifically/substantially retard development/progression and thus improve prognosis of PDAC.

Cored in the act: the use of models to understand core myopathies

The core myopathies are a group of congenital myopathies with variable clinical expression - ranging from early-onset skeletal-muscle weakness to later-onset disease of variable severity - that are identified by characteristic 'core-like' lesions in myofibers and the presence of hypothonia and slowly or rather non-progressive muscle weakness. The genetic causes are diverse; central core disease is most often caused by mutations in ryanodine receptor 1 (RYR1), whereas multi-minicore disease is linked to pathogenic variants of several genes, including selenoprotein N (SELENON), RYR1 and titin (TTN). Understanding the mechanisms that drive core development and muscle weakness remains challenging due to the diversity of the excitation-contraction coupling (ECC) proteins involved and the differential effects of mutations across proteins. Because of this, the use of representative models expressing a mature ECC apparatus is crucial. Animal models have facilitated the identification of disease progression mechanisms for some mutations and have provided evidence to help explain genotype-phenotype correlations. However, many unanswered questions remain about the common and divergent pathological mechanisms that drive disease progression, and these mechanisms need to be understood in order to identify therapeutic targets. Several new transgenic animals have been described recently, expanding the spectrum of core myopathy models, including mice with patient-specific mutations. Furthermore, recent developments in 3D tissue engineering are expected to enable the study of core myopathy disease progression and the effects of potential therapeutic interventions in the context of human cells. In this Review, we summarize the current landscape of core myopathy models, and assess the hurdles and opportunities of future modeling strategies.

Ventilator-induced diaphragm dysfunction in critical illness

IMPACT STATEMENT

Mechanical ventilation (MV) is life-saving for patients with acute respiratory failure but also causes difficult liberation of patients from ventilator due to rapid decrease of diaphragm muscle endurance and strength, which is termed ventilator-induced diaphragmatic damage (VIDD). Numerous studies have revealed that VIDD could increase extubation failure, ICU stay, ICU mortality, and healthcare expenditures. However, the mechanisms of VIDD, potentially involving a multistep process including muscle atrophy, oxidative loads, structural damage, and muscle fiber remodeling, are not fully elucidated. Further research is necessary to unravel mechanistic framework for understanding the molecular mechanisms underlying VIDD, especially mitochondrial dysfunction and increased mitochondrial oxidative stress, and develop better MV strategies, rehabilitative programs, and pharmacologic agents to translate this knowledge into clinical benefits.

ADAM17 activation and regulation of profibrotic responses by high glucose requires its C-terminus and FAK kinase

Glomerular matrix accumulation is the hallmark of diabetic nephropathy. The metalloprotease ADAM17 mediates high glucose (HG)-induced matrix production by kidney mesangial cells through release of ligands for the epidermal growth factor receptor. Here we study the mechanism by which HG activates ADAM17. We find that the C-terminus is essential for ADAM17 activation and the profibrotic response to HG. In the C-terminus, Src-mediated Y702 phosphorylation and PI3K/MEK/Erk-mediated T735 phosphorylation are critical to ADAM17 activation, but play divergent roles in ADAM17 trafficking in response to HG. While T735 phosphorylation is required for the HG-induced increase in cell surface mature ADAM17, Y702 phosphorylation is dispensable. Src, however, enables trafficking independently of its phosphorylation of ADAM17. The nonreceptor tyrosine kinase FAK is a central mediator of these processes. These data not only support a critical role for the C-terminus in ADAM17 activation and downstream profibrotic responses to HG, but also highlight FAK as a potential alternate therapeutic target for diabetic nephropathy.

