Insights into Skeletal Muscle Development and Applications in Regenerative Medicine

Bioinformatics analysis and identification of genes and molecular pathways involved in Parkinson's disease in patients with mutations in the glucocerebrosidase gene

Glucocerebrosidase (GBA) mutations occur frequently in Parkinson's disease (PD) patients. This study aims to identify potential crucial genes and pathways associated with GBA mutations in patients with PD and to further analyze new molecular mechanisms related to the occurrence of gene mutations from the perspective of bioinformatics. Gene expression profiles of datasets GSE53424 and GSE99142 were acquired from the Gene Expression Ominibus database. Differentially expressed genes (DEGs) were detected, using the 'limma' package in R, comparing IDI-PD 1 (idiopathic PD patients) and GBA-PD 1 [PD patients with heterozygous GBA mutations (GBA N370S)] group samples. The functions of top modules were assessed using the DAVID, whereas gene ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses were performed. Protein-protein interaction networks were assembled with Cytoscape software and separated into subnetworks using the Molecular Complex Detection Algorithm. Data from GSE53424 and GSE99142 were also extracted to verify our findings. There were 283 DEGs identified in PD patients heterozygous for GBA mutations. Module analysis revealed that GBA mutations in PD patients were associated with significant pathways, including Calcium signaling pathway, Rap1 signaling pathway and Cytokine-cytokine receptor interaction. Hub genes of the two modules were corticotropin-releasing hormone (CRH) and Melatonin receptor 1B (MTNR1B). The expression of CRH was downregulated, whereas that of MTNR1B was upregulated in PD patients with GBA mutations. The expression of CRH and MTNR1B has diagnostic value for PD patients with heterozygous GBA mutations. Novel DEGs and pathways identified herein might provide new insights into the underlying molecular mechanisms of heterozygous GBA mutations in PD patients.

Notch Signaling in Development, Tissue Homeostasis, and Disease

Notch signaling is an evolutionarily highly conserved signaling mechanism, but in contrast to signaling pathways such as Wnt, Sonic Hedgehog, and BMP/TGF-β, Notch signaling occurs via cell-cell communication, where transmembrane ligands on one cell activate transmembrane receptors on a juxtaposed cell. Originally discovered through mutations in Drosophila more than 100 yr ago, and with the first Notch gene cloned more than 30 yr ago, we are still gaining new insights into the broad effects of Notch signaling in organisms across the metazoan spectrum and its requirement for normal development of most organs in the body. In this review, we provide an overview of the Notch signaling mechanism at the molecular level and discuss how the pathway, which is architecturally quite simple, is able to engage in the control of cell fates in a broad variety of cell types. We discuss the current understanding of how Notch signaling can become derailed, either by direct mutations or by aberrant regulation, and the expanding spectrum of diseases and cancers that is a consequence of Notch dysregulation. Finally, we explore the emerging field of Notch in the control of tissue homeostasis, with examples from skin, liver, lung, intestine, and the vasculature.

Making muscle: skeletal myogenesis in vivo and in vitro

Skeletal muscle is the largest tissue in the body and loss of its function or its regenerative properties results in debilitating musculoskeletal disorders. Understanding the mechanisms that drive skeletal muscle formation will not only help to unravel the molecular basis of skeletal muscle diseases, but also provide a roadmap for recapitulating skeletal myogenesis in vitro from pluripotent stem cells (PSCs). PSCs have become an important tool for probing developmental questions, while differentiated cell types allow the development of novel therapeutic strategies. In this Review, we provide a comprehensive overview of skeletal myogenesis from the earliest premyogenic progenitor stage to terminally differentiated myofibers, and discuss how this knowledge has been applied to differentiate PSCs into muscle fibers and their progenitors in vitro.

Generation of human muscle fibers and satellite-like cells from human pluripotent stem cells in vitro

Progress toward finding a cure for muscle diseases has been slow because of the absence of relevant cellular models and the lack of a reliable source of muscle progenitors for biomedical investigation. Here we report an optimized serum-free differentiation protocol to efficiently produce striated, millimeter-long muscle fibers together with satellite-like cells from human pluripotent stem cells (hPSCs) in vitro. By mimicking key signaling events leading to muscle formation in the embryo, in particular the dual modulation of Wnt and bone morphogenetic protein (BMP) pathway signaling, this directed differentiation protocol avoids the requirement for genetic modifications or cell sorting. Robust myogenesis can be achieved in vitro within 1 month by personnel experienced in hPSC culture. The differentiating culture can be subcultured to produce large amounts of myogenic progenitors amenable to numerous downstream applications. Beyond the study of myogenesis, this differentiation method offers an attractive platform for the development of relevant in vitro models of muscle dystrophies and drug screening strategies, as well as providing a source of cells for tissue engineering and cell therapy approaches.

