Pax3 and Pax7 have distinct and overlapping functions in adult muscle progenitor cells

Enhanced differentiation of human embryonic stem cells to mesenchymal progenitors by inhibition of TGF-beta/activin/nodal signaling using SB-431542

Directing differentiation of human embryonic stem cells (hESCs) into specific cell types using an easy and reproducible protocol is a prerequisite for the clinical use of hESCs in regenerative-medicine procedures. Here, we report a protocol for directing the differentiation of hESCs into mesenchymal progenitor cells. We demonstrate that inhibition of transforming growth factor beta (TGF-beta)/activin/nodal signaling during embryoid body (EB) formation using SB-431542 (SB) in serum-free medium markedly upregulated paraxial mesodermal markers (TBX6, TBX5) and several myogenic developmental markers, including early myogenic transcriptional factors (Myf5, Pax7), as well as myocyte-committed markers [NCAM, CD34, desmin, MHC (fast), alpha-smooth muscle actin, Nkx2.5, cTNT]. Continuous inhibition of TGF-beta signaling in EB outgrowth cultures (SB-OG) enriched for myocyte progenitor cells; markers were PAX7(+) (25%), MYOD1(+) (52%), and NCAM(+) (CD56) (73%). DNA microarray analysis revealed differential upregulation of 117 genes (>2-fold compared with control cells) annotated to myogenic development and function. Moreover, these cells showed the ability to contract (80% of the population) and formed myofibers when implanted intramuscularly in vivo. Interestingly, SB-OG cells cultured in 10% fetal bovine serum (FBS) developed into a homogeneous population of mesenchymal progenitors that expressed CD markers characteristic of mesenchymal stem cells (MSCs): CD44(+) (100%), CD73(+) (98%), CD146(+) (96%), and CD166(+) (88%) with the ability to differentiate into osteoblasts, adipocytes, and chondrocytes in vitro and in vivo. Furthermore, microarray analysis of these cells revealed downregulation of genes related to myogenesis: MYH3 (-167.9-fold), ACTA1 (-161-fold), MYBPH (-139-fold), ACTC (-100.3-fold), MYH8 (-45.5-fold), and MYOT (-41.8-fold) and marked upregulation of genes related to mesoderm-derived cell lineages. In conclusion, our data provides a simple and versatile protocol for directing the differentiation of hESCs into a myogenic lineage and then further into mesenchymal progenitors by blocking the TGF-beta signaling pathway.

Adult muscle 'stem' cells can be sustained in culture as free-floating myospheres

The effectiveness of cell-based therapy to treat muscle disease has been hampered by difficulties in isolating, maintaining and propagating the stem cells that are needed for treatment. Here we report the isolation of muscle-derived stem cells from both young and old mice and their propagation over extended periods of time in culture as "free-floating" myospheres. Analysis of these sphere-forming cells showed that they express stem cell antigen-1 (Sca-1), beta1 integrin (CD29), Thy-1 (CD90), and CD34, but did not express CD45, CD31, or myogenic markers (Pax7, Myf5, and MyoD). We found that cells derived from myospheres and then grown adherently (MDACs) behaved similar to primary myoblasts, in that these cells expressed myogenic markers and were able to easily form multinucleated myotubes. Unlike the parental myospheres but analogous to primary myoblasts, MDACs expressed Pax7, Myf5, and MyoD, indicating that the parent myosphere cells were a more primitive type of cell. In support of this we demonstrated that myospheres were also able to differentiate into adipogenic and osteogenic cells in culture, as well as being able to contribute to injured muscle in vivo. In summary, we report that primitive adult muscle stem cells can be easily isolated and sustained in culture as myospheres.

A Pax3/Dmrt2/Myf5 regulatory cascade functions at the onset of myogenesis

All skeletal muscle progenitor cells in the body derive from the dermomyotome, the dorsal epithelial domain of developing somites. These multipotent stem cells express Pax3, and this expression is maintained in the myogenic lineage where Pax3 plays an important role. Identification of Pax3 targets is therefore important for understanding the mechanisms that underlie the onset of myogenesis. In a microarray screen of Pax3-GFP sorted cells, with analysis on Pax3 gain and loss of function genetic backgrounds, we identify Dmrt2, expressed in the dermomyotome, as a Pax3 target. In vitro gel shift analysis and chromatin immunoprecipitation with in vivo extracts show that Pax3 binds to a conserved 286 bp sequence, situated at −18 kb from Dmrt2. This sequence directs reporter transgene expression to the somite, and this is severely affected when the Pax3 site is mutated in the context of the locus. In Dmrt2 mutant embryos, somite maturation is perturbed and the skeletal muscle of the myotome is abnormal. We now report that the onset of myogenesis is also affected. This depends on activation, in the epaxial dermomyotome, of the myogenic determination gene, Myf5, through its early epaxial enhancer. This sequence contains sites that bind Dmrt2, which belongs to the DM class of DNA–binding proteins. Mutation of these sites compromises activity of the enhancer in transgenic embryos where the reporter transgene is under the control of the Myf5 epaxial enhancer. Transactivation of this site by Dmrt2 is demonstrated in vitro, and conditional overexpression of Dmrt2 in Pax3 expressing cells in the somite confirms the role of this factor in the activation of Myf5. These results reveal a novel genetic network, comprising a Pax3/Dmrt2/Myf5 regulatory cascade that operates in stem cells of the epaxial dermomyotome to initiate skeletal muscle formation.

It is well established that skeletal muscle derives from segmented structures called somites that form on either side of the axis of the embryo. The part of the somite that contains muscle stem cells is called the dermomyotome. These cells express the transcription factor Pax3, which regulates muscle stem cell behaviour. We now show that the Dmrt2 gene, also expressed in the dermomyotome, is directly controlled by Pax3. Since Dmrt2 has been implicated in maintaining the integrity of the dermomyotome, this therefore indicates an upstream role for Pax3 in this structure as well as in controlling cells that form skeletal muscle. Furthermore Dmrt2 directly regulates early activation of the myogenic determination gene, Myf5, required for the formation of the first skeletal muscle in the somite. This is a novel function for Dmrt2 and shows that this transcription factor controls both structure and cell fate. Our results reveal a Pax3/Dmrt2/Myf5 regulatory cascade through which Pax3 orchestrates the onset of myogenesis in the muscle stem cells of the dermomyotome.