Inhibition of Src and forkhead box O1 signaling by induced pluripotent stem-cell therapy attenuates hyperoxia-augmented ventilator-induced diaphragm dysfunction

Mechanical ventilation (MV) with hyperoxia is required for providing life support to patients with acute lung injury (ALI). However, MV may cause diaphragm weakness through muscle injury and atrophy, an effect termed ventilator-induced diaphragm dysfunction (VIDD). Src protein tyrosine kinase and class O of forkhead box 1 (FoxO1) mediate acute inflammatory responses and muscle protein degradation induced by oxidative stress. Induced pluripotent stem cells (iPSCs) have been reported to improve hyperoxia-augmented ALI; however, the mechanisms regulating the interactions among VIDD, hyperoxia, and iPSCs are unclear. In this study, we hypothesized that iPSC therapy can ameliorate hyperoxia-augmented VIDD by suppressing the Src-FoxO1 pathway. Male C57BL/6 mice, either wild-type or Src-deficient, aged between 6 and 8 weeks were exposed to MV (6 or 10 mL/kg) with or without hyperoxia for 2-8 h after the administration of 5 × 10(7) cells/kg Oct4/Sox2/Parp1 mouse iPSCs or iPSC-derived conditioned medium (iPSC-CM). Nonventilated mice were used as controls. MV during hyperoxia aggravated VIDD, as demonstrated by the increases in Src activation, FoxO1 dephosphorylation, malondialdehyde, caspase-3, atrogin-1 and muscle ring finger-1 production, microtubule-associated protein light chain 3-II, disorganized myofibrils, disrupted mitochondria, autophagy, and myonuclear apoptosis; however, MV with hyperoxia reduced mitochondrial cytochrome C, diaphragm muscle fiber size, and contractility (P < 0.05). Hyperoxia-exacerbated VIDD was attenuated in Src-deficient mice and by iPSCs and iPSC-CM (P < 0.05). Our data indicate that iPSC therapy attenuates MV-induced diaphragmatic injury that occurs during hyperoxia-augmented VIDD by inhibiting the Src-FoxO1 signaling pathway.

Genetic variants of ADAM17 are implicated in the pathological process of Kawasaki disease and secondary coronary artery lesions via the TGF-β/SMAD3 signaling pathway

Kawasaki disease (KD) is a systemic vasculitis childhood disease frequently complicating coronary artery lesions (CALs). Recently, the gene encoding a disintegrin and metalloprotease 17 (ADAM17) was found to modify vascular pathology in humans by differentially regulating the transforming growth factor-β (TGF-β) signaling pathway, which affects KD/CAL susceptibility. To explore the potential role of ADAM17 in KD occurrence and outcomes, we investigated the association of 28 single nucleotide polymorphisms (SNPs) in ADAM17 and three pathway genes of TGF-β signaling with KD phenotypes in a Han Chinese population, including 392 KD patients and 421 non-KD controls. Three ADAM17 SNPs showed an association with KD risk, which was further confirmed by haplotype analysis. The effect of ADAM17 on KD was also shown by multi-variable logistic regression analysis. In two-locus model analyses with SNPs in ADAM17 and TGF-β signaling pathway genes, stronger compound effects on the risk of KD and secondary CAL formation were observed relative to comparable single SNPs.

CONCLUSION

Our results suggest that ADAM17 contributes to the KD risk and is involved in secondary CAL formation via the TGF-β/SMAD3 signaling pathway. This further enriches our understanding of the importance of the signaling pathway in KD occurrence and outcomes.

WHAT IS KNOWN

• The transforming growth factor (TGF)-β/SMAD3 signaling pathway greatly influences susceptibility to Kawasaki disease (KD) and secondary coronary artery lesions (CALs) and/or the treatment response of intravenous immunoglobulin. • A disintegrin and metalloprotease 17 (ADAM17) effectively reduces TGF-β signaling by cleaving TGF-β receptor type-1, while ADAM17 genetic variants modify human vascular pathology by differentially regulating this signaling although it is unknown whether ADAM17 contributes to KD phenotypes. What is New: • ADAM17 genetic variants were shown to be associated with KD risk, even when excluding the influence of TGF-β signaling pathway genes, suggesting that ADAM17 is an important KD susceptibility-related genetic locus. • The more significant compound effects of two-locus models, combining single nucleotide polymorphisms (SNPs) in ADAM17 and other TGF-β signaling pathway genes including TGFB2 and SMAD3, on KD phenotypes relative to single SNPs suggest that ADAM17 is also involved in secondary CAL formation and confers the risk of KD/CALs via the TGF-β/SMAD3 signaling pathway.

