Characterization of DLK1+ cells emerging during skeletal muscle remodeling in response to myositis, myopathies, and acute injury

Network-based modelling reveals cell-type enriched patterns of non-coding RNA regulation during human skeletal muscle remodelling

A majority of human genes produce non-protein-coding RNA (ncRNA), and some have roles in development and disease. Neither ncRNA nor human skeletal muscle is ideally studied using short-read sequencing, so we used a customised RNA pipeline and network modelling to study cell-type specific ncRNA responses during muscle growth at scale. We completed five human resistance-training studies (n=144 subjects), identifying 61% who successfully accrued muscle-mass. We produced 288 transcriptome-wide profiles and found 110 ncRNAs linked to muscle growth in vivo, while a transcriptome-driven network model demonstrated interactions via a number of discrete functional pathways and single-cell types. This analysis included established hypertrophy-related ncRNAs, including CYTOR - which was leukocyte-associated (FDR = 4.9 ×10-7). Novel hypertrophy-linked ncRNAs included PPP1CB-DT (myofibril assembly genes, FDR = 8.15 × 10-8), and EEF1A1P24 and TMSB4XP8 (vascular remodelling and angiogenesis genes, FDR = 2.77 × 10-5). We also discovered that hypertrophy lncRNA MYREM shows a specific myonuclear expression pattern in vivo. Our multi-layered analyses established that single-cell-associated ncRNA are identifiable from bulk muscle transcriptomic data and that hypertrophy-linked ncRNA genes mediate their association with muscle growth via multiple cell types and a set of interacting pathways.

Large scale serum proteomics identifies proteins associated with performance decline and clinical milestones in Duchenne muscular dystrophy

Serum biomarkers are promising minimally invasive outcome measures in clinical studies in Duchenne muscular dystrophy (DMD). However, biomarkers strongly associated with clinical progression and predicting performance decline are lacking. In this study we aimed to identify serum biomarkers associated with clinical performance and able to predict clinical milestones in DMD. Towards this aim we present a retrospective multi-center cohort study including serum samples and clinical data collected in research participants with DMD as part of a natural history study at the University of Florida (UF) and real-world observations at Leiden University Medical Center (LUMC) between 2009-2022. The 7K SomaScan® assay was used to analyse protein levels in in individual serum samples. Serum biomarkers predicted age at loss of ambulation (LoA), age at loss of overhead reach (OHR) and age at loss of hand to mouth function (HTM). Secondary outcomes were the association of biomarkers with age, corticosteroid (CS) usage, and clinical performance based on the North Star Ambulatory Assessment (NSAA), 10 meter run velocity (10mrv), 6 minute walk (6MWT) and Performance of the Upper Limb (PUL2.0). A total of 716 serum samples were collected in 79 participants at UF and 74 at LUMC (mean[SD] age; 10.9[3.2] vs 8.4[3.4]). 244 serum proteins showed an association with CS usage in both cohorts independent of CS type and regimen, including MMP3 and IGLL1. 318 probes (corresponding to 294 proteins) showed significant associations with NSAA, 10mrv, 6MWT and/or PUL2.0 across both cohorts. The expression of 38 probes corresponding to 36 proteins such as RGMA, EHMT2, ART3, ANTXR2 and DLK1 was associated with risk of both lower and upper limb clinical milestones in both the LUMC and UF cohort. In conclusion, multiple biomarkers were associated with CS use, motor function and upper lower and upper limb disease milestones in DMD. These biomarkers were validated across two independent cohorts, increasing their likelihood of translation for use within the broader DMD population.

Pericardial delta like non-canonical NOTCH ligand 1 (Dlk1) augments fibrosis in the heart through epithelial to mesenchymal transition

BACKGROUND

Heart failure due to myocardial infarction (MI) involves fibrosis driven by epicardium-derived cells (EPDCs) and cardiac fibroblasts, but strategies to inhibit and provide cardio-protection remains poor. The imprinted gene, non-canonical NOTCH ligand 1 (Dlk1), has previously been shown to mediate fibrosis in the skin, lung and liver, but very little is known on its effect in the heart.

METHODS

Herein, human pericardial fluid/plasma and tissue biopsies were assessed for DLK1, whereas the spatiotemporal expression of Dlk1 was determined in mouse hearts. The Dlk1 heart phenotype in normal and MI hearts was assessed in transgenic mice either lacking or overexpressing Dlk1. Finally, in/ex vivo cell studies provided knowledge on the molecular mechanism.

RESULTS

Dlk1 was demonstrated in non-myocytes of the developing human myocardium but exhibited a restricted pericardial expression in adulthood. Soluble DLK1 was twofold higher in pericardial fluid (median 45.7 [34.7 (IQR)) μg/L] from cardiovascular patients (n = 127) than in plasma (median 26.1 μg/L [11.1 (IQR)]. The spatial and temporal expression pattern of Dlk1 was recapitulated in mouse and rat hearts. Similar to humans lacking Dlk1, adult Dlk1-/- mice exhibited a relatively mild developmental, although consistent cardiac phenotype with some abnormalities in heart size, shape, thorax orientation and non-myocyte number, but were functionally normal. However, after MI, scar size was substantially reduced in Dlk1-/- hearts as compared with Dlk1+/+ littermates. In line, high levels of Dlk1 in transgenic mice Dlk1fl/fl xWT1GFPCre and Dlk1fl/fl xαMHCCre/+Tam increased scar size following MI. Further mechanistic and cellular insight demonstrated that pericardial Dlk1 mediates cardiac fibrosis through epithelial to mesenchymal transition (EMT) of the EPDC lineage by maintaining Integrin β8 (Itgb8), a major activator of transforming growth factor β and EMT.

CONCLUSIONS

Our results suggest that pericardial Dlk1 embraces a, so far, unnoticed role in the heart augmenting cardiac fibrosis through EMT. Monitoring DLK1 levels as well as targeting pericardial DLK1 may thus offer new venues for cardio-protection.

