Dlk1 is necessary for proper skeletal muscle development and regeneration

Functional specialisation and coordination of myonuclei

Myofibres serve as the functional unit for locomotion, with the sarcomere as fundamental subunit. Running the entire length of this structure are hundreds of myonuclei, located at the periphery of the myofibre, juxtaposed to the plasma membrane. Myonuclear specialisation and clustering at the centre and ends of the fibre are known to be essential for muscle contraction, yet the molecular basis of this regionalisation has remained unclear. While the 'myonuclear domain hypothesis' helped explain how myonuclei can independently govern large cytoplasmic territories, novel technologies have provided granularity on the diverse transcriptional programs running simultaneously within the syncytia and added a new perspective on how myonuclei communicate. Building upon this, we explore the critical cellular and molecular sources of transcriptional and functional heterogeneity within myofibres, discussing the impact of intrinsic and extrinsic factors on myonuclear programs. This knowledge provides new insights for understanding muscle development, repair, and disease, but also opens avenues for the development of novel and precise therapeutic approaches.

Aging and Autophagy: Roles in Musculoskeletal System Injury

Aging is a multifactorial process that ultimately leads to a decline in physiological function and a consequent reduction in the health span, and quality of life in elderly population. In musculoskeletal diseases, aging is often associated with a gradual loss of skeletal muscle mass and strength, resulting in reduced functional capacity and an increased risk of chronic metabolic diseases, leading to impaired function and increased mortality. Autophagy is a highly conserved physiological process by which cells, under the regulation of autophagy-related genes, degrade their own organelles and large molecules by lysosomal degradation. This process is unique to eukaryotic cells and is a strict regulator of homeostasis, the maintenance of energy and substance balance. Autophagy plays an important role in a wide range of physiological and pathological processes such as cell homeostasis, aging, immunity, tumorigenesis and neurodegenerative diseases. On the one hand, under mild stress conditions, autophagy mediates the restoration of homeostasis and proliferation, reduction of the rate of aging and delay of the aging process. On the other hand, under more intense stress conditions, an inadequate suppression of autophagy can lead to cellular aging. Conversely, autophagy activity decreases during aging. Due to the interrelationship between aging and autophagy, limited literature exists on this topic. Therefore, the objective of this review is to summarize the current concepts on aging and autophagy in the musculoskeletal system. The aim is to better understand the mechanisms of age-related changes in bone, joint and muscle, as well as the interaction relationship between autophagy and aging. Its goal is to provide a comprehensive perspective for the improvement of diseases of the musculoskeletal system.

Muscle stem cell niche dynamics during muscle homeostasis and regeneration

The process of skeletal muscle regeneration involves a coordinated interplay of specific cellular and molecular interactions within the injury site. This review provides an overview of the cellular and molecular components in regenerating skeletal muscle, focusing on how these cells or molecules in the niche regulate muscle stem cell functions. Dysfunctions of muscle stem cell-to-niche cell communications during aging and disease will also be discussed. A better understanding of how niche cells coordinate with muscle stem cells for muscle repair will greatly aid the development of therapeutic strategies for treating muscle-related disorders.

A Fanca knockout mouse model reveals novel Fancd2 function

Fanconi anemia (FA) is a genetically and clinically heterogenous inherited disorder. Clinically, Fanca subtype patients exhibited milder phenotypes compared to Fancd2 subtypes. Increasing evidence suggests that Fancd2 perform independent functions, but the detailed mechanisms are not well characterized. In this study, we developed a Fanca KO mice model in C57BL/6 background with ATG region deletion, then performed a detailed FA phenotypes characterization and analysis with Fanca KO mice and Fancd2 KO mice in the same congenic background. We found that both the Fanca KO and Fancd2 KO cause severe FA phenotypes in mice. However, Fanca KO mice exhibited milder FA phenotypes comparing to Fancd2 KO mice. Fanca KO mice showed higher embryonic and postnatal survival rate, less congenital eye defects in early development. At adult stage, Fanca KO mice showed increased HSC number and reconstitution function. Furthermore, we did RNA-seq study and identified differential expression of Dlk1 and Dlk1 pathway genes in Fanca KO and Fancd2 KO embryonic cells and adult HSCs. Finally, we revealed that Fancd2 was expressed and physically interact with Dlk1 in Fanca KO cells. Collectively, our findings suggested that Fancd2 has distinct functions in the absence of Fanca.

Effects of Adipose Tissue-Specific Knockout of Delta-like Non-Canonical Notch Ligand 1 on Lipid Metabolism in Mice

Delta-like non-canonical Notch ligand 1 (DLK1), which inhibits the differentiation of precursor adipocytes, is a recognized marker gene for precursor adipocytes. Lipids play a crucial role in energy storage and metabolism as a vital determinant of beef quality. In this study, we investigated the mechanism of the DLK1 gene in lipid metabolism by constructing adipose tissue-specific knockout mice. We examined some phenotypic traits, including body weight, liver coefficient, fat index, the content of triglyceride (TG) and cholesterol (CHOL) in abdominal white adipose tissue (WAT) and blood. Subsequently, the fatty acid content and genes related to lipid metabolism expression were detected in DLK1-/- and wild-type mice via GC-MS/MS analysis and quantitative real-time PCR (qRT-PCR), respectively. The results illustrated that DLK1-/- mice exhibited significant abdominal fat deposition compared to wild-type mice. HE staining and immunohistochemistry (IHC) results showed that the white adipocytes of DLK1-/- mice were larger, and the protein expression level of DLK1-/- was significantly lower. Regarding the blood biochemical parameters of female mice, DLK1-/- mice had a strikingly higher triglyceride content (p < 0.001). The fatty acid content in DLK1-/- mice was generally reduced. There was a significant reduction in the expression levels of the majority of genes that play a crucial role in lipid metabolism. This study reveals the molecular regulatory mechanism of fat metabolism in mice and provides a molecular basis and reference for the future application of the DLK1 gene in the breeding of beef cattle with an excellent meat quality traits. It also provides a molecular basis for unravelling the complex and subtle relationship between adipose tissue and health.

Intermuscular adipose tissue in obesity and related disorders: cellular origins, biological characteristics and regulatory mechanisms

Intermuscular adipose tissue (IMAT) is a unique adipose depot interspersed between muscle fibers (myofibers) or muscle groups. Numerous studies have shown that IMAT is strongly associated with insulin resistance and muscular dysfunction in people with metabolic disease, such as obesity and type 2 diabetes. Moreover, IMAT aggravates obesity-related muscle metabolism disorders via secretory factors. Interestingly, researchers have discovered that intermuscular brown adipocytes in rodent models provide new hope for obesity treatment by acting on energy dissipation, which inspired researchers to explore the underlying regulation of IMAT formation. However, the molecular and cellular properties and regulatory processes of IMAT remain debated. Previous studies have suggested that muscle-derived stem/progenitor cells and other adipose tissue progenitors contribute to the development of IMAT. Adipocytes within IMAT exhibit features that are similar to either white adipocytes or uncoupling protein 1 (UCP1)-positive brown adipocytes. Additionally, given the heterogeneity of skeletal muscle, which comprises myofibers, satellite cells, and resident mesenchymal progenitors, it is plausible that interplay between these cellular components actively participate in the regulation of intermuscular adipogenesis. In this context, we review recent studies associated with IMAT to offer insights into the cellular origins, biological properties, and regulatory mechanisms of IMAT. Our aim is to provide novel ideas for the therapeutic strategy of IMAT and the development of new drugs targeting IMAT-related metabolic diseases.

Canonical Wnt signaling regulates soft palate development by mediating ciliary homeostasis

Craniofacial morphogenesis requires complex interactions involving different tissues, signaling pathways, secreted factors and organelles. The details of these interactions remain elusive. In this study, we have analyzed the molecular mechanisms and homeostatic cellular activities governing soft palate development to improve regenerative strategies for individuals with cleft palate. We have identified canonical Wnt signaling as a key signaling pathway primarily active in cranial neural crest (CNC)-derived mesenchymal cells surrounding soft palatal myogenic cells. Using Osr2-Cre;β-cateninfl/fl mice, we show that Wnt signaling is indispensable for mesenchymal cell proliferation and subsequently for myogenesis through mediating ciliogenesis. Specifically, we have identified that Wnt signaling directly regulates expression of the ciliary gene Ttll3. Impaired ciliary disassembly leads to differentiation defects in mesenchymal cells and indirectly disrupts myogenesis through decreased expression of Dlk1, a mesenchymal cell-derived pro-myogenesis factor. Moreover, we show that siRNA-mediated reduction of Ttll3 expression partly rescues mesenchymal cell proliferation and myogenesis in the palatal explant cultures from Osr2-Cre;β-cateninfl/fl embryos. This study highlights the role of Wnt signaling in palatogenesis through the control of ciliary homeostasis, which establishes a new mechanism for Wnt-regulated craniofacial morphogenesis.