Muscular dystrophy in a dish: engineered human skeletal muscle mimetics for disease modeling and drug discovery

Engineered in vitro models using human cells, particularly patient-derived induced pluripotent stem cells (iPSCs), offer a potential solution to issues associated with the use of animals for studying disease pathology and drug efficacy. Given the prevalence of muscle diseases in human populations, an engineered tissue model of human skeletal muscle could provide a biologically accurate platform to study basic muscle physiology, disease progression, and drug efficacy and/or toxicity. Such platforms could be used as phenotypic drug screens to identify compounds capable of alleviating or reversing congenital myopathies, such as Duchene muscular dystrophy (DMD). Here, we review current skeletal muscle modeling technologies with a specific focus on efforts to generate biomimetic systems for investigating the pathophysiology of dystrophic muscle.

Cyclic Stretch Facilitates Myogenesis in C2C12 Myoblasts and Rescues Thiazolidinedione-Inhibited Myotube Formation

Thiazolidinedione (TZD), a specific peroxisome proliferator-activated receptor γ (PPARγ) agonist, was developed to control blood glucose in diabetes patients. However, several side effects were reported that increased the risk of heart failure. We used C2C12 myoblasts to investigate the role of PPARs and their transcriptional activity during myotube formation. The role of mechanical stretch during myogenesis was also explored by applying cyclic stretch to the differentiating C2C12 myoblasts with 10% strain deformation at 1 Hz. The myogenesis medium (MM), composed of Dulbecco’s modified Eagle’s medium with 2% horse serum, facilitated myotube formation with increased myosin heavy chain and α-smooth muscle actin (α-SMA) protein expression. The PPARγ protein and PPAR response element (PPRE) promoter activity decreased during MM induction. Cyclic stretch further facilitated the myogenesis in MM with increased α-SMA and decreased PPARγ protein expression and inhibited PPRE promoter activity. Adding a PPARγ agonist (TZD) to the MM stopped the myogenesis and restored the PPRE promoter activity, whereas a PPARγ antagonist (GW9662) significantly increased the myotube number and length. During the myogenesis induction, application of cyclic stretch rescued the inhibitory effects of TZD. These results provide novel perspectives for mechanical stretch to interplay and rescue the dysfunction of myogenesis with the involvement of PPARγ and its target drugs.

Caveolin-1-dependent activation of the metalloprotease TACE/ADAM17 by TGF-β in hepatocytes requires activation of Src and the NADPH oxidase NOX1

Transforming growth factor-β (TGF-β) plays a dual role in hepatocytes, inducing both pro- and anti-apoptotic responses, the balance between which decides cell fate. Survival signals are mediated by the epidermal growth factor receptor (EGFR) pathway, which is activated by TGF-β. We have previously shown that caveolin-1 (CAV1) is required for activation of the metalloprotease tumour necrosis factor (TNF)-α-converting enzyme/a disintegrin and metalloproteinase 17 (TACE/ADAM17), and hence transactivation of the EGFR pathway. The specific mechanism by which TACE/ADAM17 is activated has not yet been determined. Here we show that TGF-β induces phosphorylation of sarcoma kinase (Src) in hepatocytes, a process that is impaired in Cav1(-/-) hepatocytes, coincident with a decrease in phosphorylated Src in detergent-resistant membrane fractions. TGF-β-induced activation of TACE/ADAM17 and EGFR phosphorylation were blocked using the Src inhibitor PP2. Cav1(+/+) hepatocytes showed early production of reactive oxygen species (ROS) induced by TGF-β, which was not seen in Cav1(-/-) cells. Production of ROS was inhibited by both the NADPH oxidase 1 (NOX1) inhibitor STK301831 and NOX1 knock-down, which also impaired TACE/ADAM17 activation and thus EGFR phosphorylation. Finally, neither STK301831 nor NOX1 silencing impaired Src phosphorylation, but PP2 blocked early ROS production, showing that Src is involved in NOX1 activation. As expected, inhibition of Src or NOX1 increased TGF-β-induced cell death in Cav1(+/+) cells. In conclusion, CAV1 is required for TGF-β-mediated activation of TACE/ADAM17 through a mechanism that involves phosphorylation of Src and NOX1-mediated ROS production.