Identification and Quantification of Proliferating Cells in Skeletal Muscle of Glutamine Supplemented Low- and Normal-Birth-Weight Piglets

One way to improve the growth of low-birth-weight (LBW) piglets can be stimulation of the cellular development of muscle by optimized amino acid supply. In the current study, it was investigated how glutamine (Gln) supplementation affects muscle tissue of LBW and normal-birth-weight (NBW) piglets. Longissimus and semitendinosus muscles of 96 male piglets, which were supplemented with 1 g Gln/kg body weight or alanine, were collected at slaughter on day 5 or 26 post natum (dpn), one hour after injection with Bromodeoxyuridine (BrdU, 12 mg/kg). Immunohistochemistry was applied to detect proliferating, BrdU-positive cells in muscle cross-sections. Serial stainings with cell type specific antibodies enabled detection and subsequent quantification of proliferating satellite cells and identification of further proliferating cell types, e.g., preadipocytes and immune cells. The results indicated that satellite cells and macrophages comprise the largest fractions of proliferating cells in skeletal muscle of piglets early after birth. The Gln supplementation somewhat stimulated satellite cells. We observed differences between the two muscles, but no influence of the piglets' birth weight was observed. Thus, Gln supplements may not be considered as effective treatment in piglets with low birth weight for improvement of muscle growth.

Lung Cancer Stage Prediction Using Multi-Omics Data

Lung cancer is one of the leading causes of cancer death. Patients with early-stage lung cancer can be treated by surgery, while patients in the middle and late stages need chemotherapy or radiotherapy. Therefore, accurate staging of lung cancer is crucial for doctors to formulate accurate treatment plans for patients. In this paper, the random forest algorithm is used as the lung cancer stage prediction model, and the accuracy of lung cancer stage prediction is discussed in the microbiome, transcriptome, microbe, and transcriptome fusion groups, and the accuracy of the model is measured by indicators such as ACC, recall, and precision. The results showed that the prediction accuracy of microbial combinatorial transcriptome fusion analysis was the highest, reaching 0.809. The study reveals the role of multimodal data and fusion algorithm in accurately diagnosing lung cancer stage, which could aid doctors in clinics.

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.

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.

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.

Intrinsic High Aerobic Capacity in Male Rats Protects Against Diet-Induced Insulin Resistance

Low aerobic capacity increases the risk for insulin resistance but the mechanisms are unknown. In this study, we tested susceptibility to acute (3-day) high-fat, high-sucrose diet (HFD)-induced insulin resistance in male rats selectively bred for divergent intrinsic aerobic capacity, that is, high-capacity running (HCR) and low-capacity running (LCR) rats. We employed hyperinsulinemic-euglycemic clamps, tracers, and transcriptome sequencing of skeletal muscle to test whether divergence in aerobic capacity impacted insulin resistance through systemic and tissue-specific metabolic adaptations. An HFD evoked decreased insulin sensitivity and insulin signaling in muscle and liver in LCR rats, whereas HCR rats were protected. An HFD led to increased glucose transport in skeletal muscle (twofold) of HCR rats while increasing glucose transport into adipose depots of the LCR rats (twofold). Skeletal muscle transcriptome revealed robust differences in the gene profile of HCR vs LCR on low-fat diet and HFD conditions, including robust differences in specific genes involved in lipid metabolism, adipogenesis, and differentiation. HCR transcriptional adaptations to an acute HFD were more robust than for LCR and included genes driving mitochondrial energy metabolism. In conclusion, intrinsic aerobic capacity robustly impacts systemic and skeletal muscle adaptations to HFD-induced alterations in insulin resistance, an effect that is likely driven by baseline differences in oxidative capacity, gene expression profile, and transcriptional adaptations to an HFD.

LncRNA-MEG3 promotes bovine myoblast differentiation by sponging miR-135

Long noncoding RNA maternally expressed gene 3 (lncRNA-MEG3) is an important regulator in multiple biological functions. However, lncRNA-MEG3's function in cattle growth and regulatory mechanism on bovine skeletal muscle development has not yet been well studied. In this project, we first investigated lncRNA-MEG3's expression profile and detected that it was highly expressed in bovine skeletal muscle tissue and its RNA level was kept increasingly during the early phase of bovine primary myoblast differentiation. Using luciferase reporter assays, we identified the lncRNA-MEG3 core promoter containing putative transcription factor binding site for myocyte enhancer factor 2C (MEF2C). Interestingly, we found that LncRNA-MEG3 could significantly upregulate and downregulate myosin heavy chain ( MHC), myogenin ( MyoG), and MEF2C through overexpression and RNAi strategies, respectively. Using luciferase reporter assays, we also verified lncRNA-MEG3 as a miR-135 sponge. Overexpression of miR-135 markedly inhibited the wild type of lncRNA-MEG3, but not the mutant lncRNA-MEG3 reporter. The luciferase activity of miR-135 sensor could be rescued by lncRNA-MEG3 via competing for miRNA-135. In addition, the luciferase activity of MEF2C was significantly upregulated by the wild type of lncRNA-MEG3. This study, for the first time, revealed that lncRNA-MEG3 could promote bovine skeletal muscle differentiation via interacting with miRNA-135 and MEF2C. The results were valuable for further studies and applications of lncRNA related roles in beef molecular breeding.

Role of protein delta homolog 1 in the proliferation and differentiation of ameloblasts

Protein delta homolog 1 (DLK1) regulates the odontoblastic differentiation of human dental pulp stem cells. It was hypothesized that DLK1 may exert regulatory effects on epithelial‑mesenchymal interactions in tooth development. The present study investigated the expression of DLK1 during the development of mouse enamel and its role in the proliferation and differentiation of ameloblast‑lineage cells (ALCs). DLK1 expression was upregulated in ameloblasts in the first mandibular molar during the entire process of enamel development. The mRNA and protein levels of DLK1 were significantly upregulated following ameloblastic induction in ALCs. In addition, overexpression of DLK1 promoted the proliferation of ALCs, inhibited ameloblastic differentiation, upregulated the expression of amelogenin and enamelin, and downregulated the expression of odontogenic ameloblast‑associated protein and kallikrein 4. The results of the present study suggested that DLK1 may be a potent regulator of ameloblast proliferation and differentiation, and may regulate enamel formation during tooth development.

Alteration in methylation level at differential methylated regions of MEST and DLK1 in fetus of preeclampsia

OBJECTIVES

Offspring born to preeclamptic women are at high risk for metabolic diseases in later life, but the mechanisms are not known. The purposes of the current investigation were to clarify the changes in DNA methylation at MEST and DLK1 DMRs in fetus of preeclampsia and to explore the possible mechanisms behind the high risk of adult diseases in the offspring of preeclampsia.