Lipid metabolism mRNA expression and cellularity of intramuscular adipocytes within the Longissimus muscle of Angus- and Wagyu-sired cattle fed for a similar days on feed or body weight endpoint

This study investigates intramuscular (IM) adipocyte development in the Longissimus muscle (LM) between Wagyu- and Angus-sired steers compared at a similar age and days on feed (D) endpoint or similar body weight (B) endpoint by measuring IM adipocyte cell area and lipid metabolism mRNA expression. Angus-sired steers (AN, n = 6) were compared with steers from two different Wagyu sires (WA), selected for either growth (G) or marbling (M), to be compared at a similar days on feed (DOF; 258 ± 26.7 d; WA-GD, n = 5 and WA-MD, n = 5) in Exp. 1 or body weight (BW; 613 ± 18.0 kg; WA-GB, n = 4 and WA-MB, n = 5) in Exp. 2, respectively. In Exp. 1, WA-MD steers had a greater (P ≤ 0.01) percentage of IM fat in the LM compared with AN and WA-GD steers. In Exp. 2, WA-MB steers had a greater (P ≤ 0.01) percentage of IM fat in the LM compared with AN and WA-GB steers. The distribution of IM adipocyte area was unimodal at all biopsy collections, with IM adipocyte area becoming progressively larger as cattle age (P ≤ 0.01) and BW increased (P ≤ 0.01). Peroxisome proliferator activated receptor delta (PPARd) was upregulated earlier for WA-MD and WA-MB cattle compared with other steers at a similar DOF and BW (P ≤ 0.02; treatment × biopsy interaction). Peroxisome proliferator activated receptor gamma was upregulated (PPARg) at a lesser BW for WA-MB steers (P = 0.09; treatment × biopsy interaction), while WA-MD steers had a greater (P ≤ 0.04) overall mean PPARg mRNA expression compared with other steers. Glycerol-3-phosphate acyltransferase, lipin 1, and hormone sensitive lipase demonstrated mRNA expression patterns similar to PPARg and PPARd or CCAAT enhancer binding protein beta, which emphasizes their importance in marbling development and growth. Additionally, WA-MD and WA-MB steers often had a greater early mRNA expression of fatty acid transporters (fatty acid transport protein 1; P < 0.02; treatment × biopsy interaction) and binding proteins (fatty acid binding protein 4) compared with other steers. Cattle with a greater marbling propensity appear to upregulate adipogenesis at a younger chronological and physiological maturity through PPARd, PPARg, and possibly adipogenic regulating compounds, lysophosphatidic acid, and diacylglycerol. These genes and compounds could be used as potential markers for marbling propensity of cattle in the future.

Protective Effect of Delta-Like 1 Homolog Against Muscular Atrophy in a Mouse Model

BACKGRUOUND

Muscle atrophy is caused by an imbalance between muscle growth and wasting. Delta-like 1 homolog (DLK1), a protein that modulates adipogenesis and muscle development, is a crucial regulator of myogenic programming. Thus, we investigated the effect of exogenous DLK1 on muscular atrophy.

METHODS

We used muscular atrophy mouse model induced by dexamethasone (Dex). The mice were randomly divided into three groups: (1) control group, (2) Dex-induced muscle atrophy group, and (3) Dex-induced muscle atrophy group treated with DLK1. The effects of DLK1 were also investigated in an in vitro model using C2C12 myotubes.

RESULTS

Dex-induced muscular atrophy in mice was associated with increased expression of muscle atrophy markers and decreased expression of muscle differentiation markers, while DLK1 treatment attenuated these degenerative changes together with reduced expression of the muscle growth inhibitor, myostatin. In addition, electron microscopy revealed that DLK1 treatment improved mitochondrial dynamics in the Dex-induced atrophy model. In the in vitro model of muscle atrophy, normalized expression of muscle differentiation markers by DLK1 treatment was mitigated by myostatin knockdown, implying that DLK1 attenuates muscle atrophy through the myostatin pathway.

CONCLUSION

DLK1 treatment inhibited muscular atrophy by suppressing myostatin-driven signaling and improving mitochondrial biogenesis. Thus, DLK1 might be a promising candidate to treat sarcopenia, characterized by muscle atrophy and degeneration.

Pituitary Tumor-Transforming Gene 1/Delta like Non-Canonical Notch Ligand 1 Signaling in Chronic Liver Diseases

The management of chronic liver diseases (CLDs) remains a challenge, and identifying effective treatments is a major unmet medical need. In the current review we focus on the pituitary tumor transforming gene (PTTG1)/delta like non-canonical notch ligand 1 (DLK1) axis as a potential therapeutic target to attenuate the progression of these pathological conditions. PTTG1 is a proto-oncogene involved in proliferation and metabolism. PTTG1 expression has been related to inflammation, angiogenesis, and fibrogenesis in cancer and experimental fibrosis. On the other hand, DLK1 has been identified as one of the most abundantly expressed PTTG1 targets in adipose tissue and has shown to contribute to hepatic fibrosis by promoting the activation of hepatic stellate cells. Here, we extensively analyze the increasing amount of information pointing to the PTTG1/DLK1 signaling pathway as an important player in the regulation of these disturbances. These data prompted us to hypothesize that activation of the PTTG1/DLK1 axis is a key factor upregulating the tissue remodeling mechanisms characteristic of CLDs. Therefore, disruption of this signaling pathway could be useful in the therapeutic management of CLDs.

Functional Identification of Porcine DLK1 during Muscle Development

Simple Summary

Skeletal muscle is the largest tissue and serves as a protein reservoir and energy reservoir in the human and animal body. It also serves as the main metabolic activity site. The formation of skeletal muscle mainly depends on the differentiation and fusion of myocytes and other complex ordered processes; each step is regulated by various factors. In this study, we investigated the expression profiles, functional identification, and regulatory pathways of Delta-like 1 homolog (DLK1) in pigs and myocytes. We found that DLK1 was highly expressed in the muscle tissues of pigs. DLK1 promoted myocyte proliferation, migration, differentiation, fusion, and muscular hypertrophy, but suppressed muscle degradation. DLK1 also inhibited the Notch signaling pathway by regulating the expression of key factors in the pathway, thereby producing a phenotype in which DLK1 promotes muscle development. These findings provide valuable information to improve our understanding of the functional mechanisms of DLK1 that underly myogenesis to accelerate the process of animal genetic improvement.

Abstract

DLK1 is paternally expressed and is involved in metabolism switching, stem cell maintenance, cell proliferation, and differentiation. Porcine DLK1 was identified in our previous study as a candidate gene that regulates muscle development. In the present study, we characterized DLK1 expression in pigs, and the results showed that DLK1 was highly expressed in the muscles of pigs. In-vitro cellular tests showed that DLK1 promoted myoblast proliferation, migration, and muscular hypertrophy, and at the same time inhibited muscle degradation. The expression of myogenic and fusion markers and the formation of multinucleated myotubes were both upregulated in myoblasts with DLK1 overexpression. DLK1 levels in cultured myocytes were negatively correlated with the expression of key factors in the Notch pathway, suggesting that the suppression of Notch signaling pathways may mediate these processes. Collectively, our results suggest a biological function of DLK1 as an enhancer of muscle development by the inhibition of Notch pathways.

Role of MicroRNAs and Long Non-Coding RNAs in Sarcopenia

Sarcopenia is an age-related pathological process characterized by loss of muscle mass and function, which consequently affects the quality of life of the elderly. There is growing evidence that non-coding RNAs, including microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), play a key role in skeletal muscle physiology. Alterations in the expression levels of miRNAs and lncRNAs contribute to muscle atrophy and sarcopenia by regulating various signaling pathways. This review summarizes the recent findings regarding non-coding RNAs associated with sarcopenia and provides an overview of sarcopenia pathogenesis promoted by multiple non-coding RNA-mediated signaling pathways. In addition, we discuss the impact of exercise on the expression patterns of non-coding RNAs involved in sarcopenia. Identifying non-coding RNAs associated with sarcopenia and understanding the molecular mechanisms that regulate skeletal muscle dysfunction during aging will provide new insights to develop potential treatment strategies.