Testing the effects of FSHD candidate gene expression in vertebrate muscle development

The genetic lesion leading to facioscapulohumeral muscular dystrophy (FSHD) is a dominant deletion at the 4q35 locus. The generally accepted disease model involves an epigenetic dysregulation in the region resulting in the upregulation of one or more proximal genes whose overexpression specifically affects skeletal muscle. However, multiple FSHD candidate genes have been proposed without clear consensus. Using Xenopus laevis as a model for vertebrate development our lab has studied the effects of overexpression of the FSHD candidate gene ortholog, frg1 (FSHD region gene 1), showing that increased levels of frg1 systemically led specifically to an abnormal musculature and increased angiogenesis, the two most prominent clinical features of FSHD. Here we studied the overexpression effects of three other promising FSHD candidate genes, DUX4, DUX4c, and PITX1 using the same model system and methods for direct comparison. Expression of even very low levels of either DUX4 or pitx1 early in development led to massive cellular loss and severely abnormal development. These abnormalities were not muscle specific. In contrast, elevated levels of DUX4c resulted in no detectable adverse affects on muscle and DUX4c levels did not alter the expression of myogenic regulators. This data supports a model for DUX4 and PITX1 in FSHD only as pro-apoptotic factors if their expression in FSHD is confined to cells within the myogenic pathway; neither could account for the vascular pathology prevalent in FSHD. Taken together, increased frg1 expression alone leads to a phenotype that most closely resembles the pathophysiology observed in FSHD patients.

Sprouty1 regulates reversible quiescence of a self-renewing adult muscle stem cell pool during regeneration

Satellite cells are a heterogeneous population of skeletal muscle specific stem cells capable of self-renewal and differentiation after transplantation. Whether quiescent satellite cells can self-renew and contribute to muscle fiber repair in their endogenous environment in normal regenerating muscle has remained unknown. The transcription factor Pax7 is expressed in satellite cells and is critical for establishing the adult satellite cell pool. Using a temporally-inducible genetic lineage tracing approach (Pax7-CreER^tm; R26R-lacZ) to fate-map adult satellite cells, we show that in response to injury quiescent adult Pax7⁺ cells enter the cell cycle; a subpopulation return to quiescence to fully replenish the satellite cell compartment and the others contribute to de novo muscle fiber formation. We demonstrate that Sprouty1 (Spry1), an inhibitor of receptor tyrosine kinase signaling, is robustly expressed in quiescent Pax7⁺ satellite cells in uninjured adult muscle, down-regulated in proliferating myogenic cells in injured muscles, and re-induced as Pax7⁺ cells return to quiescence in regenerated muscles. We show through deletion of Spry1 specifically in cycling adult Pax7⁺ satellite cells, that Spry1 is required for the return to quiescence and homeostasis of the self-renewing Pax7⁺ satellite cell pool during repair. Satellite cells unable to return to quiescence succumb to apoptosis leading to a diminished self-renewing Pax7-derived satellite cell pool. Our results define a novel role for Spry1 in adult stem cell biology and tissue repair.

Identification and characterization of a non-satellite cell muscle resident progenitor during postnatal development

Satellite cells are resident myogenic progenitors in postnatal skeletal muscle involved in muscle postnatal growth and adult regenerative capacity. Here, we identify and describe a population of muscle-resident stem cells, which are located in the interstitium, that express the cell stress mediator PW1 but do not express other markers of muscle stem cells such as Pax7. PW1(+)/Pax7(-) interstitial cells (PICs) are myogenic in vitro and efficiently contribute to skeletal muscle regeneration in vivo as well as generating satellite cells and PICs. Whereas Pax7 mutant satellite cells show robust myogenic potential, Pax7 mutant PICs are unable to participate in myogenesis and accumulate during postnatal growth. Furthermore, we found that PICs are not derived from a satellite cell lineage. Taken together, our findings uncover a new and anatomically identifiable population of muscle progenitors and define a key role for Pax7 in a non-satellite cell population during postnatal muscle growth.

Repairing skeletal muscle: regenerative potential of skeletal muscle stem cells

Skeletal muscle damaged by injury or by degenerative diseases such as muscular dystrophy is able to regenerate new muscle fibers. Regeneration mainly depends upon satellite cells, myogenic progenitors localized between the basal lamina and the muscle fiber membrane. However, other cell types outside the basal lamina, such as pericytes, also have myogenic potency. Here, we discuss the main properties of satellite cells and other myogenic progenitors as well as recent efforts to obtain myogenic cells from pluripotent stem cells for patient-tailored cell therapy. Clinical trials utilizing these cells to treat muscular dystrophies, heart failure, and stress urinary incontinence are also briefly outlined.

Notch signaling in the regulation of stem cell self-renewal and differentiation

Stem cells are rare and unique precursor cells that participate in the building and rebuilding of tissues and organs during embryogenesis, postnatal growth, and injury repair. Stem cells are distinctively endowed with the ability to both self-renew and differentiate, such that they can replenish the stem cell pool while continuing to produce the differentiated daughter cells that are essential for tissue function. Stem cell self-renewal/differentiation decisions must be carefully controlled during organogenesis, tissue homeostasis, and regeneration, as failure in stem cell maintenance or activation can lead to progressive tissue wasting, while unchecked self-renewal is a hallmark of many cancers. Here, we review evidence implicating the Notch signaling pathway, an evolutionarily conserved cell fate determinant with widespread roles in a variety of tissues and organisms, as a crucial regulator of stem cell behavior. As discussed below, this pathway plays varied and critical roles at multiple stages of organismal development, in lineage-specific differentiation of pluripotent embryonic stem cells, and in controlling stem cell numbers and activity in the context of age-related tissue degeneration, injury-induced tissue repair, and malignancy.

Retention of Pax3 expression in satellite cells of muscle spindles

Intrafusal fibers within muscle spindles retain features characteristic of immaturity, unlike the larger and more numerous extrafusal fibers constituting the bulk of skeletal muscle. Satellite cells (SCs), myogenic progenitors, are detected on the surfaces of both intrafusal and extrafusal fibers, but little is known of spindle SCs. We have recently demonstrated that, like their extrafusal counterparts, SCs in muscle spindles of posthatch chickens express paired box transcription factor 7 (Pax7) protein. During vertebrate embryogenesis, myogenic progenitors express both Pax7 and Pax3 proteins. In postnatal mice, Pax3 appears in rare SC subsets, whereas Pax7 is expressed by all SCs within extrafusal fibers. Here we test the hypothesis that Pax3 protein maintains localized expression within SCs of muscle spindles. Immunohistochemical techniques were used to identify SCs by their Pax7 expression within anterior latissimus dorsi muscle excised from posthatch chickens of various ages. A greater percentage of SCs express Pax3 within intrafusal than extrafusal fibers at each age, and the proportion of SCs expressing Pax3 declines with aging. This is the first study to localize Pax3 expression in posthatch avian muscle and within SCs of muscle spindles. We suggest that Pax3-positive SCs are involved in fiber maintenance.