F-BOX proteins in cancer cachexia and muscle wasting: Emerging regulators and therapeutic opportunities

Cancer cachexia is a debilitating metabolic syndrome accounting for fatigue, an impairment of normal activities, loss of muscle mass associated with body weight loss eventually leading to death in majority of patients with advanced disease. Cachexia patients undergoing skeletal muscle atrophy show consistent activation of the SCF ubiquitin ligase (F-BOX) family member Atrogin-1 (also known as MAFBx/FBXO32) alongside the activation of the muscle ring finger protein1 (MuRF1). Other lesser known F-BOX family members are also emerging as key players supporting muscle wasting pathways. Recent work highlights a spectrum of different cancer signaling mechanisms impacting F-BOX family members that feed forward muscle atrophy related genes during cachexia. These novel players provide unique opportunities to block cachexia induced skeletal muscle atrophy by therapeutically targeting the SCF protein ligases. Conversely, strategies that induce the production of proteins may be helpful to counter the effects of these F-BOX proteins. Through this review, we bring forward some novel targets that promote atrogin-1 signaling in cachexia and muscle wasting and highlight newer therapeutic opportunities that can help in the better management of patients with this devastating and fatal disorder.

Generation of Functional Neuromuscular Junctions from Human Pluripotent Stem Cell Lines

Several neuromuscular diseases involve dysfunction of neuromuscular junctions (NMJs), yet there are no patient-specific human models for electrophysiological characterization of NMJ. We seeded cells of neurally-induced embryoid body-like spheres derived from induced pluripotent stem cell (iPSC) or embryonic stem cell (ESC) lines as monolayers without basic fibroblast factor (bFGF) and observed differentiation of neuronal as well as spontaneously contracting, multinucleated skeletal myotubes. The myotubes showed striation, immunoreactivity for myosin heavy chain, actin bundles typical for myo-oriented cells, and generated spontaneous and evoked action potentials (APs). The myogenic differentiation was associated with expression of MyoD1, myogenin and type I ryanodine receptor. Neurons formed end plate like structures with strong binding of α-bungarotoxin, a marker of nicotinic acetylcholine receptors highly expressed in the postsynaptic membrane of NMJs, and expressed SMI-32, a motoneuron marker, as well as SV2, a marker for synapses. Pharmacological stimulation of cholinergic receptors resulted in strong depolarization of myotube membrane and raised Ca²⁺ concentration in sarcoplasm, while electrical stimulation evoked Ca²⁺ transients in myotubes. Stimulation of motoneurons with N-Methyl-D-aspartate resulted in reproducible APs in myotubes and end plates displayed typical mEPPs and tonic activity depolarizing myotubes of about 10 mV. We conclude that simultaneous differentiation of neurons and myotubes from patient-specific iPSCs or ESCs results also in the development of functional NMJs. Our human model of NMJ may serve as an important tool to investigate normal development, mechanisms of diseases and novel drug targets involving NMJ dysfunction and degeneration.

Acute and temporal expression of tumor necrosis factor (TNF)-α-stimulated gene 6 product, TSG6, in mesenchymal stem cells creates microenvironments required for their successful transplantation into muscle tissue

Previously, we demonstrated that when mesenchymal stem cells (MSCs) from mouse ES cells were transplanted into skeletal muscle, more than 60% of them differentiated into muscles in the crush-injured tibialis anterior muscle in vivo, although MSCs neither differentiated nor settled in the intact muscle. Microenvironments, including the extracellular matrix between the injured and intact muscle, were quite different. In the injured muscle, hyaluronan (HA), heavy chains of inter-α-inhibitor (IαI), CD44, and TNF-α-stimulated gene 6 product (TSG-6) increased 24-48 h after injury, although basement membrane components of differentiated muscle such as perlecan, laminin, and type IV collagen increased gradually 4 days after the crush. We then investigated the microenvironments crucial for cell transplantation, using the lysate of C2C12 myotubules for mimicking injured circumstances in vivo. MSCs settled in the intact muscle when they were transplanted together with the C2C12 lysate or TSG6. MSCs produced and released TSG6 when they were cultured with C2C12 lysates in vitro. MSCs pretreated with the lysate also settled in the intact muscle. Furthermore, MSCs whose TSG6 was knocked down by shRNA, even if transplanted or pretreated with the lysate, could not settle in the muscle. Immunofluorescent staining showed that HA and IαI always co-localized or were distributed closely, suggesting formation of covalent complexes, i.e. the SHAP-HA complex in the presence of TSG6. Thus, TSG6, HA, and IαI were crucial factors for the settlement and probably the subsequent differentiation of MSCs.