METHODS

Fetal lymphocytes were isolated from umbilical cord blood of 78 women with preeclampsia and 95 women with normal pregnancy. Genomic DNA was extracted and then DNA methylation levels of MEST and DLK1 DMRs were determined by MassARRAY quantitative methylation analysis.

RESULTS

The methylation levels were detected in 20 CpG sites of MEST DMR and 16 sites of DLK1 DMR. Methylation changes were significantly different at CPG1, 3, 4, 7.8, 15, 18.19, and 20 of MEST between preeclampsia and normal pregnancy (P = 0.014, 0.001, <0.001, <0.001, = 0.001,  = 0.005, and = 0.003, respectively). Significant differences were also observed at CPG 3 and 9 of DLK1 (P = 0.002 and 0.027, respectively). However, overall methylation at these DMRs were not affected.

CONCLUSION

We conclude methylation changes at some CpG sites of MEST and DLK DMRs in preeclamptic group. This may be among the mechanisms behind the high risk of adult diseases in the later life of offspring born to preeclamptic pregnancies.

ABBREVIATIONS

DMR: Differentially Methylated Region; MEST: Mesoderm Specific Transcript.

The effect of delta-like 1 homologue on the proliferation and odontoblastic differentiation in human dental pulp stem cells

INTRODUCTION

This study aimed to investigate the functions of delta-like homologue 1 (DLK1) in the proliferation and differentiation of human dental pulp stem cells (hDPSCs).

METHODS

Immunohistochemical analysis was used to determine the expression of alkaline phosphatase (ALP), dentin sialophosphoprotein (DSPP), DLK1, NOTCH1 and p-ERK1/2 in the mouse first maxillary molar. Recombinant lentivirus was constructed to overexpress DLK1 stably in hDPSCs. The cell viability and proliferation of hDPSCs were examined by CCK8 and EdU incorporation assay respectively. The odontoblastic differentiation of hDPSCs was determined by detection of ALPase activity assay, ALP and alizarin red staining and the expression of mineralization-related genes including ALP, DSPP and dental matrix protein. The mRNA and protein levels of DLK1 and p-ERK1/2 protein expression were detected. ERK inhibitor was used to test the differentiation effect of DLK1 on hDPSCs.

RESULTS

Delta-like homologue 1 was highly expressed on the odontoblasts and dental pulp cells on the first maxillary molar; the expression of p-ERK1/2 is similar with the DLK1 in the same process. The expression level of DLK1 increased significantly after the odontoblastic induction of hDPSCs. DLK1 overexpression increased the proliferation ability of hDPSCs and inhibited odontoblastic differentiation of hDPSCs. The protein level of p-ERK1/2 significantly increased in hDPSCs/dlk1-oe group. ERK signalling pathway inhibitor reversed the odontoblastic differentiation effects of DLK1 on hDPSCs.

CONCLUSIONS

The proliferation of hDPSCs was promoted after DLK1 overexpression. DLK1 inhibited the odontoblastic differentiation of hDPSCs, which maybe through ERK signalling pathway.

Neonatal epicardial-derived progenitors aquire myogenic traits in skeletal muscle, but not cardiac muscle

BACKGROUND/OBJECTIVES

Epicardium-derived progenitor cells (EPDCs) differentiate into all heart cell types in the embryonic heart, yet their differentiation into cardiomyocytes in the adult heart is limited and poorly described. This may be due to EPDCs lacking myogenic potential or the inert adult heart missing regenerative signals essential for directed differentiation of EPDCs. Herein, we aimed to evaluate the myogenic potential of neonatal EPDCs in adult and neonatal mouse myocardium, as well as in skeletal muscle. The two latter tissues have an intrinsic capability to develop and regenerate, in contrast to the adult heart.

METHODS

Highly purified mouse EPDCs were transplanted into damaged neonatal and adult myocardium as well as regenerating skeletal muscle. Co-cultures with skeletal myoblasts were used to distinguish fusion independent myogenic conversion.

RESULTS

No donor EPDC-derived cardiomyocytes were observed in hearts. In contrast, a remarkable contribution of EPDCs to skeletal muscle myofiber formation was evident in vivo. Furthermore, co-cultures of EPDCs with myoblasts showed that EPDCs became part of multinucleated fibers and appeared to acquire myogenic traits independent of a fusion event. Fluorescence activated cell sorting of EPDCs co-cultured with and without myoblasts and subsequent qRT-PCR of 64 transcripts established that the myogenic phenotype conversion was accomplished through induction of a transcriptional myogenic program.

CONCLUSION

These results suggest that EPDCs may be more myogenic than previously anticipated. But, the heart may lack factors for induction of myogenesis of EPDCs, a scenario that should be taken into consideration when aiming for repair of damaged myocardium by stem cell transplantation.

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.

Early myogenic responses to acute exercise before and after resistance training in young men

To enable dynamic regulation of muscle mass and myofiber repair following injury, a satellite cell precursor population exists to supply additional nuclei. Activated satellite cells express many genes and associated proteins necessary for maturation and incorporation into the damaged fiber. There is little knowledge about the response of these markers following whole-body resistance exercise training. We investigated the impact of 12 weeks of progressive whole-body resistance training on the expression of MRFs, PAX7, NCAM, and FA1, incorporating both acute and chronic resistance exercise components. Ten young recreationally active males (21.2 ± 3.5 years) performed 12 weeks of whole-body resistance training at 70-85% of their predetermined one-repetition maximum (1RM). At the initiation and completion of the training period, muscular strength was assessed by RM and dynamometer testing, and vastus lateralis samples were obtained prior to and 3 h following an acute resistance exercise test (both whole-body and isometric exercises). Increased mRNA expression of PAX7 (threefold), NCAM (threefold), MYF5 (threefold), MYOD (threefold) and MYOGENIN (twofold) was observed 3 h after the acute resistance exercise test, both pre and posttraining. Similarly, PAX7 (11-fold) and FA1 (twofold) protein abundance increased after acute exercise, while resting NCAM (eightfold) and FA1 (threefold) protein abundance increased following 12 weeks of resistance training. It is possible that these molecular changes are primarily due to the preceding exercise bout, and are not modified by long-term or whole-body exercise training.

