Distinct regulatory cascades govern extraocular and pharyngeal arch muscle progenitor cell fates

Defective angiogenesis in CXCL12 mutant mice impairs skeletal muscle regeneration

BACKGROUND

During muscle regeneration, the chemokine CXCL12 (SDF-1) and the synthesis of some specific heparan sulfates (HS) have been shown to be critical. CXCL12 activity has been shown to be heavily influenced by its binding to extracellular glycosaminoglycans (GAG) by modulating its presentation to its receptors and by generating haptotactic gradients. Although CXCL12 has been implicated in several phases of tissue repair, the influence of GAG binding under HS influencing conditions such as acute tissue destruction remains understudied.

METHODS

To investigate the role of the CXCL12/HS proteoglycan interactions in the pathophysiology of muscle regeneration, we performed two models of muscle injuries (notexin and freeze injury) in mutant CXCL12^Gagtm/Gagtm mice, where the CXCL12 gene having been selectively mutated in critical binding sites of CXCL12 to interact with HS. Histological, cytometric, functional transcriptomic, and ultrastructure analysis focusing on the satellite cell behavior and the vessels were conducted on muscles before and after injuries. Unless specified, statistical analysis was performed with the Mann-Whitney test.

RESULTS

We showed that despite normal histology of the resting muscle and normal muscle stem cell behavior in the mutant mice, endothelial cells displayed an increase in the angiogenic response in resting muscle despite the downregulated transcriptomic changes induced by the CXCL12 mutation. The regenerative capacity of the CXCL12-mutated mice was only delayed after a notexin injury, but a severe damage by freeze injury revealed a persistent defect in the muscle regeneration of CXCL12 mutant mice associated with vascular defect and fibroadipose deposition with persistent immune cell infiltration.

CONCLUSION

The present study shows that CXCL12 is crucial for proper muscle regeneration. We highlight that this homing molecule could play an important role in drastic muscle injuries and that the regeneration defect could be due to an impairment of angiogenesis, associated with a long-lasting fibro-adipogenic scar.

Molecular and cellular mechanisms underlying the evolution of form and function in the amniote jaw

The amniote jaw complex is a remarkable amalgamation of derivatives from distinct embryonic cell lineages. During development, the cells in these lineages experience concerted movements, migrations, and signaling interactions that take them from their initial origins to their final destinations and imbue their derivatives with aspects of form including their axial orientation, anatomical identity, size, and shape. Perturbations along the way can produce defects and disease, but also generate the variation necessary for jaw evolution and adaptation. We focus on molecular and cellular mechanisms that regulate form in the amniote jaw complex, and that enable structural and functional integration. Special emphasis is placed on the role of cranial neural crest mesenchyme (NCM) during the species-specific patterning of bone, cartilage, tendon, muscle, and other jaw tissues. We also address the effects of biomechanical forces during jaw development and discuss ways in which certain molecular and cellular responses add adaptive and evolutionary plasticity to jaw morphology. Overall, we highlight how variation in molecular and cellular programs can promote the phenomenal diversity and functional morphology achieved during amniote jaw evolution or lead to the range of jaw defects and disease that affect the human condition.

Direct Reprogramming of Mouse Fibroblasts into Functional Skeletal Muscle Progenitors

Skeletal muscle harbors quiescent stem cells termed satellite cells and proliferative progenitors termed myoblasts, which play pivotal roles during muscle regeneration. However, current technology does not allow permanent capture of these cell populations in vitro. Here, we show that ectopic expression of the myogenic transcription factor MyoD, combined with exposure to small molecules, reprograms mouse fibroblasts into expandable induced myogenic progenitor cells (iMPCs). iMPCs express key skeletal muscle stem and progenitor cell markers including Pax7 and Myf5 and give rise to dystrophin-expressing myofibers upon transplantation in vivo. Notably, a subset of transplanted iMPCs maintain Pax7 expression and sustain serial regenerative responses. Similar to satellite cells, iMPCs originate from Pax7⁺ cells and require Pax7 itself for maintenance. Finally, we show that myogenic progenitor cell lines can be established from muscle tissue following small-molecule exposure alone. This study thus reports on a robust approach to derive expandable myogenic stem/progenitor-like cells from multiple cell types.

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MyoD and small molecules reprogram fibroblasts to myogenic progenitors termed iMPCs

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iMPCs self-renew and express key satellite cell and myoblast markers

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iMPC growth is driven by Pax7⁺ cells and requires Pax7 gene function

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Transplanted iMPCs engraft and sustain muscle regeneration in vivo

Beneficial role of adipose-derived mesenchymal stem cells from microfragmented fat in a murine model of duchenne muscular dystrophy

INTRODUCTION

No etiologic therapy is available for Duchenne muscular dystrophy (DMD), but mesenchymal stem cells were shown to be effective in preclinical models of DMD. The objective of this study is to investigate the effect of microfragmented fat extracted on a murine model of DMD.

METHODS

Fat tissue was extracted from healthy human participants and injected IM into DMD mice. Histological analysis, cytokines, and force measurement were performed up to 4 weeks after injection.

RESULTS

Duchenne muscular dystrophy mice injected with microfragmented fat exhibited an improved muscle phenotype (decreased necrosis and fibrosis), a decrease of inflammatory cytokines, and increased strength.

DISCUSSION

Administration of microfragmented fat in key muscles may improve muscular phenotype in patients with DMD. Muscle Nerve, 2019.

A periodic table of cell types

Single cell biology is currently revolutionizing developmental and evolutionary biology, revealing new cell types and states in an impressive range of biological systems. With the accumulation of data, however, the field is grappling with a central unanswered question: what exactly is a cell type? This question is further complicated by the inherently dynamic nature of developmental processes. In this Hypothesis article, we propose that a 'periodic table of cell types' can be used as a framework for distinguishing cell types from cell states, in which the periods and groups correspond to developmental trajectories and stages along differentiation, respectively. The different states of the same cell type are further analogous to 'isotopes'. We also highlight how the concept of a periodic table of cell types could be useful for predicting new cell types and states, and for recognizing relationships between cell types throughout development and evolution.

PAX3 Confers Functional Heterogeneity in Skeletal Muscle Stem Cell Responses to Environmental Stress

Muscle satellite cells (MuSCs) are the quiescent muscle stem cells required for adult skeletal muscle repair. The impact of environmental stress such as pollution on MuSC behavior remains unexplored. We evaluated the impact of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure, a ubiquitous and highly toxic pollutant, on MuSCs by combining in vivo mouse molecular genetic models with ex vivo studies. While all MuSCs express the transcription factor PAX7, we show that a subset also express PAX3 and exhibit resistance to environmental stress. Upon systemic TCDD treatment, PAX3-negative MuSCs display impaired survival, atypical activation, and sporadic differentiation through xenobiotic aryl hydrocarbon receptor signaling. We further show that PAX3-positive MuSCs become sensitized to environmental stress when PAX3 function is impaired and that PAX3-mediated induction of mTORC1 is required for protection. Our study, therefore, identifies a functional heterogeneity of MuSCs in response to environmental stress controlled by PAX3.