Fennel (Foeniculum vulgare) seed powder increases Delta-Like Non-Canonical Notch Ligand 1 gene expression in testis, liver, and humeral muscle tissues of growing lambs

Delta-Like Non-Canonical Notch Ligand 1 (DLK1) is one of the key genes involved in the development of muscle, liver, pancreas, and lung cells; adipocytes production; and the improvement of digestion, growth performance, and meat quality. It has been documented that fennel is effective on increasing the DLK1 gene (DLK1) expression in the testis, liver, and muscle tissues, which may consequently have important implications for sheep production. Hence, the aim of the current investigation was to evaluate the fennel seed powder's effect on DLK1 expression in testis, liver, and humeral muscle tissues in growing lambs. For the purpose of this study, 30 male Kermani sheep were fed with three different group of diets (number of animals in each group was 10), including control (without any fennel seed powder), treatment 1 (with 10 g/kg of dry matter (DM) fennel seed powder), and treatment 2 (with 20 g/kg of DM fennel seed powder) during a 3-month period. Thereafter, total RNA was extracted, cDNA was synthesized, and Real-Time PCR was performed. The addition of fennel seed powder (in the treatment 1 and treatment 2 groups) in the growing lambs diets consequently resulted in greater expression of DLK1 in both the liver and humeral muscle tissues compared to the testis tissue (P < 0.05). Furthermore, the increased DLK1 expression was higher in the tissue of humeral muscle (P < 0.05) in comparison to the other two tissues. As well, the concentration of blood testosterone was greater (P < 0.05) for the animals fed with fennel powder compared to growing lambs fed with the control diet. However, the concentrations of blood liver enzymes, including serum glutamic oxaloacetic transaminase (SGOT) and serum glutamic pyruvic transaminase (SGPT), decreased by the addition of 10 g/kg DM fennel to diets of lambs compared to the control diet (no fennel). Therefore, it can be concluded that using fennel seed powder in the diet of growing lamb by affecting the expression of DLK1, can improve the concentrations of blood testosterone, SGOT, SGPT, and muscle structure (increased mass of muscle and size of muscle fiber).

Emerging Roles of DLK1 in the Stem Cell Niche and Cancer Stemness

DLK1 is a maternally imprinted, paternally expressed gene coding for the transmembrane protein Delta-like homologue 1 (DLK1), a non-canonical NOTCH ligand with well-described roles during development, and tumor-supportive functions in several aggressive cancer forms. Here, we review the many functions of DLK1 as a regulator of stem cell pools and tissue differentiation in tissues such as brain, muscle, and liver. Furthermore, we review recent evidence supporting roles for DLK1 in the maintenance of aggressive stem cell characteristics of tumor cells, specifically focusing on central nervous system tumors, neuroblastoma, and hepatocellular carcinoma. We discuss NOTCH -dependent as well as NOTCH-independent functions of DLK1, and focus particularly on the complex pattern of DLK1 expression and cleavage that is finely regulated from a spatial and temporal perspective. Progress in recent years suggest differential functions of extracellular, soluble DLK1 as a paracrine stem cell niche-secreted factor, and has revealed a role for the intracellular domain of DLK1 in cell signaling and tumor stemness. A better understanding of DLK1 regulation and signaling may enable therapeutic targeting of cancer stemness by interfering with DLK1 release and/or intracellular signaling.

Deletion of delta-like 1 homologue accelerates renal inflammation by modulating the Th17 immune response

Preclinical studies have demonstrated that activation of the NOTCH pathway plays a key role in the pathogenesis of kidney damage. There is currently no information on the role of the Delta-like homologue 1 (DLK1), a NOTCH inhibitor, in the regulation of renal damage. Here, we investigated the contribution of DLK1 to experimental renal damage and the underlying molecular mechanisms. Using a Dlk1-null mouse model in the experimental renal damage of unilateral ureteral obstruction, we found activation of NOTCH, as shown by increased nuclear translocation of the NOTCH1 intracellular domain, and upregulation of Dlk2/hey-1 expression compared to wild-type (WT) littermates. NOTCH1 over-activation in Dlk1-null injured kidneys was associated with a higher inflammatory response, characterized by infiltration of inflammatory cells, mainly CD4/IL17A + lymphocytes, and activation of the Th17 immune response. Furthermore, pharmacological NOTCH blockade inhibited the transcription factors controlling Th17 differentiation and gene expression of the Th17 effector cytokine IL-17A and other related-inflammatory factors, linked to a diminution of inflammation in the injured kidneys. We propose that the non-canonical NOTCH ligand DLK1 acts as a NOTCH antagonist in renal injury regulating the Th17-mediated inflammatory response.

Role of fibro-adipogenic progenitor cells in muscle atrophy and musculoskeletal diseases

Maintaining muscle mass is clinically important as muscle helps to regulate metabolic systems of the body as well as support activities of daily living that require mobility, strength, and power. Losing muscle mass decreases an individual's independence and quality of life, and at the same time increases the risk of disease burden. Fibro-adipogenic progenitor (FAP) cells are a group of muscle progenitor cells that play an important role in muscle regeneration and maintenance of skeletal muscle fiber size. These important functions of FAPs are mediated by a complex secretome that interacts in a paracrine manner to stimulate muscle satellite cells to divide and differentiate. Dysregulation of FAP differentiation leads to fibrosis, fatty infiltration, muscle atrophy, and impaired muscle regeneration. Functional deficits in skeletal muscle resulting from atrophy, fibrosis, or fatty infiltration will reduce biomechanical stresses on the skeleton, and both FAP-derived adipocytes and FAPs themselves are likely to secrete factors that can induce bone loss. These findings suggest that FAPs represent a cell population to be targeted therapeutically to improve both muscle and bone health in settings of aging, injury, and disease.

Stromal DLK1 promotes proliferation and inhibits differentiation of the intestinal epithelium during development

The stem/progenitor cells of the developing intestine are biologically distinct from their adult counterparts. Here, we examine the microenvironmental cues that regulate the embryonic stem/progenitor population, focusing on the role of Notch pathway factor delta-like protein-1 (DLK1). mRNA-seq analyses of intestinal mesenchymal cells (IMCs) collected from embryonic day 14.5 (E14.5) or adult IMCs and a novel coculture system with E14.5 intestinal epithelial organoids were used. Following addition of recombinant DLK1 (rDLK) or Dlk1 siRNA (siDlk1), epithelial characteristics were compared using imaging, replating efficiency assays, qPCR, and immunocytochemistry. The intestinal phenotypes of littermate Dlk1+/+ and Dlk1-/- mice were compared using immunohistochemistry. Using transcriptomic analyses, we identified morphogens derived from the embryonic mesenchyme that potentially regulate the developing epithelial cells, to focus on Notch family candidate DLK1. Immunohistochemistry indicated that DLK1 was expressed exclusively in the intestinal stroma at E14.5 at the top of emerging villi, decreased after birth, and shifted to the intestinal epithelium in adulthood. In coculture experiments, addition of rDLK1 to adult IMCs inhibited organoid differentiation, whereas Dlk1 knockdown in embryonic IMCs increased epithelial differentiation to secretory lineage cells. Dlk1-/- mice had restricted Ki67+ cells in the villi base and increased secretory lineage cells compared with Dlk1+/+ embryos. Mesenchyme-derived DLK1 plays an important role in the promotion of epithelial stem/precursor expansion and prevention of differentiation to secretory lineages in the developing intestine.NEW & NOTEWORTHY Using a novel coculture system, transcriptomics, and transgenic mice, we investigated differential molecular signaling between the intestinal epithelium and mesenchyme during development and in the adult. We show that the Notch pathway factor delta-like protein-1 (DLK1) is stromally produced during development and uncover a new role for DLK1 in the regulation of intestinal epithelial stem/precursor expansion and differentiation to secretory lineages.

Dlk1 regulates quiescence in calcitonin receptor-mutant muscle stem cells

Muscle stem cells, also called muscle satellite cells (MuSCs), are responsible for skeletal muscle regeneration and are sustained in an undifferentiated and quiescent state under steady conditions. The calcitonin receptor (CalcR)-protein kinase A (PKA)-Yes-associated protein 1 (Yap1) axis is one pathway that maintains quiescence in MuSCs. Although CalcR signaling in MuSCs has been identified, the critical CalcR signaling targets are incompletely understood. Here, we show the relevance between the ectopic expression of delta-like non-canonical Notch ligand 1 (Dlk1) and the impaired quiescent state in CalcR-conditional knockout (cKO) MuSCs. Dlk1 expression was rarely detected in both quiescent and proliferating MuSCs in control mice, whereas Dlk1 expression was remarkably increased in CalcR-cKO MuSCs at both the mRNA and protein levels. It is noteworthy that all Ki67+ non-quiescent CalcR-cKO MuSCs express Dlk1, and non-quiescent CalcR-cKO MuSCs are enriched in the Dlk1+ fraction by cell sorting. Using mutant mice, we demonstrated that PKA-activation or Yap1-depletion suppressed Dlk1 expression in CalcR-cKO MuSCs, which suggests that the CalcR-PKA-Yap1 axis inhibits the expression of Dlk1 in quiescent MuSCs. Moreover, the loss of Dlk1 rescued the quiescent state in CalcR-cKO MuSCs, which indicates that the ectopic expression of Dlk1 disturbs quiescence in CalcR-cKO. Collectively, our results suggest that ectopically expressed Dlk1 is responsible for the impaired quiescence in CalcR-cKO MuSCs.