Characterization of DLK1(PREF1)+/CD34+ cells in vascular stroma of human white adipose tissue

Sorting of native (unpermeabilized) SVF-cells from human subcutaneous (s)WAT for cell surface staining (cs) of DLK1 and CD34 identified three main populations: ~10% stained cs-DLK1+/cs-CD34-, ~20% cs-DLK1+/cs-CD34+dim and ~45% cs-DLK1-/cs-CD34+. FACS analysis after permeabilization showed that all these cells stained positive for intracellular DLK1, while CD34 was undetectable in cs-DLK1+/cs-CD34- cells. Permeabilized cs-DLK1-/cs-CD34+ cells were positive for the pericyte marker α-SMA and the mesenchymal markers CD90 and CD105, albeit CD105 staining was dim (cs-DLK1-/cs-CD34+/CD90+/CD105+dim/α-SMA+/CD45-/CD31-). Only these cells showed proliferative and adipogenic capacity. Cs-DLK1+/cs-CD34- and cs-DLK1+/cs-CD34+dim cells were also α-SMA+ but expressed CD31, had a mixed hematopoietic and mesenchymal phenotype, and could neither proliferate nor differentiate into adipocytes. Histological analysis of sWAT detected DLK1+/CD34+ and DLK1+/CD90+ cells mainly in the outer ring of vessel-associated stroma and at capillaries. DLK1+/α-SMA+ cells were localized in the CD34- perivascular ring and in adventitial vascular stroma. All these DLK1+ cells possess a spindle-shaped morphology with extremely long processes. DLK1+/CD34+ cells were also detected in vessel endothelium. Additionally, we show that sWAT contains significantly more DLK1+ cells than visceral (v)WAT. We conclude that sWAT has more DKL1+ cells than vWAT and contains different DLK1/CD34 populations, and only cs-DLK1-/cs-CD34+/CD90+/CD105+dim/α-SMA+/CD45-/CD31- cells in the adventitial vascular stroma exhibit proliferative and adipogenic capacity.

DLK1-DIO3 imprinted cluster in induced pluripotency: landscape in the mist

DLK1-DIO3 represents an imprinted cluster which genes are involved in physiological cell biology as early as the stem cell level and in the pathogenesis of several diseases. Transcription factor-mediated induced pluripotent cells (iPSCs) are considered an unlimited source of patient-specific hematopoietic stem cells for clinical application in patient-tailored regenerative medicine. However, to date there is no marker established able to distinguish embryonic stem cell-equivalent iPSCs or safe human iPSCs. Recent findings suggest that the DLK1-DIO3 locus possesses the potential to represent such a marker but there are also contradictory data. This review aims to report the current data on the topic describing both sides of the coin.

DLK1/PREF1 regulates nutrient metabolism and protects from steatosis

Nonalcoholic fatty liver disease (NAFLD) is associated with insulin resistance and obesity, as well as progressive liver dysfunction. Recent animal studies have underscored the importance of hepatic growth hormone (GH) signaling in the development of NAFLD. The imprinted Delta-like homolog 1 (Dlk1)/preadipocyte factor 1 (Pref1) gene encodes a complex protein producing both circulating and membrane-tethered isoforms whose expression dosage is functionally important because even modest elevation during embryogenesis causes lethality. DLK1 is up-regulated during embryogenesis, during suckling, and in the mother during pregnancy. We investigated the normal role for elevated DLK1 dosage by overexpressing Dlk1 from endogenous control elements. This increased DLK1 dosage caused improved glucose tolerance with no primary defect in adipose tissue expansion even under extreme metabolic stress. Rather, Dlk1 overexpression caused reduced fat stores, pituitary insulin-like growth factor 1 (IGF1) resistance, and a defect in feedback regulation of GH. Increased circulatory GH culminated in a switch in whole body fuel metabolism and a reduction in hepatic steatosis. We propose that the function of DLK1 is to shift the metabolic mode of the organism toward peripheral lipid oxidation and away from lipid storage, thus mediating important physiological adaptations associated with early life and with implications for metabolic disease resistance.

Delta-like 1 homolog (DLK1) inhibits proliferation and myotube formation of avian QM7 myoblasts

Delta-like 1 homolog (DLK1) has been implicated as an important regulator in mammalian muscle development. Our previous studies showed that different alternative splicing isoforms have distinct functions in the regulation of myogenesis in mice. Unlike most mammals, including mice, pigs, cattle, and sheep, DLK1 mRNA for avian species has a single form without alternative splicing. In the current study, we have used QM7 cells, a quail myoblast, to study the role of DLK1 in the regulation of avian myogenesis. Overexpression of DLK1 inhibited myogenesis with a lower fusion rate and thinner myotube compared to the control QM7 cells. Comparison of relative levels of protein and mRNA showed down-regulation of PAX7, MYOG, and MHC, and up-regulation of MYOD by DLK1, suggesting that quail DLK1 inhibits myogenesis at later stages of myogenic differentiation and myotube formation. DLK1 reduced the QM7 cell growth rate which is accompanied by a lower percentage of bromodeoxyuridine positive cells, indicating an inhibitory role of DLK1 in proliferation. During the early post-hatch ages, the relatively slower increase in the amount of total DNA mass in breast muscle of the heavy weight quail line, that has been selected for over 40 generations, could be partially explained by the higher expression of DLK1 compared to the control quail. Taken together, DLK1 inhibits myogenic differentiation and proliferation by regulating the expression levels of myogenic factors in quail. In addition, the regulation of expression level and cleavage of full-length DLK1 may be important factors for regulating myogenesis in quail having no splicing variants of DLK1.