YY1 regulates skeletal muscle regeneration through controlling metabolic reprogramming of satellite cells

Skeletal muscle satellite cells (SCs) are adult muscle stem cells responsible for muscle regeneration after acute or chronic injuries. The lineage progression of quiescent SC toward activation, proliferation, and differentiation during the regeneration is orchestrated by cascades of transcription factors (TFs). Here, we elucidate the function of TF Yin Yang1 (YY1) in muscle regeneration. Muscle-specific deletion of YY1 in embryonic muscle progenitors leads to severe deformity of diaphragm muscle formation, thus neonatal death. Inducible deletion of YY1 in SC almost completely blocks the acute damage-induced muscle repair and exacerbates the chronic injury-induced dystrophic phenotype. Examination of SC revealed that YY1 loss results in cell-autonomous defect in activation and proliferation. Mechanistic search revealed that YY1 binds and represses mitochondrial gene expression. Simultaneously, it also stabilizes Hif1α protein and activates Hif1α-mediated glycolytic genes to facilitate a metabolic reprogramming toward glycolysis which is needed for SC proliferation. Altogether, our findings have identified YY1 as a key regulator of SC metabolic reprogramming through its dual roles in modulating both mitochondrial and glycolytic pathways.

High-Dimensional Single-Cell Cartography Reveals Novel Skeletal Muscle-Resident Cell Populations

Adult tissue repair and regeneration require stem-progenitor cells that can self-renew and generate differentiated progeny. Skeletal muscle regenerative capacity relies on muscle satellite cells (MuSCs) and their interplay with different cell types within the niche. However, our understanding of skeletal muscle tissue cellular composition is limited. Here, using a combined approach of single-cell RNA sequencing and mass cytometry, we precisely mapped 10 different mononuclear cell types in adult mouse muscle. We also characterized gene signatures and determined key discriminating markers of each cell type. We identified two previously understudied cell populations in the interstitial compartment. One expresses the transcription factor scleraxis and generated tenocytes in vitro. The second expresses markers of smooth muscle and mesenchymal cells (SMMCs) and, while distinct from MuSCs, exhibited myogenic potential and promoted MuSC engraftment following transplantation. The blueprint presented here yields crucial insights into muscle-resident cell-type identities and can be exploited to study muscle diseases.

Modular co-option of cardiopharyngeal genes during non-embryonic myogenesis

Background

In chordates, cardiac and body muscles arise from different embryonic origins. In addition, myogenesis can be triggered in adult organisms, during asexual development or regeneration. In non-vertebrate chordates like ascidians, muscles originate from embryonic precursors regulated by a conserved set of genes that orchestrate cell behavior and dynamics during development. In colonial ascidians, besides embryogenesis and metamorphosis, an adult can propagate asexually via blastogenesis, skipping embryo and larval stages, and form anew the adult body, including the complete body musculature.

Results

To investigate the cellular origin and mechanisms that trigger non-embryonic myogenesis, we followed the expression of ascidian myogenic genes during Botryllus schlosseri blastogenesis and reconstructed the dynamics of muscle precursors. Based on the expression dynamics of Tbx1/10, Ebf, Mrf, Myh3 for body wall and of FoxF, Tbx1/10, Nk4, Myh2 for heart development, we show that the embryonic factors regulating myogenesis are only partially co-opted in blastogenesis, and that markers for muscle precursors are expressed in two separate domains: the dorsal tube and the ventral mesenchyma.

Conclusions

Regardless of the developmental pathway, non-embryonic myogenesis shares a similar molecular and anatomical setup as embryonic myogenesis, but implements a co-option and loss of molecular modules. We then propose that the cellular precursors contributing to heart and body muscles may have different origins and may be coordinated by different developmental pathways.

Electronic supplementary material

The online version of this article (10.1186/s13227-019-0116-7) contains supplementary material, which is available to authorized users.

Combined Notch and PDGF Signaling Enhances Migration and Expression of Stem Cell Markers while Inducing Perivascular Cell Features in Muscle Satellite Cells

Satellite cells are responsible for skeletal muscle regeneration. Upon activation, they proliferate as transient amplifying myoblasts, most of which fuse into regenerating myofibers. Despite their remarkable differentiation potential, these cells have limited migration capacity, which curtails clinical use for widespread forms of muscular dystrophy. Conversely, skeletal muscle perivascular cells have less myogenic potential but better migration capacity than satellite cells. Here we show that modulation of Notch and PDGF pathways, involved in developmental specification of pericytes, induces perivascular cell features in adult mouse and human satellite cell-derived myoblasts. DLL4 and PDGF-BB-treated cells express markers of perivascular cells and associate with endothelial networks while also upregulating markers of satellite cell self-renewal. Moreover, treated cells acquire trans-endothelial migration ability while remaining capable of engrafting skeletal muscle upon intramuscular transplantation. These results extend our understanding of muscle stem cell fate plasticity and provide a druggable pathway with clinical relevance for muscle cell therapy.

Age-related alterations in swallowing biomechanics

BACKGROUND

Aging rodent models allow for the discovery of underlying mechanisms of cranial muscle dysfunction. Methods are needed to allow quantification of complex, multivariate biomechanical movements during swallowing. Videofluoroscopic swallow studies (VSS) are the standard of care in assessment of swallowing disorders in patients and validated quantitative, kinematic, and morphometric analysis methods have been developed. Our purpose was to adapt validated morphometric techniques to the rodent to computationally analyze swallowing dysfunction in the aging rodent.

METHODS

VSS, quantitative analyses (bolus area, bolus velocity, mastication rate) and a rodent specific multivariate, morphometric computational analysis of swallowing biomechanics were performed on 20 swallows from 5 young adult and 5 old Fischer 344/Brown Norway rats. Eight anatomical landmarks were used to track the relative change in position of skeletal levers (cranial base, vertebral column, mandible) and soft tissue landmarks (upper esophageal sphincter, base of tongue).

RESULTS

Bolus area significantly increased and mastication rate significantly decreased with age. Aging accounted for 77.1% of the variance in swallow biomechanics, and 18.7% of the variance was associated with swallow phase (oral vs pharyngeal). Post hoc analyses identified age-related alterations in tongue base retraction, mastication, and head posture during the swallow.

CONCLUSION

Geometric morphometric analysis of rodent swallows suggests that swallow biomechanics are altered with age. When used in combination with biological assays of age-related adaptations in neuromuscular systems, this multivariate analysis may increase our understanding of underlying musculoskeletal dysfunction that contributes to swallowing disorders with aging.

Skeletal muscle stem cells in comfort and stress

Investigations on developmental and regenerative myogenesis have led to major advances in decrypting stem cell properties and potential, as well as their interactions within the evolving niche. As a consequence, regenerative myogenesis has provided a forum to investigate intrinsic regulators of stem cell properties as well as extrinsic factors, including stromal cells, during normal growth and following injury and disease. Here we review some of the latest advances in the field that have exposed fundamental processes including regulation of stress following trauma and ageing, senescence, DNA damage control and modes of symmetric and asymmetric cell divisions. Recent studies have begun to explore the nature of the niche that is distinct in different muscle groups, and that is altered from prenatal to postnatal stages, and during ageing. We also discuss heterogeneities among muscle stem cells and how distinct properties within the quiescent and proliferating cell states might impact on homoeostasis and regeneration. Interestingly, cellular quiescence, which was thought to be a passive cell state, is regulated by multiple mechanisms, many of which are deregulated in various contexts including ageing. These and other factors including metabolic activity and genetic background can impact on the efficiency of muscle regeneration.