A reference single-cell transcriptomic atlas of human skeletal muscle tissue reveals bifurcated muscle stem cell populations

Single-cell RNA-sequencing (scRNA-seq) facilitates the unbiased reconstruction of multicellular tissue systems in health and disease. Here, we present a curated scRNA-seq dataset of human muscle samples from 10 adult donors with diverse anatomical locations. We integrated ~ 22,000 single-cell transcriptomes using Scanorama to account for technical and biological variation and resolved 16 distinct populations of muscle-resident cells using unsupervised clustering of the data compendium. These cell populations included muscle stem/progenitor cells (MuSCs), which bifurcated into discrete "quiescent" and "early-activated" MuSC subpopulations. Differential expression analysis identified transcriptional profiles altered in the activated MuSCs including genes associated with aging, obesity, diabetes, and impaired muscle regeneration, as well as long non-coding RNAs previously undescribed in human myogenic cells. Further, we modeled ligand-receptor cell-communication interactions and observed enrichment of the TWEAK-FN14 pathway in activated MuSCs, a characteristic signature of muscle wasting diseases. In contrast, the quiescent MuSCs have enhanced expression of the EGFR receptor, a recognized human MuSC marker. This work provides a new benchmark reference resource to examine human muscle tissue heterogeneity and identify potential targets in MuSC diversity and dysregulation in disease contexts.

Analysis of DNA methylation profiles during sheep skeletal muscle development using whole-genome bisulfite sequencing

BACKGROUND

DNA methylation is an epigenetic regulatory form that plays an important role in regulating the gene expression and the tissues development.. However, DNA methylation regulators involved in sheep muscle development remain unclear. To explore the functional importance of genome-scale DNA methylation during sheep muscle growth, this study systematically investigated the genome-wide DNA methylation profiles at key stages of Hu sheep developmental (fetus and adult) using deep whole-genome bisulfite sequencing (WGBS).

RESULTS

Our study found that the expression levels of DNA methyltransferase (DNMT)-related genes were lower in fetal muscle than in the muscle of adults. The methylation levels in the CG context were higher than those in the CHG and CHH contexts, and methylation levels were highest in introns, followed by exons and downstream regions. Subsequently, we identified 48,491, 17, and 135 differentially methylated regions (DMRs) in the CG, CHG, and CHH sequence contexts and 11,522 differentially methylated genes (DMGs). The results of bisulfite sequencing PCR (BSP) correlated well with the WGBS-Seq data. Moreover, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) functional annotation analysis revealed that some DMGs were involved in regulating skeletal muscle development and fatty acid metabolism. By combining the WGBS-Seq and previous RNA-Seq data, a total of 159 overlap genes were obtained between differentially expressed genes (DEGs) and DMGs (FPKM > 10 and fold change > 4). Finally, we found that 9 DMGs were likely to be involved in muscle growth and metabolism of Hu sheep.

CONCLUSIONS

We systemically studied the global DNA methylation patterns of fetal and adult muscle development in Hu sheep, which provided new insights into a better understanding of the epigenetic regulation of sheep muscle development.

Myogenic Progenitor Cell Lineage Specification by CRISPR/Cas9-Based Transcriptional Activators

Engineered CRISPR/Cas9-based transcriptional activators can potently and specifically activate endogenous fate-determining genes to direct differentiation of pluripotent stem cells. Here, we demonstrate that endogenous activation of the PAX7 transcription factor results in stable epigenetic remodeling and differentiates human pluripotent stem cells into skeletal myogenic progenitor cells. Compared with exogenous overexpression of PAX7 cDNA, we find that endogenous activation results in the generation of more proliferative myogenic progenitors that can maintain PAX7 expression over multiple passages in serum-free conditions while preserving the capacity for terminal myogenic differentiation. Transplantation of human myogenic progenitors derived from endogenous activation of PAX7 into immunodeficient mice resulted in a greater number of human dystrophin⁺ myofibers compared with exogenous PAX7 overexpression. RNA-sequencing analysis also revealed transcriptome-wide differences between myogenic progenitors generated via CRISPR-based endogenous activation of PAX7 and exogenous PAX7 cDNA overexpression. These studies demonstrate the utility of CRISPR/Cas9-based transcriptional activators for controlling cell-fate decisions.

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CRISPR activation generates myogenic progenitors from human iPSCs/ESCs

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Activation of endogenous PAX7 leads to autonomously maintained gene expression

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Endogenous activation outperforms cDNA expression in generating a stable phenotype

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CRISPRa myogenic progenitors engraft and regenerate muscle fibers in vivo in mice

Differential gene responses 3 days following infarction in the fetal and adolescent sheep heart

There are critical molecular mechanisms that can be activated to induce myocardial repair, and in humans this is most efficient during fetal development. The timing of heart development in relation to birth and the size/electrophysiology of the heart are similar in humans and sheep, providing a model to investigate the repair capacity of the mammalian heart and how this can be applied to adult heart repair. Myocardial infarction was induced by ligation of the left anterior descending coronary artery in fetal (105 days gestation when cardiomyocytes are proliferative) and adolescent sheep (6 mo of age when all cardiomyocytes have switched to an adult phenotype). An ovine gene microarray was used to compare gene expression in sham and infarcted (remote, border and infarct areas) cardiac tissue from fetal and adolescent hearts. The gene response to myocardial infarction was less pronounced in fetal compared with adolescent sheep hearts and there were unique gene responses at each age. There were also region-specific changes in gene expression between each age, in the infarct tissue, tissue bordering the infarct, and tissue remote from the infarction. In total, there were 880 genes that responded to MI uniquely in the adolescent samples compared with 170 genes in the fetal response, as well as 742 overlap genes that showed concordant direction of change responses to infarction at both ages. In response to myocardial infarction, there were specific changes in genes within pathways of mitochondrial oxidation, muscle contraction, and hematopoietic cell lineages, suggesting that the control of energy utilization and immune function are critical for effective heart repair. The more restricted gene response in the fetus may be an important factor in its enhanced capacity for cardiac repair.

Expression and Functional Analyses of Dlk1 in Muscle Stem Cells and Mesenchymal Progenitors during Muscle Regeneration

Delta like non-canonical Notch ligand 1 (Dlk1) is a paternally expressed gene which is also known as preadipocyte factor 1 (Pref-1). The accumulation of adipocytes and expression of Dlk1 in regenerating muscle suggests a correlation between fat accumulation and Dlk1 expression in the muscle. Additionally, mice overexpressing Dlk1 show increased muscle weight, while Dlk1-null mice exhibit decreased body weight and muscle mass, indicating that Dlk1 is a critical factor in regulating skeletal muscle mass during development. The muscle regeneration process shares some features with muscle development. However, the role of Dlk1 in regeneration processes remains controversial. Here, we show that mesenchymal progenitors also known as adipocyte progenitors exclusively express Dlk1 during muscle regeneration. Eliminating developmental effects, we used conditional depletion models to examine the specific roles of Dlk1 in muscle stem cells or mesenchymal progenitors. Unexpectedly, deletion of Dlk1 in neither the muscle stem cells nor the mesenchymal progenitors affected the regenerative ability of skeletal muscle. In addition, fat accumulation was not increased by the loss of Dlk1. Collectively, Dlk1 plays essential roles in muscle development, but does not greatly impact regeneration processes and adipogenic differentiation in adult skeletal muscle regeneration.