New insights into polar overdominance in callipyge sheep

The callipyge phenotype in sheep involves substantial postnatal muscle hypertrophy and other changes to carcass composition. A single nucleotide polymorphism in the DLK1-DIO3 imprinted gene cluster alters gene expression of the paternal allele-specific protein-coding genes and several maternal allele-specific long noncoding RNA and microRNA when the mutation is inherited in cis. The inheritance pattern of the callipyge phenotype is polar overdominant because muscle hypertrophy only occurs in heterozygous animals that inherit a normal maternal allele and the callipyge SNP on the paternal allele (+/C). We examined the changes of gene expression of four major transcripts from the DLK1-DIO3 cluster and four myosin isoforms during the development of muscle hypertrophy in the semimembranosus as well as in the supraspinatus that does not undergo hypertrophy. The homozygous (C/C) animals had an intermediate gene expression pattern for the paternal allele-specific genes and two myosin isoforms, indicating a biological activity that was insufficient to change muscle mass. Transcriptome analysis was conducted by RNA sequencing in the four callipyge genotypes. The data show that homozygous animals (C/C) have lower levels of gene expression at many loci relative to the other three genotypes. A number of the downregulated genes are putative targets of the maternal allele-specific microRNA with gene ontology, indicating regulatory and cell signaling functions. These results suggest that the trans-effect of the maternal noncoding RNA and associated miRNA is to stabilize the expression of a number of regulatory genes at a functional, but low level to make the myofibers of homozygous (C/C) lambs less responsive to hypertrophic stimuli of the paternal allele-specific genes.

Do neonatal mouse hearts regenerate following heart apex resection?

The mammalian heart has generally been considered nonregenerative, but recent progress suggests that neonatal mouse hearts have a genuine capacity to regenerate following apex resection (AR). However, in this study, we performed AR or sham surgery on 400 neonatal mice from inbred and outbred strains and found no evidence of complete regeneration. Ideally, new functional cardiomyocytes, endothelial cells, and vascular smooth muscle cells should be formed in the necrotic area of the damaged heart. Here, damaged hearts were 9.8% shorter and weighed 14% less than sham controls. In addition, the resection border contained a massive fibrotic scar mainly composed of nonmyocytes and collagen disposition. Furthermore, there was a substantial reduction in the number of proliferating cardiomyocytes in AR hearts. Our results thus question the usefulness of the AR model for identifying molecular mechanisms underlying regeneration of the adult heart after damage.

Membrane-bound delta-like 1 homolog (Dlk1) promotes while soluble Dlk1 inhibits myogenesis in C2C12 cells

Delta-like 1 homolog (Dlk1) is important in myogenesis. However, the roles of different Dlk1 isoforms have not been investigated. In C2C12 cell lines producing different Dlk1 isoforms, membrane-bound Dlk1 promoted the hypertrophic phenotype and a higher fusion rate, whereas soluble Dlk1 inhibited myotube formation. Inversed expression patterns of genes related to myogenic differentiation further support these phenotypic changes. In addition, temporal expression and balance between the Dlk1 isoforms have a regulatory role in myogenesis in vivo. Collectively, Dlk1 isoforms have distinctive effects on myogenesis, and its regulation during myogenesis is critical for normal muscle development.

Dual role of delta-like 1 homolog (DLK1) in skeletal muscle development and adult muscle regeneration

Muscle development and regeneration is tightly orchestrated by a specific set of myogenic transcription factors. However, factors that regulate these essential myogenic inducers remain poorly described. Here, we show that delta-like 1 homolog (Dlk1), an imprinted gene best known for its ability to inhibit adipogenesis, is a crucial regulator of the myogenic program in skeletal muscle. Dlk1-/- mice were developmentally retarded in their muscle mass and function owing to inhibition of the myogenic program during embryogenesis. Surprisingly however, Dlk1 depletion improves in vitro and in vivo adult skeletal muscle regeneration by substantial enhancement of the myogenic program and muscle function, possibly by means of an increased number of available myogenic precursor cells. By contrast, Dlk1 fails to alter the adipogenic commitment of muscle-derived progenitors in vitro, as well as intramuscular fat deposition during in vivo regeneration. Collectively, our results suggest a novel and surprising dual biological function of DLK1 as an enhancer of muscle development, but as an inhibitor of adult muscle regeneration.

Horse serum reduces expression of membrane-bound and soluble isoforms of the preadipocyte marker Delta-like 1 homolog (Dlk1), but is inefficient for adipogenic differentiation of mouse preadipocytes

Downregulation of the preadipocyte marker Delta-like 1 homologue (Dlk1), an inhibitor of adipogenesis, has been suggested to be a prerequisite for adipogenic differentiation to occur, and low Dlk1 levels are often used to verify adipogenesis. Mouse preadipocytic cell lines such as 3T3-L1, as well as primary derived preadipocytes, are important models to study adipogenic differentiation and obesity. However, in vitro adipogenic differentiation of primary derived preadipocytes remains incomplete, and identification of factors that will improve the adipogenic differentiation process is thus of high value. In this study we show that horse serum fails to improve adipogenic differentiation of mouse preadipocytes (both 3T3-L1 cells and primary derived mouse preadipocytes) as otherwise reported for bone marrow derived adipogenic precursors. Unexpectedly, while Dlk1 levels were indeed decreased using horse serum, this did not correlate with a high degree of adipogenic differentiation. In conclusion, our novel results thus reveal that horse serum clearly is insufficient for adipogenic differentiation of mouse preadipocytes and that low levels of Dlk1 alone are a poor marker of mouse in vitro adipogenesis. We would also like to emphasize that it is very important for the field of cellular differentiation that researchers thoroughly investigate the effect of individual reagents in their protocols. Such data will increase understanding of the limitations and possibilities of individual systems.

Delta-like 1 homolog (dlk1): a marker for rhabdomyosarcomas implicated in skeletal muscle regeneration