Muscle stem cells

Despite being mainly composed of highly differentiated contractile fibers, the adult skeletal muscle possesses the remarkable ability to regenerate, following injury. The cells that are responsible for this capacity are the satellite cells, a small population of adult stem cells positioned under the basal lamina of muscle fibers and that can give rise to both differentiated myogenic cells while maintaining a stem cell pool by a self-renewal mechanism. We will discuss here recent publications on the developmental origin of muscle stem cells, on the signaling pathways that affect their proliferation and differentiation, with reference to works on skeletal muscle formation in the embryo as well as the adult, using the mouse and chick as reference models.

An adult tissue-specific stem cell in its niche: a gene profiling analysis of in vivo quiescent and activated muscle satellite cells

The satellite cell of skeletal muscle provides a paradigm for quiescent and activated tissue stem cell states. We have carried out transcriptome analyses on satellite cells purified by flow cytometry from Pax3(GFP/+) mice. We compared samples from adult skeletal muscles where satellite cells are mainly quiescent, with samples from growing muscles or regenerating (mdx) muscles, where they are activated. Analysis of regulation that is shared by both activated states avoids other effects due to immature or pathological conditions. This in vivo profile differs from that of previously analyzed satellite cells activated after cell culture. It reveals how the satellite cell protects itself from damage and maintains quiescence, while being primed for activation on receipt of the appropriate signal. This is illustrated by manipulation of the corepressor Dach1, and by the demonstration that quiescent satellite cells are better protected from oxidative stress than those from mdx or 1-week-old muscles. The quiescent versus in vivo activated comparison also gives new insights into how the satellite cell controls its niche on the muscle fiber through cell adhesion and matrix remodeling. The latter also potentiates growth factor activity through proteoglycan modification. Dismantling the extracellular matrix is important for satellite cell activation when the expression of proteinases is up-regulated, whereas transcripts for their inhibitors are high in quiescent cells. In keeping with this, we demonstrate that metalloproteinase function is required for efficient regeneration in vivo.

Skeletal muscle stem cells in developmental versus regenerative myogenesis

Tissue and organ regeneration proceed in a coordinated manner to restore proper function after trauma. Vertebrate skeletal muscle has a remarkable ability to regenerate after repeated and complete destruction of the tissue, yet limited information is available on how muscle stem and progenitor cells, and other nonmuscle cells, reestablish homeostasis after the regenerative process. The genetic pathways that regulate the establishment of skeletal muscle in the embryo have been studied extensively, and many of the genes that govern muscle stem cell maintenance and commitment are redeployed during adult homeostasis and regeneration. Therefore, correlates can be made between embryonic muscle development and postnatal regeneration. However, there are some important distinctions between prenatal development and regeneration - in the context of the cells, niche, anatomy and the regulatory genes employed. The similarities and distinctions between these two scenarios are the focus of this review.

Pax-3 and Pax-7 label muscle progenitor cells during myotomal myogenesis in Coregonus lavaretus (Teleostei: Coregonidae)

In Coregonus lavaretus, prior the mesoderm segmentation, in cells adjacent to the notochord called adaxial cells MyoD and slow myosin heavy chain (MyHC-slow) proteins were observed. After somite formation, adaxial cells migrate towards the lateral part of the myotome and form a layer of red muscles. Deeper cells differentiate into white muscle fibres. In situ hybridization using Pax-3 molecular probe revealed, that after somitogenesis, Pax-3 is expressed in a layer of cells superficial to the myotome resembling the "external cells" (found in many teleosts species) or dermomyotome described in Amniota. During later developmental stages Pax-3 gene is expressed in cells in intermyotomal space and then in myoblasts between myotubes. In these cells Pax-7 protein was also observed. Pax-3/7 positive cells which have migrated into the myotomes differentiate into satellite cells/secondary myoblasts and participate in hypertrophic and hyperplastic growth of muscles.

The origin, molecular regulation and therapeutic potential of myogenic stem cell populations

Satellite cells, originating in the embryonic dermamyotome, reside beneath the myofibre of mature adult skeletal muscle and constitute the tissue-specific stem cell population. Recent advances following the identification of markers for these cells (including Pax7, Myf5, c-Met and CD34) (CD, cluster of differentiation; c-Met, mesenchymal epithelial transition factor) have led to a greater understanding of the role played by satellite cells in the regeneration of new skeletal muscle during growth and following injury. In response to muscle damage, satellite cells harbour the ability both to form myogenic precursors and to self-renew to repopulate the stem cell niche following myofibre damage. More recently, other stem cell populations including bone marrow stem cells, skeletal muscle side population cells and mesoangioblasts have also been shown to have myogenic potential in culture, and to be able to form skeletal muscle myofibres in vivo and engraft into the satellite cell niche. These cell types, along with satellite cells, have shown potential when used as a therapy for skeletal muscle wasting disorders where the intrinsic stem cell population is genetically unable to repair non-functioning muscle tissue. Accurate understanding of the mechanisms controlling satellite cell lineage progression and self-renewal as well as the recruitment of other stem cell types towards the myogenic lineage is crucial if we are to exploit the power of these cells in combating myopathic conditions. Here we highlight the origin, molecular regulation and therapeutic potential of all the major cell types capable of undergoing myogenic differentiation and discuss their potential therapeutic application.

Adult muscle stem cells avoid death and Paxes

In a recent Nature publication, Lepper et al. (2009) use cell-specific lineage tracing and temporally controllable gene deletion approaches to study muscle stem cell function and reveal age-dependent requirements for Pax gene expression.