The HDAC3 enzymatic activity regulates skeletal muscle fuel metabolism

Histone deacetylase 3 (HDAC3) is a major HDAC, whose enzymatic activity is targeted by small molecule inhibitors for treating a variety of conditions. However, its enzymatic activity is largely dispensable for its function in embryonic development and hepatic lipid metabolism. HDAC3 plays a pivotal role in regulating muscle fuel metabolism and contractile function. Here, we address whether these muscular functions of HDAC3 require its enzymatic activity. By mutating the NCoR/SMRT corepressors in a knock-in mouse model named NS-DADm, we ablated the enzymatic activity of HDAC3 without affecting its protein levels. Compared to the control mice, skeletal muscles from NS-DADm mice showed lower force generation, enhanced fatigue resistance, enhanced fatty acid oxidation, reduced glucose uptake during exercise, upregulated expression of metabolic genes involved in branched-chain amino acids catabolism, and reduced muscle mass during aging, without changes in the muscle fiber-type composition or mitochondrial protein content. These muscular phenotypes are similar to those observed in the HDAC3-depleted skeletal muscles, which demonstrates that, unlike that in the liver or embryonic development, the metabolic function of HDAC3 in skeletal muscles requires its enzymatic activity. These results suggest that drugs specifically targeting HDAC3 enzyme activity could be developed and tested to modulate muscle energy metabolism and exercise performance.

MicroRNA and Long Non-coding RNA Regulation in Skeletal Muscle From Growth to Old Age Shows Striking Dysregulation of the Callipyge Locus

MicroRNAs (miRNAs) undergo high levels of regulation in skeletal muscle development and control skeletal muscle mass, function and metabolism over the lifespan. More recently, the role of long non-coding RNAs (lncRNAs) in skeletal muscle regulation has started to emerge. Following up on our recent study describing the expression pattern and putative roles of 768 miRNAs in the quadriceps muscle of mice at early life stages, we used a high-throughput miRNA qPCR-based array to assess the expression of the same miRNAs in 28-month old male mouse quadriceps muscle. In addition, we report the expression patterns of lncRNAs playing a putative role in muscle development and adaptation from growth to old age. Twelve miRNAs were significantly downregulated in 28-month old muscle when compared with 12-week old muscle. Ten of them clustered at the Dlk1-Dio3 locus, known as ‘Callipyge,’ which is associated with muscle development and hypertrophy. This collective downregulation was paralleled by decreases in the expression levels of the maternally expressed imprinted LncRNA coding genes Meg3 and Rian stemming from the same chromosomal region. In contrast, the paternally expressed imprinted Dlk1-Dio3 locus members Rtl1, Dio3, and Dlk1 and the muscle related lncRNAs lncMyoD1, Neat_v1, Neat_v2, and Malat1 underwent significant changes during growth, but their expression levels were not altered past the age of 12 weeks, suggesting roles limited to hyperplasia and early hypertrophy. In conclusion, collective muscle miRNA expression gradually decreases over the lifespan and a cluster of miRNAs and maternally expressed lncRNAs stemming from the Callipyge locus is significantly dysregulated in aging muscle. The Dlk1-Dio3 locus therefore represents a potential new mechanism for age-related muscle decline.

Proteome and transcriptome profiling of equine myofibrillar myopathy identifies diminished peroxiredoxin 6 and altered cysteine metabolic pathways

Equine myofibrillar myopathy (MFM) causes exertional muscle pain and is characterized by myofibrillar disarray and ectopic desmin aggregates of unknown origin. To investigate the pathophysiology of MFM, we compared resting and 3 h postexercise transcriptomes of gluteal muscle and the resting skeletal muscle proteome of MFM and control Arabian horses with RNA sequencing and isobaric tags for relative and absolute quantitation analyses. Three hours after exercise, 191 genes were identified as differentially expressed (DE) in MFM vs. control muscle with >1 log₂ fold change (FC) in genes involved in sulfur compound/cysteine metabolism such as cystathionine-beta-synthase ( CBS, ↓4.51), a cysteine and neutral amino acid membrane transporter ( SLC7A10, ↓1.80 MFM), and a cationic transporter (SLC24A1, ↓1.11 MFM). In MFM vs. control at rest, 284 genes were DE with >1 log₂ FC in pathways for structure morphogenesis, fiber organization, tissue development, and cell differentiation including > 1 log₂ FC in cardiac alpha actin ( ACTC1 ↑2.5 MFM), cytoskeletal desmoplakin ( DSP ↑2.4 MFM), and basement membrane usherin ( USH2A ↓2.9 MFM). Proteome analysis revealed significantly lower antioxidant peroxiredoxin 6 content (PRDX6, ↓4.14 log₂ FC MFM), higher fatty acid transport enzyme carnitine palmitoyl transferase (CPT1B, ↑3.49 MFM), and lower sarcomere protein tropomyosin (TPM2, ↓3.24 MFM) in MFM vs. control muscle at rest. We propose that in MFM horses, altered cysteine metabolism and a deficiency of cysteine-containing antioxidants combined with a high capacity to oxidize fatty acids and generate ROS during aerobic exercise causes chronic oxidation and aggregation of key proteins such as desmin.

The imprinted gene Pw1/Peg3 regulates skeletal muscle growth, satellite cell metabolic state, and self-renewal

Pw1/Peg3 is an imprinted gene expressed from the paternally inherited allele. Several imprinted genes, including Pw1/Peg3, have been shown to regulate overall body size and play a role in adult stem cells. Pw1/Peg3 is expressed in muscle stem cells (satellite cells) as well as a progenitor subset of muscle interstitial cells (PICs) in adult skeletal muscle. We therefore examined the impact of loss-of-function of Pw1/Peg3 during skeletal muscle growth and in muscle stem cell behavior. We found that constitutive loss of Pw1/Peg3 function leads to a reduced muscle mass and myofiber number. In newborn mice, the reduction in fiber number is increased in homozygous mutants as compared to the deletion of only the paternal Pw1/Peg3 allele, indicating that the maternal allele is developmentally functional. Constitutive and a satellite cell-specific deletion of Pw1/Peg3, revealed impaired muscle regeneration and a reduced capacity of satellite cells for self-renewal. RNA sequencing analyses revealed a deregulation of genes that control mitochondrial function. Consistent with these observations, Pw1/Peg3 mutant satellite cells displayed increased mitochondrial activity coupled with accelerated proliferation and differentiation. Our data show that Pw1/Peg3 regulates muscle fiber number determination during fetal development in a gene-dosage manner and regulates satellite cell metabolism in the adult.

A new approach of gene co-expression network inference reveals significant biological processes involved in porcine muscle development in late gestation

The integration of genetic information in the cellular and nuclear environments is crucial for deciphering the way in which the genome functions under different physiological conditions. Experimental techniques of 3D nuclear mapping, a high-flow approach such as transcriptomic data analyses, and statistical methods for the development of co-expressed gene networks, can be combined to develop an integrated approach for depicting the regulation of gene expression. Our work focused more specifically on the mechanisms involved in the transcriptional regulation of genes expressed in muscle during late foetal development in pig. The data generated by a transcriptomic analysis carried out on muscle of foetuses from two extreme genetic lines for birth mortality are used to construct networks of differentially expressed and co-regulated genes. We developed an innovative co-expression networking approach coupling, by means of an iterative process, a new statistical method for graph inference with data of gene spatial co-localization (3D DNA FISH) to construct a robust network grouping co-expressed genes. This enabled us to highlight relevant biological processes related to foetal muscle maturity and to discover unexpected gene associations between IGF2, MYH3 and DLK1/MEG3 in the nuclear space, genes that are up-regulated at this stage of muscle development.

The testicular transcriptome associated with spermatogonia differentiation initiated by gonadotrophin stimulation in the juvenile rhesus monkey (Macaca mulatta)

STUDY QUESTION

What is the genetic landscape within the testis of the juvenile rhesus monkey (Macaca mulatta) that underlies the decision of undifferentiated spermatogonia to commit to a pathway of differentiation when puberty is induced prematurely by exogenous LH and FSH stimulation?

SUMMARY ANSWER

Forty-eight hours of gonadotrophin stimulation of the juvenile monkey testis resulted in the appearance of differentiating B spermatogonia and the emergence of 1362 up-regulated and 225 down-regulated testicular mRNAs encoding a complex network of proteins ranging from enzymes regulating Leydig cell steroidogenesis to membrane receptors, and from juxtacrine and paracrine factors to transcriptional factors governing spermatogonial stem cell fate.

WHAT IS KNOWN ALREADY

Our understanding of the cell and molecular biology underlying the fate of undifferentiated spermatogonia is based largely on studies of rodents, particularly of mice, but in the case of primates very little is known. The present study represents the first attempt to comprehensively address this question in a highly evolved primate.