Notchless defines a stage-specific requirement for ribosome biogenesis during lineage progression in adult skeletal myogenesis

Cell fate decisions occur through the action of multiple factors, including signalling molecules and transcription factors. Recently, the regulation of translation has emerged as an important step for modulating cellular function and fate, as exemplified by ribosomes that play distinct roles in regulating cell behaviour. Notchless (Nle) is a conserved nuclear protein that is involved in a crucial step in ribosome biogenesis, and is required for the maintenance of adult haematopoietic and intestinal stem/progenitor cells. Here, we show that activated skeletal muscle satellite cells in conditional Nle mutant mice are arrested in proliferation; however, deletion of Nle in myofibres does not impair myogenesis. Furthermore, conditional deletion of Nle in satellite cells during homeostasis did not impact on their fate for up to 3 months. In contrast, loss of Nle function in primary myogenic cells blocked proliferation because of major defects in ribosome formation. Taken together, we show that muscle stem cells undergo a stage-specific regulation of ribosome biogenesis, thereby underscoring the importance of differential modulation of mRNA translation for controlling cell fate decisions.

Unique morphogenetic signatures define mammalian neck muscles and associated connective tissues

In vertebrates, head and trunk muscles develop from different mesodermal populations and are regulated by distinct genetic networks. Neck muscles at the head-trunk interface remain poorly defined due to their complex morphogenesis and dual mesodermal origins. Here, we use genetically modified mice to establish a 3D model that integrates regulatory genes, cell populations and morphogenetic events that define this transition zone. We show that the evolutionary conserved cucullaris-derived muscles originate from posterior cardiopharyngeal mesoderm, not lateral plate mesoderm, and we define new boundaries for neural crest and mesodermal contributions to neck connective tissue. Furthermore, lineage studies and functional analysis of Tbx1- and Pax3-null mice reveal a unique developmental program for somitic neck muscles that is distinct from that of somitic trunk muscles. Our findings unveil the embryological and developmental requirements underlying tetrapod neck myogenesis and provide a blueprint to investigate how muscle subsets are selectively affected in some human myopathies.

Notch-Induced miR-708 Antagonizes Satellite Cell Migration and Maintains Quiescence

Critical features of stem cells include anchoring within a niche and activation upon injury. Notch signaling maintains skeletal muscle satellite (stem) cell quiescence by inhibiting differentiation and inducing expression of extracellular components of the niche. However, the complete spectrum of how Notch safeguards quiescence is not well understood. Here, we perform Notch ChIP-sequencing and small RNA sequencing in satellite cells and identify the Notch-induced microRNA-708, which is a mirtron that is highly expressed in quiescent cells and sharply downregulated in activated cells. We employ in vivo and ex vivo functional studies, in addition to live imaging, to show that miR-708 regulates quiescence and self-renewal by antagonizing cell migration through targeting the transcripts of the focal-adhesion-associated protein Tensin3. Therefore, this study identifies a Notch-miR708-Tensin3 axis and suggests that Notch signaling can regulate satellite cell quiescence and transition to the activation state through dynamic regulation of the migratory machinery.

Muscle Satellite Cell Cross-Talk with a Vascular Niche Maintains Quiescence via VEGF and Notch Signaling

Skeletal muscle is a complex tissue containing tissue resident muscle stem cells (satellite cells) (MuSCs) important for postnatal muscle growth and regeneration. Quantitative analysis of the biological function of MuSCs and the molecular pathways responsible for a potential juxtavascular niche for MuSCs is currently lacking. We utilized fluorescent reporter mice and muscle tissue clearing to investigate the proximity of MuSCs to capillaries in 3 dimensions. We show that MuSCs express abundant VEGFA, which recruits endothelial cells (ECs) in vitro, whereas blocking VEGFA using both a vascular endothelial growth factor (VEGF) inhibitor and MuSC-specific VEGFA gene deletion reduces the proximity of MuSCs to capillaries. Importantly, this proximity to the blood vessels was associated with MuSC self-renewal in which the EC-derived Notch ligand Dll4 induces quiescence in MuSCs. We hypothesize that MuSCs recruit capillary ECs via VEGFA, and in return, ECs maintain MuSC quiescence though Dll4.

Divergent early mesoderm specification underlies distinct head and trunk muscle programmes in vertebrates

Head and trunk muscles have discrete embryological origins and are governed by distinct regulatory programmes. Whereas the developmental route of trunk muscles from mesoderm is well studied, that of head muscles is ill defined. Here, we show that, unlike the myogenic trunk paraxial mesoderm, head mesoderm development is independent of the T/Tbx6 network in mouse. We reveal that, in contrast to Wnt and FGF-driven trunk mesoderm, dual inhibition of Wnt/β-catenin and Nodal specifies head mesoderm. Remarkably, the progenitors derived from embryonic stem cells by dual inhibition efficiently differentiate into cardiac and skeletal muscle cells. This twin potential is the defining feature of cardiopharyngeal mesoderm: the head subtype giving rise to heart and branchiomeric head muscles. Therefore, our findings provide compelling evidence that dual inhibition specifies head mesoderm and unravel the mechanism that diversifies head and trunk muscle programmes during early mesoderm fate commitment. Significantly, this is the first report of directed differentiation of pluripotent stem cells, without transgenes, into progenitors with muscle/heart dual potential. Ability to generate branchiomeric muscle in vitro could catalyse efforts in modelling myopathies that selectively involve head muscles.

Tbx1 regulates inherited metabolic and myogenic abilities of progenitor cells derived from slow- and fast-type muscle

Skeletal muscle is divided into slow- and fast-type muscles, which possess distinct contractile and metabolic properties. Myogenic progenitors associated with each muscle fiber type are known to intrinsically commit to specific muscle fiber lineage during embryonic development. However, it is still unclear whether the functionality of postnatal adult myogenic cells is attributable to the muscle fiber in which they reside, and whether the characteristics of myogenic cells derived from slow- and fast-type fibers can be distinguished at the genetic level. In this study, we isolated adult satellite cells from slow- and fast-type muscle individually and observed that satellite cells from each type of muscle generated myotubes expressing myosin heavy chain isoforms similar to their original muscle, and showed different metabolic features. Notably, we discovered that slow muscle-derived cells had low potential to differentiate but high potential to self-renew compared with fast muscle-derived cells. Additionally, cell transplantation experiments of slow muscle-derived cells into fast-type muscle revealed that slow muscle-derived cells could better contribute to myofiber formation and satellite cell constitution than fast muscle-derived cells, suggesting that the recipient muscle fiber type may not affect the predetermined abilities of myogenic cells. Gene expression analyses identified T-box transcriptional factor Tbx1 as a highly expressed gene in fast muscle-derived myoblasts. Gain- and loss-of-function experiments revealed that Tbx1 modulated muscle fiber types and oxidative metabolism in myotubes, and that Tbx1 stimulated myoblast differentiation, but did not regulate myogenic cell self-renewal. Our data suggest that metabolic and myogenic properties of myogenic progenitor cells vary depending on the type of muscle from which they originate, and that Tbx1 expression partially explains the functional differences of myogenic cells derived from fast-type and slow-type muscles.