Adult satellite cells and embryonic muscle progenitors have distinct genetic requirements

Myogenic potential, survival and expansion of mammalian muscle progenitors depend on the myogenic determinants Pax3 and Pax7 embryonically, and Pax7 alone perinatally. Several in vitro studies support the critical role of Pax7 in these functions of adult muscle stem cells (satellite cells), but a formal demonstration has been lacking in vivo. Here we show, through the application of inducible Cre/loxP lineage tracing and conditional gene inactivation to the tibialis anterior muscle regeneration paradigm, that, unexpectedly, when Pax7 is inactivated in adult mice, mutant satellite cells are not compromised in muscle regeneration, they can proliferate and reoccupy the sublaminal satellite niche, and they are able to support further regenerative processes. Dual adult inactivation of Pax3 and Pax7 also results in normal muscle regeneration. Multiple time points of gene inactivation reveal that Pax7 is only required up to the juvenile period when progenitor cells make the transition into quiescence. Furthermore, we demonstrate a cell-intrinsic difference between neonatal progenitor and adult satellite cells in their Pax7-dependency. Our finding of an age-dependent change in the genetic requirement for muscle stem cells cautions against inferring adult stem-cell biology from embryonic studies, and has direct implications for the use of stem cells from hosts of different ages in transplantation-based therapy.

Muscle stem cell behavior is modified by microRNA-27 regulation of Pax3 expression

Skeletal muscle stem cells are regulated by Pax3/7. During development, Pax3 is required for the maintenance of these cells in the somite and their migration to sites of myogenesis; high levels of Pax3 interfere with muscle cell differentiation, both in the embryo and in the adult. Quantitative fine-tuning of Pax3 is critical, and microRNAs provide a potential mechanism. We identify microRNA-27b (miR-27b), which directly targets the 3'-UTR of Pax3 mRNA, as such a regulator. miR-27b is expressed in the differentiating skeletal muscle of the embryonic myotome and in activated satellite cells of adult muscle. In vivo overexpression of a miR-27b transgene in Pax3-positive cells in the embryo leads to down-regulation of Pax3, resulting in interference with progenitor cell migration and in premature differentiation. In a complementary experiment, miR-27b inhibitors were transfected into cultures of adult muscle satellite cells that normally express miR-27b at the onset of differentiation, when Pax3 protein levels undergo rapid down-regulation. Interference with miR-27b function results in continuing Pax3 expression leading to more proliferation and a delay in the onset of differentiation. Pax7 levels are not affected. Introduction of miR-27b antagomirs at a site of muscle injury in vivo also affects Pax3 expression and regeneration in vivo. We therefore conclude that miR-27b regulates Pax3 protein levels and this down-regulation ensures rapid and robust entry into the myogenic differentiation program.

Enrichment and terminal differentiation of striated muscle progenitors in vitro

Enrichment and terminal differentiation of mammalian striated muscle cells is severely hampered by fibroblast overgrowth, de-differentiation and/or lack of functional differentiation. Herein we report a new, reproducible and simple method to enrich and terminally differentiate muscle stem cells and progenitors from mice and humans. We show that a single gamma irradiation of muscle cells induces their massive differentiation into structurally and functionally intact myotubes and cardiomyocytes and that these cells can be kept in culture for many weeks. Similar results are also obtained when treating skeletal muscle-derived stem cells and progenitors with Mitomycin C.

Activation of RXR and RAR signaling promotes myogenic differentiation of myoblastic C2C12 cells

Differentiation of embryonic and adult myogenic progenitors undergoes a complex series of cell rearrangements and specification events which are controlled by distinct gene regulatory networks. Delineation of the molecular mechanisms that regulate skeletal muscle specification and formation should be important for understanding congenital myopathies and muscular degenerative diseases. Retinoic acid (RA) signaling plays an important role in development. However, the role of RA signaling in adult myogenic progenitors is poorly understood. Here, we investigate the role of RA signaling in regulating myogenic differentiation of myoblastic progenitor cells. Using the mouse myoblast progenitor C2C12 line as a model, we have found that the endogenous expression of most RAR and RXR isotypes is readily detected. While the nuclear receptor co-repressors are highly expressed, two of the three nuclear receptor co-activators and the enzymes involved in RA synthesis are expressed at low level or undetectable, suggesting that the RA signaling pathway may be repressed in myogenic progenitors. Using the alpha-myosin heavy chain promoter-driven reporter (MyHC-GLuc), we have demonstrated that either ATRA or 9CRA is able to effectively induce myogenic differentiation, which can be synergistically enhanced when both ATRA and 9CRA are used. Upon ATRA and 9CRA treatment of C2C12 cells the expression of late myogenic markers significantly increases. We have further shown that adenovirus-mediated exogenous expression of RARalpha and/or RXRalpha is able to effectively induce myogenic differentiation in a ligand-independent fashion. Morphologically, ATRA- and 9CRA-treated C2C12 cells exhibit elongated cell body and become multi-nucleated myoblasts, and even form myoblast fusion. Ultrastructural analysis under transmission electron microscope reveals that RA-treated myogenic progenitor cells exhibit an abundant presence of muscle fibers. Therefore, our results strongly suggest that RA signaling may play an important role in regulating myogenic differentiation.

Muscular dystrophy begins early in embryonic development deriving from stem cell loss and disrupted skeletal muscle formation

Examination of embryonic myogenesis of two distinct, but functionally related, skeletal muscle dystrophy mutants (mdx and cav-3(-/-)) establishes for the first time that key elements of the pathology of Duchenne muscular dystrophy (DMD) and limb-girdle muscular dystrophy type 1C (LGMD-1c) originate in the disruption of the embryonic cardiac and skeletal muscle patterning processes. Disruption of myogenesis occurs earlier in mdx mutants, which lack a functional form of dystrophin, than in cav-3(-/-) mutants, which lack the Cav3 gene that encodes the protein caveolin-3; this finding is consistent with the milder phenotype of LGMD-1c, a condition caused by mutations in Cav3, and the earlier [embryonic day (E)9.5] expression of dystrophin. Myogenesis is severely disrupted in mdx embryos, which display developmental delays; myotube morphology and displacement defects; and aberrant stem cell behaviour. In addition, the caveolin-3 protein is elevated in mdx embryos. Both cav-3(-/-) and mdx mutants (from E15.5 and E11.5, respectively) exhibit hyperproliferation and apoptosis of Myf5-positive embryonic myoblasts; attrition of Pax7-positive myoblasts in situ; and depletion of total Pax7 protein in late gestation. Furthermore, both cav-3(-/-) and mdx mutants have cardiac defects. In cav-3(-/-) mutants, there is a more restricted phenotype comprising hypaxial muscle defects, an excess of malformed hypertrophic myotubes, a twofold increase in myonuclei, and reduced fast myosin heavy chain (FMyHC) content. Several mdx mutant embryo pathologies, including myotube hypotrophy, reduced myotube numbers and increased FMyHC, have reciprocity with cav-3(-/-) mutants. In double mutant (mdxcav-3(+/-)) embryos that are deficient in dystrophin (mdx) and heterozygous for caveolin-3 (cav-3(+/-)), whereby caveolin-3 is reduced to 50% of wild-type (WT) levels, these phenotypes are severely exacerbated: intercostal muscle fibre density is reduced by 71%, and Pax7-positive cells are depleted entirely from the lower limbs and severely attenuated elsewhere; these data suggest a compensatory rather than a contributory role for the elevated caveolin-3 levels that are found in mdx embryos. These data establish a key role for dystrophin in early muscle formation and demonstrate that caveolin-3 and dystrophin are essential for correct fibre-type specification and emergent stem cell function. These data plug a significant gap in the natural history of muscular dystrophy and will be invaluable in establishing an earlier diagnosis for DMD/LGMD and in designing earlier treatment protocols, leading to better clinical outcome for these patients.