STUDY DESIGN, SIZE, DURATION

Global gene expression in the testis from juvenile rhesus monkeys that had been stimulated with recombinant monkey LH and FSH for 48 h (N = 3) or 96 h (N = 4) was compared to that from vehicle treated animals (N = 3). Testicular cell types and testosterone secretion were also monitored.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Precocious testicular puberty was initiated in juvenile rhesus monkeys, 14-24 months of age, using a physiologic mode of intermittent stimulation with i.v. recombinant monkey LH and FSH that within 48 h produced 'adult' levels of circulating LH, FSH and testosterone. Mitotic activity was monitored by immunohistochemical assays of 5-bromo-2'-deoxyuridine and 5-ethynyl-2'-deoxyuridine incorporation. Animals were bilaterally castrated and RNA was extracted from the right testis. Global gene expression was determined using RNA-Seq. Differentially expressed genes (DEGs) were identified and evaluated by pathway analysis. mRNAs of particular interest were also quantitated using quantitative RT-PCR. Fractions of the left testis were used for histochemistry or immunoflouresence.

MAIN RESULTS AND THE ROLE OF CHANCE

Differentiating type B spematogonia were observed after both 48 and 96 h of gonadotrophin stimulation. Pathway analysis identified five super categories of over-represented DEGs. Repression of GFRA1 (glial cell line-derived neurotrophic factor family receptor alpha 1) and NANOS2 (nanos C2HC-type zinc finger 2) that favor spermatogonial stem cell renewal was noted after 48 and 96 h of LH and FSH stimulation. Additionally, changes in expression of numerous genes involved in regulating the Notch pathway, cell adhesion, structural plasticity and modulating the immune system were observed. Induction of genes associated with the differentiation of spermatogonia stem cells (SOHLH1(spermatogenesis- and oogenesis-specific basic helix-loop-helix 1), SOHLH2 and KIT (V-Kit Hardy-Zuckerman 4 feline sarcoma viral oncogene homolog)) was not observed. Expression of the gene encoding STRA8 (stimulated by retinoic acid 8), a protein generally considered to mark activation of retinoic acid signaling, was below our limit of detection.

LARGE SCALE DATA

The entire mRNA data set for vehicle and gonadotrophin treated animals (N = 10) has been deposited in the GEO-NCBI repository (GSE97786).

LIMITATIONS REASONS FOR CAUTION

The limited number of monkeys per group and the dilution of low abundance germ cell transcripts by mRNAs contributed from somatic cells likely resulted in an underestimation of the number of differentially expressed germ cell genes.

WIDER IMPLICATIONS OF THE FINDINGS

The findings that expression of GDNF (a major promoter of spermatogonial stem cell renewal) was not detected in the control juvenile testes, expression of SOHLH1, SOHLH2 and KIT, promoters of spermatogonial differentiation in mice, were not up-regulated in association with the gonadotrophin-induced generation of differentiating spermatogonia, and that robust activation of the retinoic acid signaling pathway was not observed, could not have been predicted. These unexpected results underline the importance of non-human primate models in translating data derived from animal research to the human situation.

STUDY FUNDING/COMPETING INTEREST(S)

The work described was funded by NIH grant R01 HD072189 to T.M.P. P.A. was supported by an Endocrine Society Summer Research Fellowship Award and CONICET (Argentine Research Council), S.N. by a grant from Vali-e-Asr Reproductive Health Research Center of Tehran University of Medical Sciences (grant #24335-39-92) to Dr Batool Hosseini Rashidi, and M.P.H. by grants from the National Health and Medical Research Council of Australia, and the Victorian State Government's Operational Infrastructure Support Program. The authors have nothing to disclose.

Identification of genes directly responding to DLK1 signaling in Callipyge sheep

BACKGROUND

In food animal agriculture, there is a need to identify the mechanisms that can improve the efficiency of muscle growth and protein accretion. Callipyge sheep provide excellent machinery since the up-regulation of DLK1 and RTL1 results in extreme postnatal muscle hypertrophy in distinct muscles. The aim of this study is to distinguish the genes that directly respond to DLK1 and RTL1 signaling from the genes that change as the result of muscle specific effects.

RESULTS

The quantitative PCR results indicated that DLK1 expression was significantly increased in hypertrophied muscles but not in non-hypertrophied muscles. However, RTL1 was up-regulated in both hypertrophied and non-hypertrophied muscles. Five genes, including PARK7, DNTTIP1, SLC22A3, METTL21E and PDE4D, were consistently co-expressed with DLK1, and therefore were possible transcriptional target genes responding to DLK1 signaling. Treatment of myoblast and myotubes with DLK1 protein induced an average of 1.6-fold and 1.4-fold increase in Dnttip1 and Pde4d expression respectively. Myh4 expression was significantly elevated in DLK1-treated myotubes, whereas the expression of Mettl21e was significantly increased in the DLK1-treated myoblasts but reduced in DLK1-treated myotubes. DLK1 treatment had no impact on Park7 expression. In addition, Park7 and Dnttip1 increased Myh4 and decreased Myh7 promoter activity, resemble to the effects of Dlk1. In contrast, expression of Mettl21e increased Myh7 and decreased Myh4 luciferase activity.

CONCLUSION

The study provided additional supports that RTL1 alone was insufficient to induce muscle hypertrophy and concluded that DLK1 was likely the primary effector of the hypertrophy phenotype. The results also suggested that DNTTIP1 and PDE4D were secondary effector genes responding to DLK1 signaling resulting in muscle fiber switch and muscular hypertrophy in callipyge lamb.

Small-RNA sequencing identifies dynamic microRNA deregulation during skeletal muscle lineage progression

Skeletal muscle satellite cells are quiescent adult resident stem cells that activate, proliferate and differentiate to generate myofibres following injury. They harbour a robust proliferation potential and self-renewing capacity enabling lifelong muscle regeneration. Although several classes of microRNAs were shown to regulate adult myogenesis, systematic examination of stage-specific microRNAs during lineage progression from the quiescent state is lacking. Here we provide a genome-wide assessment of the expression of small RNAs during the quiescence/activation transition and differentiation by RNA-sequencing. We show that the majority of small RNAs present in quiescent, activated and differentiated muscle cells belong to the microRNA class. Furthermore, by comparing expression in distinct cell states, we report a massive and dynamic regulation of microRNAs, both in numbers and amplitude, highlighting their pivotal role in regulation of quiescence, activation and differentiation. We also identify a number of microRNAs with reliable and specific expression in quiescence including several maternally-expressed miRNAs generated at the imprinted Dlk1-Dio3 locus. Unexpectedly, the majority of class-switching miRNAs are associated with the quiescence/activation transition suggesting a poised program that is actively repressed. These data constitute a key resource for functional analyses of miRNAs in skeletal myogenesis, and more broadly, in the regulation of stem cell self-renewal and tissue homeostasis.

Myostatin-deficiency in mice increases global gene expression at the Dlk1-Dio3 locus in the skeletal muscle

Myostatin, a member of the transforming growth factor-beta superfamily, is a negative regulator of skeletal muscle growth and development. Myostatin inhibition leads to increased skeletal muscle mass in mammals; hence, myostatin is considered a potential therapeutic target for skeletal muscle wasting. However, downstream molecules of myostatin in the skeletal muscle have not been fully elucidated. Here, we identified the Dlk1-Dio3 locus at the mouse chromosome 12qF1, also called as the callipyge locus in sheep, as a novel downstream target of myostatin. In skeletal muscle of myostatin knockout mice, the expression of mature miRNAs at the Dlk1-Dio3 locus was significantly increased. The increased miRNA levels are caused by the transcriptional activation of the Dlk1-Dio3 locus, because a significant increase in the primary miRNA transcript was observed in myostatin knockout mice. In addition, we found increased expression of coding and non-coding genes (Dlk1, Gtl2, Rtl1/Rtl1as, and Rian) at the Dlk1-Dio3 locus in myostatin-deficient skeletal muscle. Moreover, epigenetic changes, associated with the regulation of the Dlk1-Dio3 locus, were observed in myostatin knockout mice. Taken together, this is the first report demonstrating the role of myostatin in regulating the Dlk1-Dio3 (the callipyge) locus in the skeletal muscle.

Efficient differentiation of human pluripotent stem cells into skeletal muscle cells by combining RNA-based MYOD1-expression and POU5F1-silencing

Direct generation of skeletal muscle cells from human pluripotent stem cells (hPSCs) would be beneficial for drug testing, drug discovery, and disease modelling in vitro. Here we show a rapid and robust method to induce myogenic differentiation of hPSCs by introducing mRNA encoding MYOD1 together with siRNA-mediated knockdown of POU5F1 (also known as OCT4 or OCT3/4). This integration-free approach generates functional skeletal myotubes with sarcomere-like structure and a fusion capacity in several days. The POU5F1 silencing facilitates MYOD1 recruitment to the target promoters, which results in the significant activation of myogenic genes in hPSCs. Furthermore, deep sequencing transcriptome analyses demonstrated that POU5F1-knockdown upregulates the genes associated with IGF- and FGF-signaling and extracellular matrix that may also support myogenic differentiation. This rapid and direct differentiation method may have potential applications in regenerative medicine and disease therapeutics for muscle disorders such as muscular dystrophy.