A robust Pax7EGFP mouse that enables the visualization of dynamic behaviors of muscle stem cells

Background

Pax7 is a transcription factor involved in the specification and maintenance of muscle stem cells (MuSCs). Upon injury, MuSCs leave their quiescent state, downregulate Pax7 and differentiate, contributing to skeletal muscle regeneration. In the majority of regeneration studies, MuSCs are isolated by fluorescence-activated sorting (FACS), based on cell surface markers. It is known that MuSCs are a heterogeneous population and only a small percentage of isolated cells are true stem cells that are able to self-renew. A strong Pax7 reporter line would be valuable to study the in vivo behavior of Pax7-expressing stem cells.

Methods

We generated and characterized the muscle properties of a new transgenic Pax7EGFP mouse. Utilizing traditional immunofluorescence assays, we analyzed whole embryos and muscle sections by fluorescence microscopy, in addition to whole skeletal muscles by 2-photon microscopy, to detect the specificity of EGFP expression. Skeletal muscles from Pax7EGFP mice were also evaluated in steady state and under injury conditions. Finally, MuSCs-derived from Pax7EGFP and control mice were sorted and analyzed by FACS and their myogenic activity was comparatively examined.

Results

Our studies provide a new Pax7 reporter line with robust EGFP expression, detectable by both flow cytometry and fluorescence microscopy. Pax7EGFP-derived MuSCs have identical properties to that of wild-type MuSCs, both in vitro and in vivo, excluding any positional effect due to the transgene insertion. Furthermore, we demonstrated high specificity of EGFP to label MuSCs in a temporal manner that recapitulates the reported Pax7 expression pattern. Interestingly, immunofluorescence analysis showed that the robust expression of EGFP marks cells in the satellite cell position of adult muscles in fixed and live tissues.

Conclusions

This mouse could be an invaluable tool for the study of a variety of questions related to MuSC biology, including but not limited to population heterogeneity, polarity, aging, regeneration, and motility, either by itself or in combination with mice harboring additional genetic alterations.

Electronic supplementary material

The online version of this article (10.1186/s13395-018-0169-7) contains supplementary material, which is available to authorized users.

Distinct metabolic states govern skeletal muscle stem cell fates during prenatal and postnatal myogenesis

During growth, homeostasis and regeneration, stem cells are exposed to different energy demands. Here, we characterise the metabolic pathways that mediate the commitment and differentiation of mouse skeletal muscle stem cells, and how their modulation can influence the cell state. We show that quiescent satellite stem cells have low energetic demands and perturbed oxidative phosphorylation during ageing, which is also the case for cells from post-mortem tissues. We show also that myogenic fetal cells have distinct metabolic requirements compared to those proliferating during regeneration, with the former displaying a low respiration demand relying mostly on glycolysis. Furthermore, we show distinct requirements for peroxisomal and mitochondrial fatty acid oxidation (FAO) in myogenic cells. Compromising peroxisomal but not mitochondrial FAO promotes early differentiation of myogenic cells. Acute muscle injury and pharmacological block of peroxisomal and mitochondrial FAO expose differential requirements for these organelles during muscle regeneration. Taken together, these observations indicate that changes in myogenic cell state lead to significant alterations in metabolic requirements. In addition, perturbing specific metabolic pathways impacts on myogenic cell fates and the regeneration process.

Summary: Distinct energy metabolism pathways act during mouse skeletal muscle stem cell commitment and differentiation in different physiological states.

The transcription factor Lef1 switches partners from β-catenin to Smad3 during muscle stem cell quiescence

Skeletal muscle stem cells (MuSCs), also known as satellite cells, persist in adult mammals by entering a state of quiescence (G₀) during the early postnatal period. Quiescence is reversed during damage-induced regeneration and re-established after regeneration. Entry of cultured myoblasts into G₀ is associated with a specific, reversible induction of Wnt target genes, thus implicating members of the Tcf and Lef1 (Tcf/Lef) transcription factor family, which mediate transcriptional responses to Wnt signaling, in the initiation of quiescence. We found that the canonical Wnt effector β-catenin, which cooperates with Tcf/Lef, was dispensable for myoblasts to enter quiescence. Using pharmacological and genetic approaches in cultured C2C12 myoblasts and in MuSCs, we demonstrated that Tcf/Lef activity during quiescence depended not on β-catenin but on the transforming growth factor-β (TGF-β) effector and transcriptional coactivator Smad3, which colocalized with Lef1 at canonical Wnt-responsive elements and directly interacted with Lef1 specifically in G₀ Depletion of Smad3, but not β-catenin, reduced Lef1 occupancy at target promoters, Tcf/Lef target gene expression, and self-renewal of myoblasts. In vivo, MuSCs underwent a switch from β-catenin-Lef1 to Smad3-Lef1 interactions during the postnatal switch from proliferation to quiescence, with β-catenin-Lef1 interactions recurring during damage-induced reactivation. Our findings suggest that the interplay of Wnt-Tcf/Lef and TGF-β-Smad3 signaling activates canonical Wnt target promoters in a manner that depends on β-catenin during myoblast proliferation but is independent of β-catenin during MuSC quiescence.

Recessive MYF5 Mutations Cause External Ophthalmoplegia, Rib, and Vertebral Anomalies

MYF5 is member of the Myc-like basic helix-loop-helix transcription factor family and, in cooperation with other myogenic regulatory factors MYOD and MYF5, is a key regulator of early stages of myogenesis. Here, we report three consanguineous families with biallelic homozygous loss-of-function mutations in MYF5 who define a clinical disorder characterized by congenital ophthalmoplegia with scoliosis and vertebral and rib anomalies. The clinical phenotype overlaps strikingly with that reported in several Myf5 knockout mouse models. Affected members of two families share a haploidentical region that contains a homozygous 10 bp frameshift mutation in exon 1 of MYF5 (c.23_32delAGTTCTCACC [p.Gln8Leufs^∗86]) predicted to undergo nonsense-mediated decay. Affected members of the third family harbor a homozygous missense change in exon 1 of MYF5 (c.283C>T [p.Arg95Cys]). Using in vitro assays, we show that this missense mutation acts as a loss-of-function allele by impairing MYF5 DNA binding and nuclear localization. We performed whole-genome sequencing in one affected individual with the frameshift mutation and did not identify additional rare variants in the haploidentical region that might account for differences in severity among the families. These data support the direct role of MYF5 in rib, spine, and extraocular muscle formation in humans.

In Situ Fixation Redefines Quiescence and Early Activation of Skeletal Muscle Stem Cells

State of the art techniques have been developed to isolate and analyze cells from various tissues, aiming to capture their in vivo state. However, the majority of cell isolation protocols involve lengthy mechanical and enzymatic dissociation steps followed by flow cytometry, exposing cells to stress and disrupting their physiological niche. Focusing on adult skeletal muscle stem cells, we have developed a protocol that circumvents the impact of isolation procedures and captures cells in their native quiescent state. We show that current isolation protocols induce major transcriptional changes accompanied by specific histone modifications while having negligible effects on DNA methylation. In addition to proposing a protocol to avoid isolation-induced artifacts, our study reveals previously undetected quiescence and early activation genes of potential biological interest.