Dlk1, a member of the Epidermal Growth Factor family, is expressed in multiple tissues during development, and has been detected in carcinomas and neuroendocrine tumors. Dlk1 is paternally expressed and belongs to a group of imprinted genes associated with rhabdomyosarcomas but not with other primitive childhood tumors to date. Here, we investigate the possible roles of Dlk1 in skeletal muscle tumor formation. We analyzed tumors of different mesenchymal origin for expression of Dlk1 and various myogenic markers and found that Dlk1 was present consistently in myogenic tumors. The coincident observation of Dlk1 with a highly proliferative state in myogenic tumors led us to subsequently investigate the involvement of Dlk1 in the control of the adult myogenic programme. We performed an injury study in Dlk1 transgenic mice, ectopically expressing ovine Dlk1 (membrane bound C2 variant) under control of the myosin light chain promotor, and detected an early, enhanced formation of myotubes in Dlk1 transgenic mice. We then stably transfected the mouse myoblast cell line, C2C12, with full-length Dlk1 (soluble A variant) and detected an inhibition of myotube formation, which could be reversed by adding Dlk1 antibody to the culture supernatant. These results suggest that Dlk1 is involved in controlling the myogenic programme and that the various splice forms may exert different effects. Interestingly, both in the Dlk1 transgenic mice and the DLK1-C2C12 cells, we detected reduced myostatin expression, suggesting that the effect of Dlk1 on the myogenic programme might involve the myostatin signaling pathway. In support of a relationship between Dlk1 and myostatin we detected reciprocal expression of these two transcripts during different cell cycle stages of human myoblasts. Together our results suggest that Dlk1 is a candidate marker for skeletal muscle tumors and might be involved directly in skeletal muscle tumor formation through a modulatory effect on the myogenic programme.

Membrane-tethered delta-like 1 homolog (DLK1) restricts adipose tissue size by inhibiting preadipocyte proliferation

Adipocyte renewal from preadipocytes has been shown to occur throughout life and to contribute to obesity, yet very little is known about the molecular circuits that control preadipocyte expansion. The soluble form of the preadipocyte factor (also known as pref-1) delta-like 1 homolog (DLK1(S)) is known to inhibit adipogenic differentiation; however, the impact of DLK1 isoforms on preadipocyte proliferation remains to be determined. We generated preadipocytes with different levels of DLK1 and examined differentially affected gene pathways, which were functionally tested in vitro and confirmed in vivo. Here, we demonstrate for the first time that only membrane-bound DLK1 (DLK1(M)) exhibits a substantial repression effect on preadipocyte proliferation. Thus, by independently manipulating DLK1 isoform levels, we established that DLK1(M) inhibits G1-to-S-phase cell cycle progression and thereby strongly inhibits preadipocyte proliferation in vitro. Adult DLK1-null mice exhibit higher fat amounts than wild-type controls, and our in vivo analysis demonstrates that this may be explained by a marked increase in preadipocyte replication. Together, these data imply a major dual inhibitory function of DLK1 on adipogenesis, which places DLK1 as a master regulator of preadipocyte homeostasis, suggesting that DLK1 manipulation may open new avenues in obesity treatment.

Conditional deletions refine the embryonic requirement for Dlk1

Numerous studies have implicated Delta-like 1 (DLK1), a transmembrane protein that shares homology with Notch ligands, in embryonic growth and differentiation. Dlk1 expression is widespread, though not ubiquitous, during early development, but is confined to a few specific cell types in adults. Adult Dlk1-expressing tissues include the Insulin-producing β-cells of the pancreas and the Growth hormone-producing somatotrophs of the pituitary gland. Previously generated Dlk1 null mice (Dlk1(Sul-pat)), display a partially penetrant neonatal lethality and a complex pattern of developmental and adult phenotypes. Here we describe the generation of a conditional Dlk1 mouse line (Dlk1(flox)) to facilitate cell type-specific deletion of the Dlk1 gene, providing a powerful system to explore each aspect of the Dlk1 null phenotype. Four tissue-specific Cre mouse lines were used to produce individual Dlk1 deletions in pancreatic β-cells, pituitary somatotrophs and the endothelial cells of the embryo and placenta, key candidates for the Dlk1 phenotype. Contrary to expectations, all of these conditional mice were fully viable, and none recapitulated any aspect of the Dlk1(Sul-pat) null mice. Dlk1 expression is therefore not essential for the normal development of β-cells, somatotrophs and endothelial cells, and the tissues responsible for the Dlk1 null phenotype remain to be identified. Dlk1(flox) mice will continue to provide an important tool for further research into the function of Dlk1.

Meat Science and Muscle Biology Symposium: stem cell niche and postnatal muscle growth

Stem cell niche plays a critical role in regulating the behavior and function of adult stem cells that underlie tissue growth, maintenance, and regeneration. In the skeletal muscle, stem cells, called satellite cells, contribute to postnatal muscle growth and hypertrophy, and thus, meat production in agricultural animals. Satellite cells are located adjacent to mature muscle fibers underneath a sheath of basal lamina. Microenvironmental signals from extracellular matrix mediated by the basal lamina and from the host myofiber both impinge on satellite cells to regulate their activity. Furthermore, several types of muscle interstitial cells, including intramuscular preadipocytes and connective tissue fibroblasts, have recently been shown to interact with satellite cells and actively regulate the growth and regeneration of postnatal skeletal muscles. From this regard, interstitial adipogenic cells are not only important for marbling and meat quality, but also represent an additional cellular component of the satellite cell niche. At the molecular level, these interstitial cells may interact with satellite cells through cell surface ligands, such as delta-like 1 homolog (Dlk1) protein whose overexpression is thought to be responsible for muscle hypertrophy in callipyge sheep. In fact, extracellular Dlk1 protein has been shown to promote the myogenic differentiation of satellite cells. Understanding the cellular and molecular mechanisms within the stem cell niche that regulate satellite cell differentiation and maintain muscle homeostasis may lead to promising approaches to optimizing muscle growth and composition, thus improving meat production and quality.

Possible roles of DLK1 in the Notch pathway during development and disease

The Delta-Notch pathway is an evolutionarily conserved signaling pathway which controls a broad range of developmental processes including cell fate determination, terminal differentiation and proliferation. In mammals, four Notch receptors (NOTCH1-4) and five activating canonical ligands (JAGGED1, JAGGED2, DLL1, DLL3 and DLL4) have been described. The precise function of noncanonical Notch ligands remains unclear. Delta-like 1 homolog (DLK1), the best studied noncanonical Notch ligand, has been shown to act as an inhibitor of Notch signaling in vitro, but its function in vivo is poorly understood. In this review we summarize Notch signaling during development and highlight recent studies in DLK1expression that reveal new insights into its function.