A distinct profile of myogenic regulatory factor detection within Pax7+ cells at S phase supports a unique role of Myf5 during posthatch chicken myogenesis

Satellite cells are skeletal muscle stem cells that provide myogenic progeny for myofiber growth and repair. Temporal expression of muscle regulatory factors (MRFs) and the paired box transcription factor Pax7 defines characteristic phases of proliferation (Pax7(+)/MyoD(+)/myogenin(-)) and differentiation (Pax7(-)/MyoD(+)/myogenin(+)) during myogenesis of satellite cells. Here, using bromodeoxyuridine (BrdU) labeling and triple immunodetection, we analyzed expression patterns of Pax7 and the MRFs MyoD, Myf5, or myogenin within S phase myoblasts prepared from posthatch chicken muscle. Essentially, all BrdU incorporation was restricted to Pax7(+) cells, of which the majority also expressed MyoD. The presence of a minor BrdU(+)/Pax7(+)/myogenin(+) population in proliferation stage cultures suggests that myogenin up-regulation is alone insufficient for terminal differentiation. Myf5 was detected strictly within Pax7(+) cells and decreased during S phase while MyoD presence persisted in cycling cells. This study provides novel data in support of a unique role for Myf5 during posthatch myogenesis.

Credentialing a preclinical mouse model of alveolar rhabdomyosarcoma

The highly aggressive muscle cancer alveolar rhabdomyosarcoma (ARMS) is one of the most common soft tissue sarcoma of childhood, yet the outcome for the unresectable and metastatic disease is dismal and unchanged for nearly three decades. To better understand the pathogenesis of this disease and to facilitate novel preclinical approaches, we previously developed a conditional mouse model of ARMS by faithfully recapitulating the genetic mutations observed in the human disease, i.e., activation of Pax3:Fkhr fusion gene with either p53 or Cdkn2a inactivation. In this report, we show that this model recapitulates the immunohistochemical profile and the rapid progression of the human disease. We show that Pax3:Fkhr expression increases during late preneoplasia but tumor cells undergoing metastasis are under apparent selection for Pax3:Fkhr expression. At a whole-genome level, a cross-species gene set enrichment analysis and metagene projection study showed that our mouse model is most similar to human ARMS when compared with other pediatric cancers. We have defined an expression profile conserved between mouse and human ARMS, as well as a Pax3:Fkhr signature, including the target gene, SKP2. We further identified 7 "druggable" kinases overexpressed across species. The data affirm the accuracy of this genetically engineered mouse model.

New multiple labelling method for improved satellite cell identification in human muscle: application to a cohort of power-lifters and sedentary men

Presently applied methods to identify and quantify human satellite cells (SCs) give discrepant results. We introduce a new immunofluorescence method that simultaneously monitors two SC markers (NCAM and Pax7), the basal lamina and nuclei. Biopsies from power-lifters, power-lifters using anabolic substances and untrained subjects were re-examined. Significantly different results from those with staining for NCAM and nuclei were observed. There were three subtypes of SCs; NCAM(+)/Pax7(+) (94%), NCAM(+)/Pax7(-) (4%) and NCAM(-)/Pax7(+) (1%) but large individual variability existed. The proportion of SCs per nuclei within the basal lamina of myofibres (SC/N) was similar for all groups reflecting a balance between the number of SCs and myonuclei to maintain homeostasis. We emphasise that it is important to quantify both SC/N and the number of SCs per fibre. Our multiple marker method is more reliable for SC identification and quantification and can be used to evaluate other markers of muscle progenitor cells.

Skeletal muscle as a paradigm for regenerative biology and medicine

Tissue development and regeneration share common features, since modules of regulatory pathways and transcription factors that are crucial for prenatal development are redeployed for tissue reconstruction after trauma. Regenerative medicine has therefore gained important insights through the study of developmental and regenerative biology. Moreover, diverse experimental models have been used to investigate the regeneration process in different tissues and organs. Paradoxically, little is known regarding the relative contribution of stem cells with respect to the supporting tissue during tissue regeneration. Particular attention will be given to mouse models using distinct injury paradigms to investigate the regenerative biology of skeletal muscle. An understanding of the response of stem and parenchymal cells is crucial for the development of clinical strategies to combat the normal decline in tissue performance during aging or its reconstitution after trauma and during disease. This review addresses these issues, focusing on muscle regeneration and how different factors, including genes, cells and the environment, impinge on this process.

The origin of embryonic and fetal myoblasts: a role of Pax3 and Pax7

Skeletal muscle is a heterogeneous tissue composed of individual muscle fibers, diversified in size, shape, and contractile protein content, to fulfill the different functional needs of the vertebrate body. This heterogeneity derives from and depends at least in part on distinct classes of myogenic progenitors; i.e., embryonic and fetal myoblasts and satellite cells whose origin and lineage relationship have been elusive so far. In this issue of Genes & Development, Hutcheson and colleagues (pp. 997-1013) provide a first answer to this question.