Consequences of MEGF10 deficiency on myoblast function and Notch1 interactions

Mutations in MEGF10 cause early onset myopathy, areflexia, respiratory distress, and dysphagia (EMARDD), a rare congenital muscle disease, but the pathogenic mechanisms remain largely unknown. We demonstrate that short hairpin RNA (shRNA)-mediated knockdown of Megf10, as well as overexpression of the pathogenic human p.C774R mutation, leads to impaired proliferation and migration of C2C12 cells. Myoblasts from Megf10-/- mice and Megf10-/-/mdx double knockout (dko) mice also show impaired proliferation and migration compared to myoblasts from wild type and mdx mice, whereas the dko mice show histological abnormalities that are not observed in either single mutant mouse. Cell proliferation and migration are known to be regulated by the Notch receptor, which plays an essential role in myogenesis. Reciprocal co-immunoprecipitation studies show that Megf10 and Notch1 interact via their respective intracellular domains. These interactions are impaired by the pathogenic p.C774R mutation. Megf10 regulation of myoblast function appears to be mediated at least in part via interactions with key components of the Notch signaling pathway, and defects in these interactions may contribute to the pathogenesis of EMARDD.

DLK1-DIO3 imprinted locus deregulation in development, respiratory disease, and cancer

INTRODUCTION

The imprinted DLK1-DIO3 locus at 14q32.1-32.31 holds biological significance in fetal development, whereby imprinting errors are causal to developmental disorders. Emerging evidence has implicated this locus in other diseases including cancer, highlighting the biological parallels between fetal organ and tumour development. Areas covered: Controlled regulation of gene expression from the imprinted DLK1-DIO3 locus at 14q32.1-32.31 is crucial for proper fetal development. Deregulation of locus gene expression due to imprinting errors has been mechanistically linked to the developmental disorders Kagami-Ogata Syndrome and Temple Syndrome. In adult tissues, deregulation of locus genes has been associated with multiple malignancies although the causal genetic mechanisms remain largely uncharacterised. Here, we summarize the genetic mechanisms underlying the developmental disorders that arise as a result of improper locus imprinting and the resulting developmental phenotypes, emphasizing both the coding and noncoding components of the locus. We further highlight biological parallels common to both fetal development and disease, with a specific focus on lung development, respiratory disease, and lung cancer. Expert commentary: Many commonalities between respiratory and developmental defects have emerged with respect to the 14q32 locus, emphasizing the importance of studying the effects of imprinting on gene regulation patterns at this locus in both biological settings.

Effects of transforming growth factor-β1 treatment on muscle regeneration and adipogenesis in glycerol-injured muscle

Transforming growth factor (TGF)-β1 is associated with fibrosis in many organs. Recent studies demonstrated that delivery of TGF-β1 into chemically injured muscle enhances fibrosis. In this study, we investigated the effects of exogenous TGF-β1 on muscle regeneration and adipogenesis in glycerol-injured muscle of normal mice. Tibialis anterior (TA) muscles were injured by glycerol injection. TGF-β1 was either co-injected with glycerol, as an 'early treatment' group, or injected at day 4 after glycerol, as a 'late treatment' group and the TA muscles were collected at day 7 after initial injury. Myotube density was significantly lower in the early treatment group than in the glycerol-injured group (without TGF-β1 treatment). Moreover, the Oil red O-positive area was significantly smaller in the early treatment group than in the late treatment group and glycerol-injured group. Furthermore, TGF-β1 treatment increased endomysial fibrosis and induced immunostaining of α-smooth muscle actin. The greater inhibitory effects of early TGF-β1 treatment than that of late TGF-β1 treatment during regeneration in glycerol-injured muscle suggest a more potent effect of TGF-β1 on the initial stage of muscle regeneration and adipogenesis. Combination of TGF-β1 with glycerol might be an alternative to enhance muscle fibrosis for future studies.

Small RNAome profiling from human skeletal muscle: novel miRNAs and their targets associated with cancer cachexia

BACKGROUND

MicroRNAs (miRs) are small non-coding RNAs that regulate gene (mRNA) expression. Although the pathological role of miRs have been studied in muscle wasting conditions such as myotonic and muscular dystrophy, their roles in cancer cachexia (CC) are still emerging.

OBJECTIVES

The objectives are (i) to profile human skeletal muscle expressed miRs; (ii) to identify differentially expressed (DE) miRs between cachectic and non-cachectic cancer patients; (iii) to identify mRNA targets for the DE miRs to gain mechanistic insights; and (iv) to investigate if miRs show potential prognostic and predictive value.

METHODS

Study subjects were classified based on the international consensus diagnostic criteria for CC. Forty-two cancer patients were included, of which 22 were cachectic cases and 20 were non-cachectic cancer controls. Total RNA isolated from muscle biopsies were subjected to next-generation sequencing.

RESULTS

A total of 777 miRs were profiled, and 82 miRs with read counts of ≥5 in 80% of samples were retained for analysis. We identified eight DE miRs (up-regulated, fold change of ≥1.4 at P < 0.05). A total of 191 potential mRNA targets were identified for the DE miRs using previously described human skeletal muscle mRNA expression data (n = 90), and a majority of them were also confirmed in an independent mRNA transcriptome dataset. Ingenuity pathway analysis identified pathways related to myogenesis and inflammation. qRT-PCR analysis of representative miRs showed similar direction of effect (P < 0.05), as observed in next-generation sequencing. The identified miRs also showed prognostic and predictive value.

CONCLUSIONS

In all, we identified eight novel miRs associated with CC.

Temple syndrome: comprehensive molecular and clinical findings in 32 Japanese patients

Purpose

Temple syndrome (TS14) is a rare imprinting disorder caused by aberrations at the 14q32.2 imprinted region. Here, we report comprehensive molecular and clinical findings in 32 Japanese patients with TS14.

Methods

We performed molecular studies for TS14 in 356 patients with variable phenotypes, and clinical studies in all TS14 patients, including 13 previously reported.

Results

We identified 19 new patients with TS14, and the total of 32 patients was made up of 23 patients with maternal uniparental disomy (UPD(14)mat), six patients with epimutations, and three patients with microdeletions. Clinical studies revealed both Prader-Willi syndrome (PWS)-like marked hypotonia and Silver-Russell syndrome (SRS)-like phenotype in 50% of patients, PWS-like hypotonia alone in 20% of patients, SRS-like phenotype alone in 20% of patients, and nonsyndromic growth failure in the remaining 10% of patients in infancy, and gonadotropin-dependent precocious puberty in 76% of patients who were pubescent or older.

Conclusion

These results suggest that TS14 is not only a genetically diagnosed entity but also a clinically recognizable disorder. Genetic testing for TS14 should be considered in patients with growth failure plus both PWS-like hypotonia and SRS-like phenotypes in infancy, and/or precocious puberty, as well as a familial history of Kagami-Ogata syndrome due to maternal microdeletion at 14q32.2.

Data supporting possible implication of APOBEC2 in self-renewal functions of myogenic stem satellite cells: Toward understanding the negative regulation of myoblast differentiation

This paper provides in vitro phenotypical data to show that APOBEC2, a member of apoB mRNA editing enzyme, catalytic polypeptide-like family, may implicate in self-renewal functions of myogenic stem satellite cells, namely in the re-establishment of quiescent status after activation and proliferation of myoblasts in single-myofiber culture.

Deletion of the Ste20-like kinase SLK in skeletal muscle results in a progressive myopathy and muscle weakness

Background

The Ste20-like kinase, SLK, plays an important role in cell proliferation and cytoskeletal remodeling. In fibroblasts, SLK has been shown to respond to FAK/Src signaling and regulate focal adhesion turnover through Paxillin phosphorylation. Full-length SLK has also been shown to be essential for embryonic development. In myoblasts, the overexpression of a dominant negative SLK is sufficient to block myoblast fusion.

Methods

In this study, we crossed the Myf5-Cre mouse model with our conditional SLK knockout model to delete SLK in skeletal muscle. A thorough analysis of skeletal muscle tissue was undertaken in order to identify defects in muscle development caused by the lack of SLK. Isometric force analysis was performed on adult knockout mice and compared to age-matched wild-type mice. Furthermore, cardiotoxin injections were performed followed by immunohistochemistry for myogenic markers to assess the efficiency muscle regeneration following SLK deletion.