Biomechanical Origins of Muscle Stem Cell Signal Transduction

Skeletal muscle, the most abundant and widespread tissue in the human body, contracts upon receiving electrochemical signals from the nervous system to support essential functions such as thermoregulation, limb movement, blinking, swallowing and breathing. Reconstruction of adult muscle tissue relies on a pool of mononucleate, resident muscle stem cells, known as "satellite cells", expressing the paired-box transcription factor Pax7 necessary for their specification during embryonic development and long-term maintenance during adult life. Satellite cells are located around the myofibres in a niche at the interface of the basal lamina and the host fibre plasma membrane (i.e., sarcolemma), at a very low frequency. Upon damage to the myofibres, quiescent satellite cells are activated and give rise to a population of transient amplifying myogenic progenitor cells, which eventually exit the cell cycle permanently and fuse to form new myofibres and regenerate the tissue. A subpopulation of satellite cells self-renew and repopulate the niche, poised to respond to future demands. Harnessing the potential of satellite cells relies on a complete understanding of the molecular mechanisms guiding their regulation in vivo. Over the past several decades, studies revealed many signal transduction pathways responsible for satellite cell fate decisions, but the niche cues driving the activation and silencing of these pathways are less clear. Here we explore the scintillating possibility that considering the dynamic changes in the biophysical properties of the skeletal muscle, namely stiffness, and the stretch and shear forces to which a myofibre can be subjected to may provide missing information necessary to gain a full understanding of satellite cell niche regulation.

Exercise training and return to a well-balanced diet activate the neuregulin 1/ErbB pathway in skeletal muscle of obese rats

KEY POINTS

Some studies suggest that neuregulin 1 (NRG1) could be involved in the regulation of skeletal muscle energy metabolism in rodents. Here we assessed whether unbalanced diet is associated with alterations of the NRG1 signalling pathway and whether exercise and diet might restore NRG1 signalling in skeletal muscle of obese rats. We show that diet-induced obesity does not impair NRG1 signalling in rat skeletal muscle. We also report that endurance training and a well-balanced diet activate the NRG1 signalling in skeletal muscle of obese rats, possibly via a new mechanism mediated by the protease ADAM17. These results suggest that some beneficial effects of physical activity and diet in obese rats could be partly explained by stimulation of the NRG1 signalling pathway.

ABSTRACT

Some studies suggest that the signalling pathway of neuregulin 1 (NRG1), a protein involved in the regulation of skeletal muscle metabolism, could be altered by nutritional and exercise interventions. We hypothesized that diet-induced obesity could lead to alterations of the NRG1 signalling pathway and that chronic exercise could improve NRG1 signalling in rat skeletal muscle. To test this hypothesis, male Wistar rats received a high fat/high sucrose (HF/HS) diet for 16 weeks. At the end of this period, NRG1 and ErbB expression/activity in skeletal muscle was assessed. The obese rats then continued the HF/HS diet or were switched to a well-balanced diet. Moreover, in both groups, half of the animals also performed low intensity treadmill exercise training. After another 8 weeks, NRG1 and ErbB expression/activity in skeletal muscle were tested again. The 16 week HF/HS diet induced obesity, but did not significantly affect the NRG1/ErbB signalling pathway in rat skeletal muscle. Conversely, after the switch to a well-balanced diet, NRG1 cleavage ratio and ErbB4 amount were increased. Chronic exercise training also promoted NRG1 cleavage, resulting in increased ErbB4 phosphorylation. This result was associated with increased protein expression and phosphorylation ratio of the metalloprotease ADAM17, which is involved in NRG1 shedding. Similarly, in vitro stretch-induced activation of ADAM17 in rat myoblasts induced NRG1 cleavage and ErbB4 activation. These results show that low intensity endurance training and well-balanced diet activate the NRG1-ErbB4 pathway, possibly via the metalloprotease ADAM17, in skeletal muscle of diet-induced obese rats.