Muscle hypertrophy driven by myostatin blockade does not require stem/precursor-cell activity

Myostatin, a member of the TGF-beta family, has been identified as a powerful inhibitor of muscle growth. Absence or blockade of myostatin induces massive skeletal muscle hypertrophy that is widely attributed to proliferation of the population of muscle fiber-associated satellite cells that have been identified as the principle source of new muscle tissue during growth and regeneration. Postnatal blockade of myostatin has been proposed as a basis for therapeutic strategies to combat muscle loss in genetic and acquired myopathies. But this approach, according to the accepted mechanism, would raise the threat of premature exhaustion of the pool of satellite cells and eventual failure of muscle regeneration. Here, we show that hypertrophy in the absence of myostatin involves little or no input from satellite cells. Hypertrophic fibers contain no more myonuclei or satellite cells and myostatin had no significant effect on satellite cell proliferation in vitro, while expression of myostatin receptors dropped to the limits of detectability in postnatal satellite cells. Moreover, hypertrophy of dystrophic muscle arising from myostatin blockade was achieved without any apparent enhancement of contribution of myonuclei from satellite cells. These findings contradict the accepted model of myostatin-based control of size of postnatal muscle and reorient fundamental investigations away from the mechanisms that control satellite cell proliferation and toward those that increase myonuclear domain, by modulating synthesis and turnover of structural muscle fiber proteins. It predicts too that any benefits of myostatin blockade in chronic myopathies are unlikely to impose any extra stress on the satellite cells.

Further characterisation of the molecular signature of quiescent and activated mouse muscle satellite cells

Satellite cells are the resident stem cells of adult skeletal muscle. To date though, there is a paucity of native markers that can be used to easily identify quiescent satellite cells, with Pax7 probably being the best that is currently available. Here we have further characterized a number of recently described satellite cell markers, and also describe novel ones. Caveolin-1, integrin α7 and the calcitonin receptor proved reliable markers for quiescent satellite cells, being expressed by all satellite cells identified with Pax7. These three markers remained expressed as satellite cells were activated and underwent proliferation. The nuclear envelope proteins lamin A/C and emerin, mutations in which underlie Emery-Dreifuss muscular dystrophy, were also expressed in both quiescent and proliferating satellite cells. Conversely, Jagged-1, a Notch ligand, was not expressed in quiescent satellite cells but was induced upon activation. These findings further contribute to defining the molecular signature of muscle satellite cells.

Embryonic and fetal limb myogenic cells are derived from developmentally distinct progenitors and have different requirements for beta-catenin

Vertebrate muscle arises sequentially from embryonic, fetal, and adult myoblasts. Although functionally distinct, it is unclear whether these myoblast classes develop from common or different progenitors. Pax3 and Pax7 are expressed by somitic myogenic progenitors and are critical myogenic determinants. To test the developmental origin of embryonic and fetal myogenic cells in the limb, we genetically labeled and ablated Pax3(+) and Pax7(+) cells. Pax3(+)Pax7(-) cells contribute to muscle and endothelium, establish and are required for embryonic myogenesis, and give rise to Pax7(+) cells. Subsequently, Pax7(+) cells give rise to and are required for fetal myogenesis. Thus, Pax3(+) and Pax7(+) cells contribute differentially to embryonic and fetal limb myogenesis. To investigate whether embryonic and fetal limb myogenic cells have different genetic requirements we conditionally inactivated or activated beta-catenin, an important regulator of myogenesis, in Pax3- or Pax7-derived cells. beta-Catenin is necessary within the somite for dermomyotome and myotome formation and delamination of limb myogenic progenitors. In the limb, beta-catenin is not required for embryonic myoblast specification or myofiber differentiation but is critical for determining fetal progenitor number and myofiber number and type. Together, these studies demonstrate that limb embryonic and fetal myogenic cells develop from distinct, but related progenitors and have different cell-autonomous requirements for beta-catenin.

Mechanisms of lipase maturation

Lipases are acyl hydrolases that represent a diverse group of enzymes present in organisms ranging from prokaryotes to humans. This article focuses on an evolutionarily related family of extracellular lipases that include lipoprotein lipase, hepatic lipase and endothelial lipase. As newly synthesized proteins, these lipases undergo a series of co- and post-translational maturation steps occurring in the endoplasmic reticulum, including glycosylation and glycan processing, and protein folding and subunit assembly. This article identifies and discusses mechanisms that direct early and late events in lipase folding and assembly. Lipase maturation employs the two general chaperone systems operating in the endoplasmic reticulum, as well as a recently identified lipase-specific chaperone termed lipase maturation factor 1. We propose that the two general chaperone systems act in a coordinated manner early in lipase maturation in order to help create partially folded monomers; lipase maturation factor 1 then facilitates final monomer folding and subunit assembly into fully functional homodimers. Once maturation is complete, the lipases exit the endoplasmic reticulum and are secreted to extracellular sites, where they carry out a number of functions related to lipoprotein and lipid metabolism.

Extrinsic regulation of domestic animal-derived myogenic satellite cells II

The existence of myogenic satellite cells was reported some 47 years ago, and, since that time, satellite cell research has flourished. So much new information is generated (daily) on these cells that it can be difficult for individuals to keep abreast of important issues related to their activation and proliferation, the modulation of the activity of other cell types, the differentiation of the cells to facilitate normal skeletal muscle growth and development, or to the repair of damaged myofibers. The intent of this review is to summarize new information about the extrinsic regulation of myogenic satellite cells and to provide specific mechanisms involved in altering satellite cell physiology. Where possible, examples from agriculturally important animals are used for illustrative purposes.

Analysis of human muscle stem cells reveals a differentiation-resistant progenitor cell population expressing Pax7 capable of self-renewal

Studies using mouse models have established a critical role for resident satellite stem cells in skeletal muscle development and regeneration, but little is known about this paradigm in human muscle. Here, using human muscle stem cells, we address their lineage progression, differentiation, migration, and self-renewal. Isolated human satellite cells expressed alpha7-integrin and other definitive muscle markers, were highly motile on laminin substrates and could undergo efficient myotube differentiation and myofibrillogenesis. However, only a subpopulation of the myoblasts expressed Pax7 and displayed a variable lineage progression as measured by desmin and MyoD expression. Analysis identified a differentiation-resistant progenitor cell population that was Pax7+/desmin- and capable of self-renewal. This study extends our understanding of the role of Pax7 in regulating human satellite stem cell differentiation and self-renewal.