Local Arrangement of Fibronectin by Myofibroblasts Governs Peripheral Nuclear Positioning in Muscle Cells

Skeletal muscle cells (myofibers) are rod-shaped multinucleated cells surrounded by an extracellular matrix (ECM) basal lamina. In contrast to other cell types, nuclei in myofibers are positioned just below the plasma membrane at the cell periphery. Peripheral nuclear positioning occurs during myogenesis and is driven by myofibril crosslinking and contraction. Here we show that peripheral nuclear positioning is triggered by local accumulation of fibronectin secreted by myofibroblasts. We demonstrate that fibronectin via α5-integrin mediates peripheral nuclear positioning dependent on FAK and Src activation. Finally, we show that Cdc42, downstream of restricted fibronectin activation, is required for myofibril crosslinking but not myofibril contraction. Thus we identify that local activation of integrin by fibronectin secreted by myofibroblasts activates peripheral nuclear positioning in skeletal myofibers.

Gene expression profiling in the developing secondary palate in the absence of Tbx1 function

BACKGROUND

Microdeletion of chromosome 22q11 is associated with significant developmental anomalies, including disruption of the cardiac outflow tract, thymic/parathyroid aplasia and cleft palate. Amongst the genes within this region, TBX1 is a major candidate for many of these developmental defects. Targeted deletion of Tbx1 in the mouse has provided significant insight into the function of this transcription factor during early development of the cardiac and pharyngeal systems. However, less is known about its role during palatogenesis. To assess the influence of Tbx1 function on gene expression profile within the developing palate we performed a microarray screen using total RNA isolated from the secondary palate of E13.5 mouse embryos wild type, heterozygous and mutant for Tbx1.

RESULTS

Expression-level filtering and statistical analysis revealed a total of 577 genes differentially expressed across genotypes. Data were clustered into 3 groups based on comparison between genotypes. Group A was composed of differentially expressed genes in mutant compared to wild type (n = 89); Group B included differentially expressed genes in heterozygous compared to wild type (n = 400) and Group C included differentially expressed genes in mutant compared to heterozygous (n = 88). High-throughput quantitative real-time PCR (RT-PCR) confirmed a total of 27 genes significantly changed between wild type and mutant; and 27 genes between heterozygote and mutant. Amongst these, the majority were present in both groups A and C (26 genes). Associations existed with hypertrophic cardiomyopathy, cardiac muscle contraction, dilated cardiomyopathy, focal adhesion, tight junction and calcium signalling pathways. No significant differences in gene expression were found between wild type and heterozygous palatal shelves.

CONCLUSIONS

Significant differences in gene expression profile within the secondary palate of wild type and mutant embryos is consistent with a primary role for Tbx1 during palatogenesis.

Reciprocal signalling by Notch-Collagen V-CALCR retains muscle stem cells in their niche

The microenvironment is critical for stem cell maintenance and can be of cellular and non-cellular composition, including secreted growth factors and extracellular matrix (ECM)1–3. Although Notch and other signalling pathways have been reported to regulate quiescence4–9, the composition and source of niche molecules remain largely unknown. Here, we show that adult muscle satellite (stem) cells produce ECM collagens to maintain quiescence cell-autonomously. By ChIP-sequencing we identified NOTCH/RBPJ-bound regulatory elements adjacent to specific collagen genes, whose expression is deregulated in Notch mutant mice. Moreover, we show that satellite cell produced collagen V (COLV) is a critical component of the quiescent niche, as conditional deletion of Col5a1 leads to anomalous cell cycle entry and gradual diminution of the stem cell pool. Notably, the interaction of COLV with satellite cells is mediated by CALCR, for which COLV acts as a surrogate local ligand. Strikingly, systemic administration of a calcitonin derivative is sufficient to rescue the quiescence and self-renewal defects scored in COLV null satellite cells. This study unveils a Notch/COLV/CALCR signalling cascade that cell-autonomously maintains the satellite cell quiescent state and raises the possibility of a similar reciprocal mechanism acting in diverse stem cell populations.

Location, Location, Location: Signals in Muscle Specification

Muscles control body movement and locomotion, posture and body position and soft tissue support. Mesoderm derived cells gives rise to 700 unique muscles in humans as a result of well-orchestrated signaling and transcriptional networks in specific time and space. Although the anatomical structure of skeletal muscles is similar, their functions and locations are specialized. This is the result of specific signaling as the embryo grows and cells migrate to form different structures and organs. As cells progress to their next state, they suppress current sequence specific transcription factors (SSTF) and construct new networks to establish new myogenic features. In this review, we provide an overview of signaling pathways and gene regulatory networks during formation of the craniofacial, cardiac, vascular, trunk, and limb skeletal muscles.

Neural crest and the patterning of vertebrate craniofacial muscles

Patterning of craniofacial muscles overtly begins with the activation of lineage-specific markers at precise, evolutionarily conserved locations within prechordal, lateral, and both unsegmented and somitic paraxial mesoderm populations. Although these initial programming events occur without influence of neural crest cells, the subsequent movements and differentiation stages of most head muscles are neural crest-dependent. Incorporating both descriptive and experimental studies, this review examines each stage of myogenesis up through the formation of attachments to their skeletal partners. We present the similarities among developing muscle groups, including comparisons with trunk myogenesis, but emphasize the morphogenetic processes that are unique to each group and sometimes subsets of muscles within a group. These groups include branchial (pharyngeal) arches, which encompass both those with clear homologues in all vertebrate classes and those unique to one, for example, mammalian facial muscles, and also extraocular, laryngeal, tongue, and neck muscles. The presence of several distinct processes underlying neural crest:myoblast/myocyte interactions and behaviors is not surprising, given the wide range of both quantitative and qualitative variations in craniofacial muscle organization achieved during vertebrate evolution.

Loss of heterogeneity, quiescence, and differentiation in muscle stem cells

Skeletal muscle stem cells in the adult display heterogeneity that has been functionally linked to their behavior, self-renewal capacity, and resistance to stress in hostile environments. Behavioral heterogeneity emerges also during developmental myogenesis. Muscle stem cell diversity may be functionally linked to the changing needs of skeletal muscle regeneration. Intriguingly, dramatic reduction of stem cell diversity, the "clonal drift", that implies loss of stem cells and related expansion of clonally related stem cells has been reported for tissue replacement in several adult tissues and suggested in the zebrafish embryo. A recent study shows clonal drift of muscle stem cells in the zebrafish embryo caused by inhibition of the cell cycle and directed by the homeobox protein Meox1. Although stem cell quiescence is associated with inhibition of the transition phase G0/G1 of the cell cycle, Meox1 triggers the muscle stem cell fate by an arrest in G2 phase. Why efficient muscle growth in the zebrafish embryo requires sacrificing stem cell heterogeneity in favor of a small number of dominant clones has not been elucidated. The significance of G2-halted stem cells, which are generally associated with robust regeneration capacity, is also intriguing. These processes are relevant for understanding organ growth and the mechanisms that govern stem cell quiescence.