Development of novel monoclonal antibodies that define differentiation stages of human stromal (mesenchymal) stem cells

Human mesenchymal stem cells (hMSC) are currently being introduced for cell therapy, yet, antibodies specific for native and differentiated MSCs are required for their identification prior to clinical use. Herein, high quality antibodies against MSC surface proteins were developed by immunizing mice with hMSC, and by using a panel of subsequent screening methods. Flow cytometry analysis revealed that 83.5, 1.1, and 8.5% of primary cultures of hMSC were double positive for STRO-1 and either of DJ 3, 9, and 18, respectively. However, none of the three DJ antibodies allowed enrichment of clonogenic hMSC from BMMNCs as single reagents. Using mass-spectrometric analysis, we identified the antigen recognised by DJ3 as CD44, whereas DJ9 and DJ18 recognized HLA-DRB1 and Collagen VI, respectively. The identified proteins were highly expressed throughout in vitro osteogenic- and adipogenic differentiation. Interestingly, undifferentiated cells revealed a sole cytoplasmic distribution pattern of Collagen VI, which however changed to an extracellular matrix appearance upon osteogenic- and adipogenic differentiation. In relation to this, we found that STRO-1(+/-)/Collagen VI(-) sorted hMSC contained fewer differentiated alkaline phosphatase(+) cells compared to STRO-1(+/-)/Collagen VI(+) hMSC, suggesting that Collagen VI on the cell membrane exclusively defines differentiated MSCs. In conclusion, we have generated a panel of high quality antibodies to be used for characterization of MSCs, and in addition our results may suggest that the DJ18 generated antibody against Collagen VI can be used for negative selection of cultured undifferentiated MSCs.

Delta-like 1 serves as a new target and contributor to liver fibrosis down-regulated by mesenchymal stem cell transplantation

Chronic liver injury always progresses to fibrosis and eventually to cirrhosis, a massive health care burden worldwide. Delta-like 1 (Dlk1) is well known as an inhibitor of adipocyte differentiation. However, whether it is involved in liver fibrosis remains unclear. Here, we provide the first evidence that Dlk1 is a critical contributor to liver fibrosis through promoting activation of hepatic stellate cells (HSCs) during chronic liver injury. We found that upon liver injury, Dlk1 was dramatically induced and initially expressed in hepatocytes and then into the HSCs by a paracrine manner. It leads to the activation of HSCs, which is considered to be a pivotal event in liver fibrogenesis. Two forms (∼50 and ∼25 kDa) of the Dlk1 protein were detected by Western blot analysis. In vitro administration of Dlk1 significantly promoted HSC activation, whereas in vivo knockdown of Dlk1 dramatically inhibited HSC activation and the subsequent fibrosis. The large soluble form (∼50 kDa) of Dlk1 was shown to contribute to HSC activation. We were encouraged to find the Dlk1-promoted HSC activation and liver fibrosis can be depressed by transplantation of bone marrow-mesenchymal stem cells (BM-MSCs). Furthermore, we demonstrated that FGF2 was up-regulated in BM-MSCs under injury stimulation, and it probably participated in the inhibition of Dlk1 by BM-MSCs. Our findings provide a novel role of Dlk1 in liver fibrosis leading to a better understanding of the molecular basis in fibrosis and cirrhosis and also give insights into the cellular and molecular mechanisms of MSC biology in liver repair.

Newly formed skeletal muscle fibers are prone to false positive immunostaining by rabbit antibodies

Reports on muscle biology and regeneration often implicate immuno(cyto/histo)chemical protein characterization using rabbit polyclonal antibodies. In this study we demonstrate that newly formed myofibers are especially prone to false positive staining by rabbit antibodies and this unwanted staining is only recognized (1) by a negative muscle tissue control that does not harbor the protein to be examined (fx. from knockout mouse) or (2) by use of a nonsense rabbit antibody that has been prepared in the same way as the antibody of interest. However, many muscle immuno(cyto/histo)chemical studies only rely on controls that reveal non-specific binding by the secondary antibody and neglect that the primary rabbit antibody itself may cause false positive staining of the muscle. We suggest that reliable immuno-based protein detection in newly formed muscle fibers at least requires a nonsense rabbit antibody and optimally a negative muscle/cell control.

Expression of DLK1 and MEG3 genes in porcine tissues during postnatal development

The Drosophila-like homolog 1 (DLK1), a transmembrane signal protein similar to other members of the Notch/Delta/Serrate family, regulates the differentiation process in many types of mammalian cells. Callipyge sheep and DLK1 knockout mice are excellent examples of a fundamental role of the gene encoding DLK1 in muscle growth and fat deposition. DLK1 is located within co-regulated imprinted clusters (the DLK1/DIO3 domain), along with other imprinted genes. Some of these, e.g. the RNA coding MEG3 gene, presumedly interfere with DLK1 transcription. The aim of our study was to analyze DLK1 and MEG3 gene expression in porcine tissues (muscle, liver, kidney, heart, brain stem) during postnatal development. The highest expression of both DLK1 and MEG3 variant 1 (MEG3 var.1) was observed in the brain-stem and muscles, whereas that of MEG3 variant 2 (MEG3var.2) was the most abundant in muscles and the heart. During development (between 60 and 210 days of age) expression of analyzed genes was down-regulated in all the tissues. An exception was the brain- stem, where there was no significant change in MEG3 (both variants) mRNA level, and relatively little decline (2-fold) in that of DLK1 transcription. This may indicate a distinct function of the DLK1 gene in the brain-stem, when compared with other tissues.

Dlk1 is necessary for proper skeletal muscle development and regeneration

Delta-like 1homolog (Dlk1) is an imprinted gene encoding a transmembrane protein whose increased expression has been associated with muscle hypertrophy in animal models. However, the mechanisms by which Dlk1 regulates skeletal muscle plasticity remain unknown. Here we combine conditional gene knockout and over-expression analyses to investigate the role of Dlk1 in mouse muscle development, regeneration and myogenic stem cells (satellite cells). Genetic ablation of Dlk1 in the myogenic lineage resulted in reduced body weight and skeletal muscle mass due to reductions in myofiber numbers and myosin heavy chain IIB gene expression. In addition, muscle-specific Dlk1 ablation led to postnatal growth retardation and impaired muscle regeneration, associated with augmented myogenic inhibitory signaling mediated by NF-κB and inflammatory cytokines. To examine the role of Dlk1 in satellite cells, we analyzed the proliferation, self-renewal and differentiation of satellite cells cultured on their native host myofibers. We showed that ablation of Dlk1 inhibits the expression of the myogenic regulatory transcription factor MyoD, and facilitated the self-renewal of activated satellite cells. Conversely, Dlk1 over-expression inhibited the proliferation and enhanced differentiation of cultured myoblasts. As Dlk1 is expressed at low levels in satellite cells but its expression rapidly increases upon myogenic differentiation in vitro and in regenerating muscles in vivo, our results suggest a model in which Dlk1 expressed by nascent or regenerating myofibers non-cell autonomously promotes the differentiation of their neighbor satellite cells and therefore leads to muscle hypertrophy.