Integrated functions of Pax3 and Pax7 in the regulation of proliferation, cell size and myogenic differentiation

Pax3 and Pax7 are paired-box transcription factors with roles in developmental and adult regenerative myogenesis. Pax3 and Pax7 are expressed by postnatal satellite cells or their progeny but are down regulated during myogenic differentiation. We now show that constitutive expression of Pax3 or Pax7 in either satellite cells or C2C12 myoblasts results in an increased proliferative rate and decreased cell size. Conversely, expression of dominant-negative constructs leads to slowing of cell division, a dramatic increase in cell size and altered morphology. Similarly to the effects of Pax7, retroviral expression of Pax3 increases levels of Myf5 mRNA and MyoD protein, but does not result in sustained inhibition of myogenic differentiation. However, expression of Pax3 or Pax7 dominant-negative constructs inhibits expression of Myf5, MyoD and myogenin, and prevents differentiation from proceeding. In fibroblasts, expression of Pax3 or Pax7, or dominant-negative inhibition of these factors, reproduce the effects on cell size, morphology and proliferation seen in myoblasts. Our results show that in muscle progenitor cells, Pax3 and Pax7 function to maintain expression of myogenic regulatory factors, and promote population expansion, but are also required for myogenic differentiation to proceed.

Activation of Pax3 target genes is necessary but not sufficient for neurogenesis in the ophthalmic trigeminal placode

Vertebrate cranial neurogenic placodes are relatively simple model systems for investigating the control of sensory neurogenesis. The ophthalmic trigeminal (opV) placode, for which the earliest specific marker is the paired domain homeodomain transcription factor Pax3, forms cutaneous sensory neurons in the ophthalmic lobe of the trigeminal ganglion. We previously showed that Pax3 expression in avian opV placode cells correlates with specification and commitment to a Pax3+, cutaneous sensory neuron fate. Pax3 can act as a transcriptional activator or repressor, depending on the cellular context. We show using mouse Splotch(2H) mutants that Pax3 is necessary for the normal neuronal differentiation of opV placode cells. Using an electroporation construct encoding a Pax3-Engrailed fusion protein, which represses Pax3 target genes, we show that activation of Pax3 target genes is required cell-autonomously within chick opV placode cells for expression of the opV placode markers FGFR4 and Ngn2, maintenance of the preplacodal marker Eya2, expression of Pax3 itself (suggesting that Pax3 autoregulates), neuronal differentiation and delamination. Mis-expression of Pax3 in head ectoderm is sufficient to induce FGFR4 and Ngn2 expression, but neurons do not differentiate, suggesting that additional signals are necessary to enable Pax3+ cells to differentiate as neurons. Mis-expression of Pax3 in the Pax2+ otic and epibranchial placodes also downregulates Pax2 and disrupts otic vesicle closure, suggesting that Pax3 is sufficient to alter the identity of these cells. Overall, our results suggest that activation of Pax3 target genes is necessary but not sufficient for neurogenesis in the opV placode.

Generation of transplantable, functional satellite-like cells from mouse embryonic stem cells

Satellite cells are myogenic stem cells responsible for the postnatal regeneration of skeletal muscle. Here we report the successful in vitro induction of Pax7-positive satellite-like cells from mouse embryonic stem (mES) cells. Embryoid bodies were generated from mES cells and cultured on Matrigel-coated dishes with Dulbecco's modified Eagle medium containing fetal bovine serum and horse serum. Pax7-positive satellite-like cells were enriched by fluorescence-activated cell sorting using a novel anti-satellite cell antibody, SM/C-2.6. SM/C-2.6-positive cells efficiently differentiate into skeletal muscle fibers both in vitro and in vivo. Furthermore, the cells demonstrate satellite cell characteristics such as extensive self-renewal capacity in subsequent muscle injury model, long-term engraftment up to 24 wk, and the ability to be secondarily transplanted with remarkably high engraftment efficiency compared to myoblast transplantation. This is the first report of transplantable, functional satellite-like cells derived from mES cells and will provide a foundation for new therapies for degenerative muscle disorders.

Stem cells and plasticity of skeletal muscle cell differentiation: potential application to cell therapy for degenerative muscular diseases

Degenerative muscular diseases, such as muscular dystrophies, have been the representative targets of regenerative cell therapy. Although satellite cells play central roles in skeletal muscle regeneration that intrinsically occurs after muscle injury, their application to cell therapy is confronted by difficulties. Other stem cells expected to be applicable to cell therapy include muscle-resident stem cells and nonmuscle-resident stem cells. Moreover, dedifferentiated cells of skeletal muscle might provide unique system for cell therapy. Terminally differentiated myotubes have plasticity of differentiation and dedifferentiate under certain experimental conditions, including the expression of SV40 large T antigen or the homeobox gene Msx1. The dedifferentiated cells exhibit multipotency to transdifferentiate into multiple mesenchymal origin cells. In addition, fibroblasts or undifferentiated myoblasts treated with a drug acquire multipotency. These cells may open new doors in cell therapy.

Identification and characterization of subpopulations of Pax3 and Pax7 expressing cells in developing chick somites and limb buds

Pax3 and Pax7 are closely related paired-boxed family transcription factors that are known to play important roles in embryonic and adult myogenesis. Previous reports describing the expression of Pax3 and Pax7 transcripts reveal expression in many overlapping domains. In this manuscript, we extend these studies by examining the protein expression profiles for Pax3 and Pax7 in developing chick somites and limbs with cellular resolution. Our studies show the existence of distinct subpopulations of cells in the somite and developing limb that are defined by the relative expression levels of Pax3 and Pax7. We also show that Pax3 and Pax7 negatively regulate each other's expression in the dermomyotome, thus providing a possible mechanism for the maintenance of observed expression patterns in the dermomyotome. Further characterization of Pax3- and/or Pax7-positive cells in the dermomyotome and myotome with respect to proliferation and differentiation reveals subpopulations of cells with distinct properties.