Reporter-Based Isolation of Developmental Myogenic Progenitors

The formation and activity of mammalian tissues entail finely regulated processes, involving the concerted organization and interaction of multiple cell types. In recent years the prospective isolation of distinct progenitor and stem cell populations has become a powerful tool in the hands of developmental biologists and has rendered the investigation of their intrinsic properties possible. In this protocol, we describe how to purify progenitors with different lineage history and degree of differentiation from embryonic and fetal skeletal muscle by fluorescence-activated cell sorting (FACS). The approach takes advantage of a panel of murine strains expressing fluorescent reporter genes specifically in the myogenic progenitors. We provide a detailed description of the dissection procedures and of the enzymatic dissociation required to maximize the yield of mononucleated cells for subsequent FACS-based purification. The procedure takes ~6–7 h to complete and allows for the isolation and the subsequent molecular and phenotypic characterization of developmental myogenic progenitors.

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.

The stepwise development of the lamprey visual system and its evolutionary implications

Lampreys, which represent the oldest group of living vertebrates (cyclostomes), show unique eye development. The lamprey larva has only eyespot-like immature eyes beneath a non-transparent skin, whereas after metamorphosis, the adult has well-developed image-forming camera eyes. To establish a functional visual system, well-organised visual centres as well as motor components (e.g. trunk muscles for locomotion) and interactions between them are needed. Here we review the available knowledge concerning the structure, function and development of the different parts of the lamprey visual system. The lamprey exhibits stepwise development of the visual system during its life cycle. In prolarvae and early larvae, the 'primary' retina does not have horizontal and amacrine cells, but does have photoreceptors, bipolar cells and ganglion cells. At this stage, the optic nerve projects mostly to the pretectum, where the dendrites of neurons in the nucleus of the medial longitudinal fasciculus (nMLF) appear to receive direct visual information and send motor outputs to the neck and trunk muscles. This simple neural circuit may generate negative phototaxis. Through the larval period, the lateral region of the retina grows again to form the 'secondary' retina and the topographic retinotectal projection of the optic nerve is formed, and at the same time, the extra-ocular muscles progressively develop. During metamorphosis, horizontal and amacrine cells differentiate for the first time, and the optic tectum expands and becomes laminated. The adult lamprey then has a sophisticated visual system for image-forming and visual decision-making. In the adult lamprey, the thalamic pathway (retina-thalamus-cortex/pallium) also transmits visual stimuli. Because the primary, simple light-detecting circuit in larval lamprey shares functional and developmental similarities with that of protochordates (amphioxus and tunicates), the visual development of the lamprey provides information regarding the evolutionary transition of the vertebrate visual system from the protochordate-type to the vertebrate-type.

To roll the eyes and snap a bite - function, development and evolution of craniofacial muscles

Craniofacial muscles, muscles that move the eyes, control facial expression and allow food uptake and speech, have long been regarded as a variation on the general body muscle scheme. However, evidence has accumulated that the function of head muscles, their developmental anatomy and the underlying regulatory cascades are distinct. This article reviews the key aspects of craniofacial muscle and muscle stem cell formation and discusses how this differs from the trunk programme of myogenesis; we show novel RNAseq data to support this notion. We also trace the origin of head muscle in the chordate ancestors of vertebrates and discuss links with smooth-type muscle in the primitive chordate pharynx. We look out as to how the special properties of head muscle precursor and stem cells, in particular their competence to contribute to the heart, could be exploited in regenerative medicine.

Comparison of multiple transcriptomes exposes unified and divergent features of quiescent and activated skeletal muscle stem cells

BACKGROUND

Skeletal muscle satellite (stem) cells are quiescent in adult mice and can undergo multiple rounds of proliferation and self-renewal following muscle injury. Several labs have profiled transcripts of myogenic cells during the developmental and adult myogenesis with the aim of identifying quiescent markers. Here, we focused on the quiescent cell state and generated new transcriptome profiles that include subfractionations of adult satellite cell populations, and an artificially induced prenatal quiescent state, to identify core signatures for quiescent and proliferating.

METHODS

Comparison of available data offered challenges related to the inherent diversity of datasets and biological conditions. We developed a standardized workflow to homogenize the normalization, filtering, and quality control steps for the analysis of gene expression profiles allowing the identification up- and down-regulated genes and the subsequent gene set enrichment analysis. To share the analytical pipeline of this work, we developed Sherpa, an interactive Shiny server that allows multi-scale comparisons for extraction of desired gene sets from the analyzed datasets. This tool is adaptable to cell populations in other contexts and tissues.

RESULTS

A multi-scale analysis comprising eight datasets of quiescent satellite cells had 207 and 542 genes commonly up- and down-regulated, respectively. Shared up-regulated gene sets include an over-representation of the TNFα pathway via NFKβ signaling, Il6-Jak-Stat3 signaling, and the apical surface processes, while shared down-regulated gene sets exhibited an over-representation of Myc and E2F targets and genes associated to the G2M checkpoint and oxidative phosphorylation. However, virtually all datasets contained genes that are associated with activation or cell cycle entry, such as the immediate early stress response genes Fos and Jun. An empirical examination of fixed and isolated satellite cells showed that these and other genes were absent in vivo, but activated during procedural isolation of cells.

CONCLUSIONS

Through the systematic comparison and individual analysis of diverse transcriptomic profiles, we identified genes that were consistently differentially expressed among the different datasets and shared underlying biological processes key to the quiescent cell state. Our findings provide impetus to define and distinguish transcripts associated with true in vivo quiescence from those that are first responding genes due to disruption of the stem cell niche.

Srf controls satellite cell fusion through the maintenance of actin architecture

This work describes a crucial role for the transcription factor Srf and F-actin scaffold to drive muscle stem cell fusion in vitro and in vivo and provides evidence of how actin cytoskeleton architecture affects myoblast fusion in vertebrates.

Satellite cells (SCs) are adult muscle stem cells that are mobilized when muscle homeostasis is perturbed. Here, we show that serum response factor (Srf) is needed for optimal SC-mediated hypertrophic growth. We identified Srf as a master regulator of SC fusion required in both fusion partners, whereas it was dispensable for SC proliferation and differentiation. We show that SC-specific Srf deletion leads to impaired actin cytoskeleton and report the existence of finger-like actin–based protrusions at fusion sites in vertebrates that were notoriously absent in fusion-defective myoblasts lacking Srf. Restoration of a polymerized actin network by overexpression of an α-actin isoform in Srf mutant SCs rescued their fusion with a control cell in vitro and in vivo and reestablished overload-induced muscle growth. These findings demonstrate the importance of Srf in controlling the organization of actin cytoskeleton and actin-based protrusions for myoblast fusion in mammals and its requirement to achieve efficient hypertrophic myofiber growth.