WNT/β-Catenin Signaling Regulates Multiple Steps of Myogenesis by Regulating Step-Specific Targets

Molecules involved in WNT/β-catenin signaling show specific spatiotemporal expression and play vital roles in myogenesis; however, it is still largely unknown how WNT/β-catenin signaling regulates each step of myogenesis. Here, we show that WNT/β-catenin signaling can control diverse biological processes of myogenesis by regulating step-specific molecules. In order to identify the temporally specific roles of WNT/β-catenin signaling molecules in muscle development and homeostasis, we used in vitro culture systems for both primary mouse myoblasts and C2C12 cells, which can differentiate into myofibers. We found that a blockade of WNT/β-catenin signaling in the proliferating cells decreases proliferation activity, but does not induce cell death, through the regulation of genes cyclin A2 (Ccna2) and cell division cycle 25C (Cdc25c). During muscle differentiation, the inhibition of WNT/β-catenin signaling blocks myoblast fusion through the inhibition of the Fermitin family homolog 2 (Fermt2) gene. Blocking WNT/β-catenin signaling in the well-differentiated myofibers results in the failure of maintenance of their structure by disruption of cadherin/β-catenin/actin complex formation, which plays a crucial role in connecting a myofiber's cytoskeleton to the surrounding extracellular matrix. Thus, our results indicate that WNT/β-catenin signaling can regulate multiple steps of myogenesis, including cell proliferation, myoblast fusion, and homeostasis, by targeting step-specific molecules.

Isolation and immortalization of patient-derived cell lines from muscle biopsy for disease modeling

The generation of patient-specific cell lines represents an invaluable tool for diagnostic or translational research, and these cells can be collected from skin or muscle biopsy tissue available during the patient's diagnostic workup. In this protocol, we describe a technique for live cell isolation from small amounts of muscle or skin tissue for primary cell culture. Additionally, we provide a technique for the immortalization of myogenic cell lines and fibroblast cell lines from primary cells. Once cell lines are immortalized, substantial expansion of patient-derived cells can be achieved. Immortalized cells are amenable to many downstream applications, including drug screening and in vitro correction of the genetic mutation. Altogether, these protocols provide a reliable tool to generate and preserve patient-derived cells for downstream applications.

Notch signaling genes: myogenic DNA hypomethylation and 5-hydroxymethylcytosine

Notch intercellular signaling is critical for diverse developmental pathways and for homeostasis in various types of stem cells and progenitor cells. Because Notch gene products need to be precisely regulated spatially and temporally, epigenetics is likely to help control expression of Notch signaling genes. Reduced representation bisulfite sequencing (RRBS) indicated significant hypomethylation in myoblasts, myotubes, and skeletal muscle vs. many nonmuscle samples at intragenic or intergenic regions of the following Notch receptor or ligand genes: NOTCH1, NOTCH2, JAG2, and DLL1. An enzymatic assay of sites in or near these genes revealed unusually high enrichment of 5-hydroxymethylcytosine (up to 81%) in skeletal muscle, heart, and cerebellum. Epigenetics studies and gene expression profiles suggest that hypomethylation and/or hydroxymethylation help control expression of these genes in heart, brain, myoblasts, myotubes, and within skeletal muscle myofibers. Such regulation could promote cell renewal, cell maintenance, homeostasis, and a poised state for repair of tissue damage.

Spontaneous calcium transients manifest in the regenerating muscle and are necessary for skeletal muscle replenishment

Tissue regeneration entails replenishing of damaged cells, appropriate cell differentiation and inclusion of regenerated cells into functioning tissues. In adult humans, the capacity of the injured spinal cord and muscle to self-repair is limited. In contrast, the amphibian larva can regenerate its tail after amputation with complete recovery of muscle, notochord and spinal cord. The cellular and molecular mechanisms underlying this phenomenon are still unclear. Here we show that upon injury muscle cell precursors exhibit Ca(2+) transients that depend on Ca(2+) release from ryanodine receptor-operated stores. Blockade of these transients impairs muscle regeneration. Furthermore, inhibiting Ca(2+) transients in the regenerating tail prevents the activation and proliferation of muscle satellite cells, which results in deficient muscle replenishment. These findings suggest that Ca(2+)-mediated activity is critical for the early stages of muscle regeneration, which may lead to developing effective therapies for tissue repair.

Methodological challenges in utilizing miRNAs as circulating biomarkers

MicroRNAs (miRNAs) have emerged as important regulators in the post-transcriptional control of gene expression. The discovery of their presence not only in tissues but also in extratissular fluids, including blood, urine and cerebro-spinal fluid, together with their changes in expression in various pathological conditions, has implicated these extracellular miRNAs as informative biomarkers of disease. However, exploiting miRNAs in this capacity requires methodological rigour. Here, we report several key procedural aspects of miRNA isolation from plasma and serum, as exemplified by research in cardiovascular and pulmonary diseases. We also highlight the advantages and disadvantages of various profiling methods to determine the expression levels of plasma- and serum-derived miRNAs. Attention to such methodological details is critical, as circulating miRNAs become diagnostic tools for various human diseases.