Signals of Ezh2, Src, and Akt Involve in myostatin-Pax7 pathways regulating the myogenic fate determination during the sheep myoblast proliferation and differentiation

Myostatin and Pax7 have been well documented individually, however, the mechanism by which Myostatin regulates Pax7 is seldom reported. Here, based on muscle transcriptome analysis in Texel (Myostatin mutant) and Ujumqin (wild type) sheep across the five fetal stages, we constructed and examined the Myostatin-Pax7 pathways in muscle. Then we validated the signals by RNAi in the proliferating and differentiating sheep myoblasts in vitro at mRNA, protein, and cell morphological levels. We reveal that Myostatin signals to Pax7 at least through Ezh2, Src, and Akt during the sheep myoblast proliferation and differentiation. Other signals such as p38MAPK, mTOR, Erk1/2, Wnt, Bmp2, Smad, Tgfb1, and p21 are most probably involved in the Myostatin-affected myogenic events. Myostatin knockdown significantly reduces the counts of nucleus and myotube, but not the fusion index of myoblasts during cell differentiation. In addition, findings also indicate that Myostatin is required for normal myogenic differentiation of the sheep myoblasts, which is different from the C2C12 myoblasts. We expand the regulatory network of Myostatin-Pax7 pathways and first illustrate that Myostatin as a global regulator participates in the epigenetic events involved in myogenesis, which contributes to understand the molecular mechanism of Myostatin in regulation of myogenesis.

Cyclic stretch-induced TGF-β1 and fibronectin expression is mediated by β1-integrin through c-Src- and STAT3-dependent pathways in renal epithelial cells

Extracellular matrix (ECM) proteins, including fibronectin, may contribute to the early development and progression of renal interstitial fibrosis associated with chronic renal disease. Recent studies showed that β1-integrin is associated with the development of renal fibrosis in a murine model of unilateral ureteral obstruction (UUO). However, the molecular events responsible for β1-integrin-mediated signaling, following UUO, have yet to be determined. In this study, we investigated the mechanism by which mechanical stretch, an in vitro model for chronic obstructive nephropathy, regulates fibronectin and transforming growth factor-β1 (TGF-β1) expression in cultured human proximal tubular epithelium (HK-2) cells. Mechanical stretch upregulated fibronectin and TGF-β1 expression and activated signal transducer and transcription factor 3 (STAT3) in a time-dependent manner. Stretch-induced fibronectin and TGF-β1 were suppressed by a STAT3 inhibitor, S3I-201, and by small interfering RNA (siRNA) targeting human STAT3 (STAT3 siRNA). Similarly, fibronectin and TGF-β1 expression and STAT3 activation induced by mechanical stretch were suppressed by the Src family kinase inhibitor PP2 and by transfection of HK-2 cells with a dominant-negative mutant of c-Src (DN-Src), whereas PP3, an inactive analog of PP2, had no significant effect. Furthermore, mechanical stretch resulted in increased β1-integrin mRNA and protein levels in HK-2 cells. Furthermore, neutralizing antibody against β1-integrin and silencing of β1-integrin expression with siRNAs resulted in decreased c-Src and STAT3 activation and TGF-β1 and fibronectin expression evoked by mechanical stretch. This work demonstrates, for the first time, a role for β1-integrin in stretch-induced renal fibrosis through the activation of c-Src and STAT3 signaling pathways.