The DUX4 homeodomains mediate inhibition of myogenesis and are functionally exchangeable with the Pax7 homeodomain

Facioscapulohumeral muscular dystrophy (FSHD) is caused by inappropriate expression of the double homeodomain protein DUX4. DUX4 has bimodal effects, inhibiting myogenic differentiation and blocking MyoD at low levels of expression, and killing myoblasts at high levels. Pax3 and Pax7, which contain related homeodomains, antagonize the cell death phenotype of DUX4 in C2C12 cells, suggesting some type of competitive interaction. Here, we show that the effects of DUX4 on differentiation and MyoD expression require the homeodomains but do not require the C-terminal activation domain of DUX4. We tested the set of equally related homeodomain proteins (Pax6, Pitx2c, OTX1, Rax, Hesx1, MIXL1 and Tbx1) and found that only Pax3 and Pax7 display phenotypic competition. Domain analysis on Pax3 revealed that the Pax3 homeodomain is necessary for phenotypic competition, but is not sufficient, as competition also requires the paired and transcriptional activation domains of Pax3. Remarkably, substitution mutants in which DUX4 homeodomains are replaced by Pax7 homeodomains retain the ability to inhibit differentiation and to induce cytotoxicity.

Rewiring of an ancestral Tbx1/10-Ebf-Mrf network for pharyngeal muscle specification in distinct embryonic lineages

Skeletal muscles arise from diverse embryonic origins in vertebrates, yet converge on extensively shared regulatory programs that require muscle regulatory factor (MRF)-family genes. Myogenesis in the tail of the simple chordate Ciona exhibits a similar reliance on its single MRF-family gene, and diverse mechanisms activate Ci-Mrf Here, we show that myogenesis in the atrial siphon muscles (ASMs) and oral siphon muscles (OSMs), which control the exhalant and inhalant siphons, respectively, also requires Mrf We characterize the ontogeny of OSM progenitors and compare the molecular basis of Mrf activation in OSM versus ASM. In both muscle types, Ebf and Tbx1/10 are expressed and function upstream of Mrf However, we demonstrate that regulatory relationships between Tbx1/10, Ebf and Mrf differ between the OSM and ASM lineages. We propose that Tbx1, Ebf and Mrf homologs form an ancient conserved regulatory state for pharyngeal muscle specification, whereas their regulatory relationships might be more evolutionarily variable.

Ocular Motor Nerve Development in the Presence and Absence of Extraocular Muscle

Purpose

To spatially and temporally define ocular motor nerve development in the presence and absence of extraocular muscles (EOMs).

Methods

Myf5^cre mice, which in the homozygous state lack EOMs, were crossed to an Isl^MN:GFP reporter line to fluorescently label motor neuron cell bodies and axons. Embryonic day (E) 11.5 to E15.5 wild-type and Myf5^cre/cre:Isl^MN:GFP whole mount embryos and dissected orbits were imaged by confocal microscopy to visualize the developing oculomotor, trochlear, and abducens nerves in the presence and absence of EOMs. E11.5 and E18.5 brainstems were serially sectioned and stained for Islet1 to determine the fate of ocular motor neurons.

Results

At E11.5, all three ocular motor nerves in mutant embryos approached the orbit with a trajectory similar to that of wild-type. Subsequently, while wild-type nerves send terminal branches that contact target EOMs in a stereotypical pattern, the Myf5^cre/cre ocular motor nerves failed to form terminal branches, regressed, and by E18.5 two-thirds of their corresponding motor neurons died. Comparisons between mutant and wild-type embryos revealed novel aspects of trochlear and oculomotor nerve development.

Conclusions

We delineated mouse ocular motor nerve spatial and temporal development in unprecedented detail. Moreover, we found that EOMs are not necessary for initial outgrowth and guidance of ocular motor axons from the brainstem to the orbit but are required for their terminal branching and survival. These data suggest that intermediate targets in the mesenchyme provide cues necessary for appropriate targeting of ocular motor axons to the orbit, while EOM cues are responsible for terminal branching and motor neuron survival.

Dynamics of transcriptional (re)-programming of syncytial nuclei in developing muscles

Background

A stereotyped array of body wall muscles enables precision and stereotypy of animal movements. In Drosophila, each syncytial muscle forms via fusion of one founder cell (FC) with multiple fusion competent myoblasts (FCMs). The specific morphology of each muscle, i.e. distinctive shape, orientation, size and skeletal attachment sites, reflects the specific combination of identity transcription factors (iTFs) expressed by its FC. Here, we addressed three questions: Are FCM nuclei naive? What is the selectivity and temporal sequence of transcriptional reprogramming of FCMs recruited into growing syncytium? Is transcription of generic myogenic and identity realisation genes coordinated during muscle differentiation?

Results

The tracking of nuclei in developing muscles shows that FCM nuclei are competent to be transcriptionally reprogrammed to a given muscle identity, post fusion. In situ hybridisation to nascent transcripts for FCM, FC-generic and iTF genes shows that this reprogramming is progressive, beginning by repression of FCM-specific genes in fused nuclei, with some evidence that FC nuclei retain specific characteristics. Transcription of identity realisation genes is linked to iTF activation and regulated at levels of both transcription initiation rate and period of transcription. The generic muscle differentiation programme is activated independently.

Conclusions

Transcription reprogramming of fused myoblast nuclei is progressive, such that nuclei within a syncytial fibre at a given time point during muscle development are heterogeneous with regards to specific gene transcription. This comprehensive view of the dynamics of transcriptional (re)programming of post-mitotic nuclei within syncytial cells provides a new framework for understanding the transcriptional control of the lineage diversity of multinucleated cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-017-0386-2) contains supplementary material, which is available to authorized users.

Notch ligands regulate the muscle stem-like state ex vivo but are not sufficient for retaining regenerative capacity

Myogenic stem cells are a promising avenue for the treatment of muscular disorders. Freshly isolated muscle stem cells have a remarkable engraftment ability in vivo, but their cell number is limited. Current conventional culture conditions do not allow muscle stem cells to expand in vitro with their bona fide engraftment efficiency, requiring the improvement of culture procedures for achieving successful cell-therapy for muscle disorders. Here we expanded mouse muscle stem cells and human myoblasts with Notch ligands, DLL1, DLL4, and JAG1 to activate Notch signaling in vitro and to investigate whether these cells could retain their engraftment efficiency. Notch signaling promotes the expansion of Pax7+MyoD- mouse muscle stem-like cells and inhibits differentiation even after passage in vitro. Treatment with Notch ligands induced the Notch target genes and generated PAX7+MYOD- stem-like cells from human myoblasts previously cultured on conventional culture plates. However, cells treated with Notch ligands exhibit a stem cell-like state in culture, yet their regenerative ability was less than that of freshly isolated cells in vivo and was comparable to that of the control. These unexpected findings suggest that artificial maintenance of Notch signaling alone is insufficient for improving regenerative capacity of mouse and human donor-muscle cells and suggest that combinatorial events are critical to achieve muscle stem cell and myoblast engraftment potential.