Results

We show here that early deletion of SLK from the myogenic lineage does not markedly impair skeletal muscle development but delays the regenerative process. Interestingly, adult mice (~6 months) display an increase in the proportion of central nuclei and increased p38 activation. Furthermore, mice as young as 3 months old present with decreased force generation, suggesting that the loss of SLK impairs myofiber stability and function. Assessment of structural components revealed aberrant localization of focal adhesion proteins, such as FAK and paxillin. Our data show that the loss of SLK results in unstable myofibers resulting in a progressive myopathy. Additionally, the loss of SLK resulted in a delay in muscle regeneration following cardiotoxin injections.

Conclusions

Our results show that SLK is dispensable for muscle development and regeneration but is required for myofiber stability and optimal force generation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-016-0119-1) contains supplementary material, which is available to authorized users.

Integrated Analysis of Long Non-coding RNAs (LncRNAs) and mRNA Expression Profiles Reveals the Potential Role of LncRNAs in Skeletal Muscle Development of the Chicken

Long non-coding RNAs (lncRNAs) play important roles in transcriptional and post-transcriptional regulation. However, little is currently known about the mechanisms by which they regulate skeletal muscle development in the chicken. In this study, we used RNA sequencing to profile the leg muscle transcriptome (lncRNA and mRNA) at three stages of skeletal muscle development in the chicken: embryonic day 11 (E11), embryonic day 16 (E16), and 1 day after hatching (D1). In total, 129, 132, and 45 differentially expressed lncRNAs, and 1798, 3072, and 1211 differentially expressed mRNAs were identified in comparisons of E11 vs. E16, E11 vs. D1, and E16 vs. D1, respectively. Moreover, we identified the cis- and trans-regulatory target genes of differentially expressed lncRNAs, and constructed lncRNA-gene interaction networks. In total, 126 and 200 cis-targets, and two and three trans-targets were involved in lncRNA-gene interaction networks that were constructed based on the E11 vs. E16, and E11 vs. D1 comparisons, respectively. The comparison of the E16 vs. D1 lncRNA-gene network comprised 25 cis-targets. We determined that lncRNA target genes are potentially involved in cellular development, and cellular growth and proliferation using Ingenuity Pathway Analysis. The gene networks identified for the E11 vs. D1 comparison were involved in embryonic development, organismal development and tissue development. The present study provides an RNA sequencing based evaluation of lncRNA function during skeletal muscle development in the chicken. Comprehensive analysis facilitated the identification of lncRNAs and target genes that might contribute to the regulation of different stages of skeletal muscle development.

Human soluble delta-like 1 homolog exerts antitumor effects in vitro and in vivo

Proteolysis of delta-like 1 homolog (DLK1), a cell-surface transmembrane protein, produces an active soluble form of DLK1 (sDLK1). Both membrane-bound DLK1 and sDLK1 modulate multiple developmental processes including adipogenesis, osteogenesis, chondrogenesis and myogenesis. However, cancer-related functions of DLK1 have not yet been established. We thus evaluated the roles of extracellular sDLK1, comprising six EGF-like domains and juxtamembrane regions, in human pancreatic cancer MIA PaCa-2 cells in vitro and in vivo. We observed that sDLK1 exerted antitumor effects not only in cancer cell migration and anchorage-independent cell growth but also in in vivo tumor growth.

Notch Signaling Mediates Skeletal Muscle Atrophy in Cancer Cachexia Caused by Osteosarcoma

Skeletal muscle atrophy in cancer cachexia is mediated by the interaction between muscle stem cells and various tumor factors. Although Notch signaling has been known as a key regulator of both cancer development and muscle stem cell activity, the potential involvement of Notch signaling in cancer cachexia and concomitant muscle atrophy has yet to be elucidated. The murine K7M2 osteosarcoma cell line was used to generate an orthotopic model of sarcoma-associated cachexia, and the role of Notch signaling was evaluated. Skeletal muscle atrophy was observed in the sarcoma-bearing mice, and Notch signaling was highly active in both tumor tissues and the atrophic skeletal muscles. Systemic inhibition of Notch signaling reduced muscle atrophy. In vitro coculture of osteosarcoma cells with muscle-derived stem cells (MDSCs) isolated from normal mice resulted in decreased myogenic potential of MDSCs, while the application of Notch inhibitor was able to rescue this repressed myogenic potential. We further observed that Notch-activating factors reside in the exosomes of osteosarcoma cells, which activate Notch signaling in MDSCs and subsequently repress myogenesis. Our results revealed that signaling between tumor and muscle via the Notch pathway may play an important role in mediating the skeletal muscle atrophy seen in cancer cachexia.

Delta-like 1 homolog in Capra hircus: molecular characteristics, expression pattern and phylogeny

To research the molecular characteristics, expression pattern and phylogeny of the Delta-like 1 homolog gene (Dlk1) in goats. Dlk1 transcripts were identified in the Jianyang Da'er goats by reverse-transcription polymerase chain reaction (RT-PCR). Phylogenetic trees were constructed by Bayesian inference and neighbor-joining methods. Quantitative real-time PCR (qPCR), western blotting and in situ hybridization were performed to analyze the expression pattern of Dlk1. Five alternatively transcripts were identified in different tissues and designated as Dlk1-AS1, 2, 3, 4 and 5. Compared with the normal transcript Dlk1-AS1, Dlk1-AS4 and Dlk1-AS5 retained the identical open reading frame (ORF) and encoded proteins with truncated epidermal-growth-factor like repeats of 121 and 83 amino acids, respectively. Using the Bayesian inference method, the consensus phylogenetic tree indicated that caprine Dlk1 had a closer relationship with bovine Dlk1 than with Dlk1 from pigs, humans and mice. qPCR revealed high expression levels of Dlk1 in the kidney (P < 0.01). However, mRNA and protein levels presented an inconsistent correlation, possibly because of post-transcriptional regulation. RNA in situ hybridization indicated that Dlk1 mRNA was localized in the interlobular bile duct and alongside the hepatocyte nuclei, in the epithelial cells of proximal and distal convoluted tubules and in the connective region between the mesothelium and myocardium in the heart. The Dlk1 gene in goats produces alternatively spliced transcripts, with specific expression and cellular localization patterns. These findings would lay the foundation for further study.

KLF7 Regulates Satellite Cell Quiescence in Response to Extracellular Signaling

Retaining muscle stem satellite cell (SC) quiescence is important for the maintenance of stem cell population and tissue regeneration. Accumulating evidence supports the model where key extracellular signals play crucial roles in maintaining SC quiescence or activation, however, the intracellular mechanisms that mediate niche signals to control SC behavior are not fully understood. Here, we reported that KLF7 functioned as a key mediator involved in low-level TGF-β signaling and canonical Notch signaling-induced SC quiescence and myoblast arrest. The data obtained showed that KLF7 was upregulated in quiescent SCs and nonproliferating myoblasts. Silence of KLF7 promoted SCs activation and myoblasts proliferation, but overexpression of KLF7 induced myogenic cell arrest. Notably, the expression of KLF7 was regulated by TGF-β and Notch3 signaling. Knockdown of KLF7 diminished low-level TGF-β and canonical Notch signaling-induced SC quiescence. Investigation into the mechanism revealed that KLF7 regulation of SC function was dependent on p21 and acetylation of Lys227 and/or 231 in the DNA binding domain of KLF7. Our study provides new insights into the regulatory network of muscle stem cell quiescence. Stem Cells 2016;34:1310-1320.

H19 controls reactivation of the imprinted gene network during muscle regeneration

The H19 locus controls fetal growth by regulating expression of several genes from the imprinted gene network (IGN). H19 is fully repressed after birth, except in skeletal muscle. Using loss-of-function H19Δ3 mice, we investigated the function of H19 in adult muscle. Mutant muscles display hypertrophy and hyperplasia, with increased Igf2 and decreased myostatin (Mstn) expression. Many imprinted genes are expressed in muscle stem cells or satellite cells. Unexpectedly, the number of satellite cells was reduced by 50% in H19Δ3 muscle fibers. This reduction occurred after postnatal day 21, suggesting a link with their entry into quiescence. We investigated the biological function of these mutant satellite cells in vivo using a regeneration assay induced by multiple injections of cardiotoxin. Surprisingly, despite their reduced number, the self-renewal capacity of these cells is fully retained in the absence of H19. In addition, we observed a better regeneration potential of the mutant muscles, with enhanced expression of several IGN genes and genes from the IGF pathway.