A1 adenosine receptor-stimulated exocytosis in bladder umbrella cells requires phosphorylation of ADAM17 Ser-811 and EGF receptor transactivation

The role of phosphorylation in ADAM17-dependent shedding is controversial. We show that the A₁ adenosine receptor stimulates exocytosis in umbrella cells by a pathway that requires phosphorylation of ADAM17–Ser-811, followed by HB-EGF shedding and EGF receptor transactivation. Preventing ADAM17 phosphorylation blocks these downstream events.

Despite the importance of ADAM17-dependent cleavage in normal biology and disease, the physiological cues that trigger its activity, the effector pathways that promote its function, and the mechanisms that control its activity, particularly the role of phosphorylation, remain unresolved. Using native bladder epithelium, in some cases transduced with adenoviruses encoding small interfering RNA, we observe that stimulation of apically localized A₁ adenosine receptors (A₁ARs) triggers a G_i-G_βγ-phospholipase C-protein kinase C (PKC) cascade that promotes ADAM17-dependent HB-EGF cleavage, EGFR transactivation, and apical exocytosis. We further show that the cytoplasmic tail of rat ADAM17 contains a conserved serine residue at position 811, which resides in a canonical PKC phosphorylation site, and is phosphorylated in response to A₁AR activation. Preventing this phosphorylation event by expression of a nonphosphorylatable ADAM17^S811A mutant or expression of a tail-minus construct inhibits A₁AR-stimulated, ADAM17-dependent HB-EGF cleavage. Furthermore, expression of ADAM17^S811A in bladder tissues impairs A₁AR-induced apical exocytosis. We conclude that adenosine-stimulated exocytosis requires PKC- and ADAM17-dependent EGFR transactivation and that the function of ADAM17 in this pathway depends on the phosphorylation state of Ser-811 in its cytoplasmic domain.

Skeletal muscle myogenesis is regulated by G protein-coupled receptor kinase 2

G protein-coupled receptor kinase 2 (GRK2) is an important serine/threonine-kinase regulating different membrane receptors and intracellular proteins. Attenuation of Drosophila Gprk2 in embryos or adult flies induced a defective differentiation of somatic muscles, loss of fibers, and a flightless phenotype. In vertebrates, GRK2 hemizygous mice contained less but more hypertrophied skeletal muscle fibers than wild-type littermates. In C2C12 myoblasts, overexpression of a GRK2 kinase-deficient mutant (K220R) caused precocious differentiation of cells into immature myotubes, which were wider in size and contained more fused nuclei, while GRK2 overexpression blunted differentiation. Moreover, p38MAPK and Akt pathways were activated at an earlier stage and to a greater extent in K220R-expressing cells or upon kinase downregulation, while the activation of both kinases was impaired in GRK2-overexpressing cells. The impaired differentiation and fewer fusion events promoted by enhanced GRK2 levels were recapitulated by a p38MAPK mutant, which was able to mimic the inhibitory phosphorylation of p38MAPK by GRK2, whereas the blunted differentiation observed in GRK2-expressing clones was rescued in the presence of a constitutively active upstream stimulator of the p38MAPK pathway. These results suggest that balanced GRK2 function is necessary for a timely and complete myogenic process.

Regulation of myogenic activation of p38 MAPK by TACE-mediated TNFα release

The activation of p38 MAPK in myogenic precursor cells (MPCs) is a key signal for their exit of cell cycle and entry of the myogenic differentiation program. Therefore, identification of the signaling mechanism that activates p38 MAPK during this process is important for the understanding of the regulatory mechanism of muscle regeneration. This article reviews recent findings regarding the role of inflammatory cytokine tumor necrosis factor-α (TNFα) as a key activator of p38 MAPK during myogenesis in an autocrine/paracrine fashion, and the signaling mechanisms that converge upon TNFα converting enzyme (TACE) to release TNFα from differentiating MPCs in response to diverse regenerative stimuli.