Retinoic acid maintains human skeletal muscle progenitor cells in an immature state

Muscle satellite cells are resistant to cytotoxic agents, and they express several genes that confer resistance to stress, thus allowing efficient dystrophic muscle regeneration after transplantation. However, once they are activated, this capacity to resist to aggressive agents is diminished resulting in massive death of transplanted cells. Although cell immaturity represents a survival advantage, the signalling pathways involved in the control of the immature state remain to be explored. Here, we show that incubation of human myoblasts with retinoic acid impairs skeletal muscle differentiation through activation of the retinoic-acid receptor family of nuclear receptor. Conversely, pharmacologic or genetic inactivation of endogenous retinoic-acid receptors improved myoblast differentiation. Retinoic acid inhibits the expression of early and late muscle differentiation markers and enhances the expression of myogenic specification genes, such as PAX7 and PAX3. These results suggest that the retinoic-acid-signalling pathway might maintain myoblasts in an undifferentiated/immature stage. To determine the relevance of these observations, we characterised the retinoic-acid-signalling pathways in freshly isolated satellite cells in mice and in siMYOD immature human myoblasts. Our analysis reveals that the immature state of muscle progenitors is correlated with high expression of several genes of the retinoic-acid-signalling pathway both in mice and in human. Taken together, our data provide evidences for an important role of the retinoic-acid-signalling pathway in the regulation of the immature state of muscle progenitors.

Pitx2 in Embryonic and Adult Myogenesis

Skeletal muscle is a heterogeneous tissue that represents between 30 and 38% of the human body mass and has important functions in the organism, such as maintaining posture, locomotor impulse, or pulmonary ventilation. The genesis of skeletal muscle during embryonic development is a process controlled by an elaborate regulatory network combining the interplay of extrinsic and intrinsic regulatory mechanisms that transform myogenic precursor cells into functional muscle fibers through a finely tuned differentiation program. However, the capacity of generating muscle still remains once these fibers have matured. Adult myogenesis resembles many of the embryonic morphogenetic episodes and depends on the activation of satellite cells that have the potential to differentiate into new muscle fibers. Pitx2 is a member of the bicoid family of homeodomain transcription factors that play an important role in morphogenesis. In the last decade, Pitx2 has emerged as a key element involved in the fine-tuning mechanism that regulates skeletal-muscle development as well as the differentiation and cell fate of satellite cells in adult muscle. Here we present an integrative view of all aspects of embryonic and adult myogenesis in which Pitx2 is involved, from embryonic development to satellite-cell proliferation, fate specification, and differentiation. Those new Pitx2 functions on satellite-cell biology might open new perspectives to develop therapeutic strategies for muscular disorders.

A novel paradigm links mitochondrial dysfunction with muscle stem cell impairment in sepsis

Sepsis is an acute systemic inflammatory response of the body to microbial infection and a life threatening condition associated with multiple organ failure. Survivors may display long-term disability with muscle weakness that remains poorly understood. Recent data suggest that long-term myopathy in sepsis survivors is due to failure of skeletal muscle stem cells (satellite cells) to regenerate the muscle. Satellite cells impairment in the acute phase of sepsis is linked to unusual mitochondrial dysfunctions, characterized by a dramatic reduction of the mitochondrial mass and hyperactivity of residual organelles. Survivors maintain the impairment of satellite cells, including alterations of the mitochondrial DNA (mtDNA), in the long-term. This condition can be rescued by treatment with mesenchymal stem cells (MSCs) that restore mtDNA alterations and mitochondrial function in satellite cells, and in fine their regenerative potential. Injection of MSCs in turn increases the force of isolated muscle fibers and of the whole animal, and improves the survival rate. These effects occur in the context of reduced inflammation markers that also raised during sepsis. Targeting muscle stem cells mitochondria, in a context of reduced inflammation, may represent a valuable strategy to reduce morbidity and long-term impairment of the muscle upon sepsis.

Characterization and isolation of highly purified porcine satellite cells

Pig is an important food source and an excellent system to model human diseases. Careful characterization of the swine skeletal muscle stem cells (satellite cells) will shed lights on generation of swine skeletal muscle disease model and efficient production of porcine meat for the food industry. Paired box protein 7 (Pax7) is a highly conserved transcription factor shared by satellite cells from various species. However, the sequence of Pax7 has not been characterized in pig. The lack of method to isolate highly purified satellite cells hinders the thorough characterization of the swine satellite cells. Here we found molecular markers for swine satellite cells and revealed that the porcine satellite cells were heterogeneous in various pieces of skeletal muscle. We further developed a method to isolate highly purified satellite cells directly from porcine muscles using fluorescence-activated cell sorting. We next characterized the proliferation and differentiation abilities of isolated satellite cells in vitro; and found that long-term culturing of satellite cells in vitro led to stemness loss.

Twist of fate for skeletal muscle mesenchymal cells

Skeletal muscles are composed of different types of fibres. Can these be thought of as distinct lineages with specific lineage-restricted progenitors? A provocative study now proposes that mesenchymal cells expressing the transcription factor Twist2 act as myogenic progenitors with selective type IIb fibre-differentiation potential.

Transformation of jaw muscle satellite cells to cardiomyocytes

In the embryo a population of progenitor cells known as the second heart field forms not just parts of the heart but also the jaw muscles of the head. Here we show that it is possible to take skeletal muscle satellite cells from jaw muscles of the adult mouse and to direct their differentiation to become heart muscle cells (cardiomyocytes). This is done by exposing the cells to extracellular factors similar to those which heart progenitors would experience during normal embryonic development. By contrast, cardiac differentiation does not occur at all from satellite cells isolated from trunk and limb muscles, which originate from the somites of the embryo. The cardiomyocytes arising from jaw muscle satellite cells express a range of specific marker proteins, beat spontaneously, display long action potentials with appropriate responses to nifedipine, norepinephrine and carbachol, and show synchronized calcium transients. Our results show the existence of a persistent cardiac developmental competence in satellite cells of the adult jaw muscles, associated with their origin from the second heart field of the embryo, and suggest a possible method of obtaining cardiomyocytes from individual patients without the need for a heart biopsy.

Isolation of Muscle Stem Cells from Mouse Skeletal Muscle

Isolation of muscle stem cells from skeletal muscle is a critical step for the study of skeletal myogenesis and regeneration. Although stem cell isolation has been performed for decades, the emergence of flow cytometry with defined cell surface markers, or transgenic mouse models, has allowed the efficient isolation of highly enriched stem cell populations. Here, we describe the isolation of mouse muscle stem cells using two different combinations of enzyme treatments allowing the release of mononucleated muscle stem cells from their niche. Mouse muscle stem cells can be further isolated as a highly enriched population by flow cytometry using fluorescent reporters or cell surface markers. We will present advantages and drawbacks of these different approaches.

Development of Bipotent Cardiac/Skeletal Myogenic Progenitors from MESP1+ Mesoderm

The branchiomeric skeletal muscles co-evolved with new chambers of the heart to enable predatory feeding in chordates. These co-evolved tissues develop from a common population in anterior splanchnic mesoderm, referred to as cardiopharyngeal mesoderm (CPM). The regulation and development of CPM are poorly understood. We describe an embryonic stem cell-based system in which MESP1 drives a PDGFRA+ population with dual cardiac and skeletal muscle differentiation potential, and gene expression resembling CPM. Using this system, we investigate the regulation of these bipotent progenitors, and find that cardiac specification is governed by an antagonistic TGFβ-BMP axis, while skeletal muscle specification is enhanced by Rho kinase inhibition. We define transcriptional signatures of the first committed CPM-derived cardiac and skeletal myogenic progenitors, and discover surface markers to distinguish cardiac (PODXL+) from the skeletal muscle (CDH4+) CPM derivatives. These tools open an accessible window on this developmentally and evolutionarily important population.