Satellite cell heterogeneity with respect to expression of MyoD, myogenin, Dlk1 and c-Met in human skeletal muscle: application to a cohort of power lifters and sedentary men

Human satellite cells (SCs) are heterogeneous with respect to markers for their identification in the niche between the muscle fibre plasma membrane and its basal lamina. We have previously shown that, in biopsies from highly competitive power lifters, power lifters with long-term use of anabolic steroids and a population of healthy sedentary men, antibodies against the neuronal cell adhesion molecule (NCAM) and the paired box transcription factor Pax7 together label 94% of the SCs, NCAM alone labels 4% and Pax7 alone labels 1%. In the present study, we have further studied these biopsies with four markers related to SC activation and differentiation. Our study unequivocally shows that staining for MyoD and myogenin are present in nuclei of SCs and of myoblasts and myotubes in areas of muscle fibre regeneration. Staining for c-Met was observed in a proportion of Pax7⁺ SCs. However, widespread labelling of the sarcolemma precluded the quantification of c-Met⁺/Pax7⁺ SCs and the use of c-Met as a reliable SC marker. Pax7⁺ SCs labelled by anti-Delta like1 (Dlk1) were present in all samples but in variable proportions, whereas muscle progenitor cells related to repair were Dlk1⁻. Staining for Dlk1 was also observed in Pax7⁻ interstitial cells and in the cytoplasm of some small muscle fibres. Interestingly, the proportion of Dlk1⁺/Pax7⁺ SCs was significantly different between the groups of power lifters. Thus, our study confirms that human SCs show marked heterogeneity and this is discussed in terms of SC activation, myonuclei turnover, muscle fibre growth and muscle fibre damage and repair.

Activation of paternally expressed genes and perinatal death caused by deletion of the Gtl2 gene

The Dlk1-Gtl2 imprinting locus is located on mouse distal chromosome 12 and consists of multiple maternally expressed non-coding RNAs and several paternally expressed protein-coding genes. The imprinting of this locus plays a crucial role in embryonic development and postnatal growth. At least one cis-element, the intergenic differentially methylated region (IG-DMR) is required for expression of maternally expressed genes and repression of silenced paternally expressed genes. The mechanism by which the IG-DMR functions is largely unknown. However, it has been suggested that the unmethylated IG-DMR acts as a positive regulator activating expression of non-coding RNAs. Gtl2 is the first non-coding RNA gene downstream of the IG-DMR. Although its in vivo function in the mouse is largely unknown, its human ortholog MEG3 has been linked to tumor suppression in human tumor-derived cell lines. We generated a knockout mouse model, in which the first five exons and adjacent promoter region of the Gtl2 gene were deleted. Maternal deletion of Gtl2 resulted in perinatal death and skeletal muscle defects, indicating that Gtl2 plays an important role in embryonic development. The maternal deletion also completely abolished expression of downstream maternally expressed genes, activated expression of silenced paternally expressed genes and resulted in methylation of the IG-DMR. By contrast, the paternal inherited deletion did not have this effect. These data strongly indicate that activation of Gtl2 and its downstream maternal genes play an essential role in regulating Dlk1-Gtl2 imprinting, possibly by maintaining active status of the IG-DMR.

MicroRNA-15a fine-tunes the level of Delta-like 1 homolog (DLK1) in proliferating 3T3-L1 preadipocytes

Delta like 1 homolog (Dlk1) exists in both transmembrane and soluble molecular forms, and is implicated in cellular growth and plays multiple roles in development, tissue regeneration, and cancer. Thus, DLK1 levels are critical for cell function, and abnormal DLK1 expression can be lethal; however, little is known about the underlying mechanisms. We here report that miR-15a modulates DLK1 levels in preadipocytes thus providing a mechanism for DLK1 regulation that further links it to cell cycle arrest and cancer since miR-15a is deregulated in these processes. In preadipocytes, miR-15a increases with cell density, and peaks at the same stage where membrane DLK1(M) and soluble DLK1(S) are found at maximum levels. Remarkably, miR-15a represses the amount of all Dlk1 variants at the mRNA level but also the level of DLK1(M) protein while it increases the amount of DLK1(S) supporting a direct repression of DLK1 and a parallel effect on the protease that cleaves off the DLK1 from the membrane. In agreement with previous studies, we found that miR-15a represses cell numbers, but additionally, we report that miR-15a also increases cell size. Conversely, anti-miR-15a treatment decreases cell size while increasing cell numbers, scenarios that were completely rescued by addition of purified DLK1(S). Our data thus imply that miR-15a regulates cell size and proliferation by fine-tuning Dlk1 among others, and further emphasize miR-15a and DLK1 levels to play important roles in growth signaling networks.

"The preadipocyte factor" DLK1 marks adult mouse adipose tissue residing vascular cells that lack in vitro adipogenic differentiation potential

Delta-like 1 (Dlk1) is expressed in 3T3-L1 preadipocytes and has frequently been referred to as "the" preadipocyte marker, yet the phenotype of DLK1(+) cells in adipose tissue remains undetermined. Herein, we demonstrate that DLK1(+) cells encompass around 1-2% of the adult mouse adipose stromal vascular fraction (SVF). Unexpectedly, the DLK1(+)SVF population was enriched for cells expressing genes generally ascribed to the vascular lineage and did not possess any adipogenic differentiation potential in vitro. Instead, DLK1(+) cells comprised an immediate ability for cobblestone formation, generation of tube-like structures on matrigel, and uptake of Acetylated Low Density-Lipoprotein, all characteristics of endothelial cells. We therefore suggest that DLK1(+)SVF cells are of a vascular origin and not them-selves committed preadipocytes as assumed hitherto.