The role of Delta-like 1 shedding in muscle cell self-renewal and differentiation

Myogenic cells have the ability to adopt two divergent fates upon exit from the cell cycle: differentiation or self-renewal. The Notch signaling pathway is a well-known negative regulator of myogenic differentiation. Using mouse primary myoblasts cultured in vitro or C2C12 myogenic cells, we found that Notch activity is essential for maintaining the expression of Pax7, a transcription factor associated with the self-renewal lineage, in quiescent undifferentiated myoblasts after they exit the cell cycle. Stimulation of the Notch pathway by expression of a constitutively active Notch-1, or co-culture of myogenic cells with CHO cells transfected with Delta like-1 (DLL1), increases the level of Pax7. DLL1, a ligand for Notch receptor, is shed by ADAM metalloproteases in a pool of Pax7+ C2C12 reserve cells, but it remains intact in differentiated myotubes. DLL1 shedding changes the receptor/ligand ratio and modulates the level of Notch signaling. Inhibition of DLL1 cleavage by a soluble, dominant-negative mutant form of ADAM12 leads to elevation of Notch signaling, inhibition of differentiation, and expansion of the pool of self-renewing Pax7+/MyoD- cells. These results suggest that ADAM-mediated shedding of DLL1 in a subset of cells during myogenic differentiation in vitro contributes to downregulation of Notch signaling in neighboring cells and facilitates their progression into differentiation. We propose that the proteolytic processing of DLL1 helps achieve an asymmetry in Notch signaling in initially equivalent myogenic cells and helps sustain the balance between differentiation and self-renewal.

An isogenetic myoblast expression screen identifies DUX4-mediated FSHD-associated molecular pathologies

Facioscapulohumeral muscular dystrophy (FSHD) is caused by an unusual deletion with neomorphic activity. This deletion derepresses genes in cis; however which candidate gene causes the FSHD phenotype, and through what mechanism, is unknown. We describe a novel genetic tool, inducible cassette exchange, enabling rapid generation of isogenetically modified cells with conditional and variable transgene expression. We compare the effects of expressing variable levels of each FSHD candidate gene on myoblasts. This screen identified only one gene with overt toxicity: DUX4 (double homeobox, chromosome 4), a protein with two homeodomains, each similar in sequence to Pax3 and Pax7. DUX4 expression recapitulates key features of the FSHD molecular phenotype, including repression of MyoD and its target genes, diminished myogenic differentiation, repression of glutathione redox pathway components, and sensitivity to oxidative stress. We further demonstrate competition between DUX4 and Pax3/Pax7: when either Pax3 or Pax7 is expressed at high levels, DUX4 is no longer toxic. We propose a hypothesis for FSHD in which DUX4 expression interferes with Pax7 in satellite cells, and inappropriately regulates Pax targets, including myogenic regulatory factors, during regeneration.

Prospective isolation of skeletal muscle stem cells with a Pax7 reporter

Muscle regeneration occurs through activation of quiescent satellite cells whose progeny proliferate, differentiate, and fuse to make new myofibers. We used a transgenic Pax7-ZsGreen reporter mouse to prospectively isolate stem cells of skeletal muscle by flow cytometry. We show that Pax7-expressing cells (satellite cells) in the limb, head, and diaphragm muscles are homogeneous in size and granularity and uniformly labeled by certain cell surface markers, including CD34 and CD29. The frequency of the satellite cells varies between muscle types and with age. Clonal analysis demonstrated that all colonies arising from single cells within the Pax7-sorted fraction have myogenic potential. In response to injury, Pax7(+) cells reduce CD34, CD29, and CXCR4 expression, increase in size, and acquire Sca-1. When directly isolated and cultured in vitro, Pax7(+) cells display the hallmarks of activation and proliferate, initially as suspension aggregates and later distributed between suspension and adherence. During in vitro expansion, Pax7 (ZsGreen) and CD34 expression decline, whereas expression of PSA-NCAM is acquired. The nonmyogenic, Pax7(neg) cells expand as Sca1(+) PDGRalpha(+) PSA-NCAM(neg) cells. Satellite cells expanded exclusively in suspension can engraft and produce dystrophin(+) fibers in mdx(-/-) mice. These results establish a novel animal model for the study of muscle stem cell physiology and a culture system for expansion of engraftable muscle progenitors.

All muscle satellite cells are equal, but are some more equal than others?

Skeletal muscle is an accessible adult stem-cell model in which differentiated myofibres are maintained and repaired by a self-renewing stem-cell compartment. These resident stem cells, which are known as satellite cells, lie on the surface of the muscle fibre, between the plasmalemma and overlying basal lamina. Although they are normally mitotically quiescent in adult muscle, satellite cells can be activated when needed to generate myoblasts, which eventually differentiate to provide new myonuclei for the homeostasis, hypertrophy and repair of muscle fibres, or fuse together to form new myofibres for regeneration. Satellite cells also self-renew in order to maintain a viable stem-cell pool that is able to respond to repeated demand. The study of the control of self-renewal has led to the idea that the satellite-cell pool might be heterogeneous: that is it might contain both self-renewing satellite `stem' cells and myogenic precursors with limited replicative potential in the same anatomical location. The regulatory circuits that control satellite-cell self-renewal are beginning to be deciphered, with Pax7, and Notch and Wnt signalling being clearly implicated. This Commentary seeks to integrate these interesting new findings into the wider context of satellite-cell biology, and to highlight some of the many outstanding questions.

A gradient of Shh establishes mutually repressing somitic cell fates induced by Nkx3.2 and Pax3

Wnt and Sonic Hedgehog (Shh) signals are known to pattern the somite into dermomyotomal, myotomal and sclerotomal cell fates. By employing explants of presomitic mesoderm cultured with constant levels of Wnt3a conditioned medium and increasing levels of Shh, we found that differing levels of Shh signaling elicit differing responses from somitic cells: the lowest level of Shh signaling allows dermomyotomal gene expression, intermediate levels induce loss of dermomyotomal markers and activation of myogenic differentiation, and higher levels induce loss of myotomal markers and activation of sclerotomal gene expression. In addition, we have found that in the presence of high levels of Wnt signaling, instead of inducing sclerotomal markers, Shh signals act to maintain the expression of dermomyotomal and myotomal markers. One of the sclerotomal genes induced by high levels of Shh signaling is Nkx3.2. Forced expression of Nkx3.2 blocks somitic expression of the dermomyotomal marker Pax3 both in vitro and in vivo. Conversely, forced expression of Pax3 in somites can block Shh-mediated induction of sclerotomal gene expression and chondrocyte differentiation in vitro. Thus we propose that varying levels of Shh signaling act in a morphogen-like manner to elicit differing responses from somitic cells, and that Pax3 and Nkx3.2 set up mutually repressing cell fates that promote either dermomyotome/myotome or sclerotome differentiation, respectively.

PAX3/7 expression coincides with MyoD during chronic skeletal muscle overload

Paired box (Pax) proteins 3 and 7 are key determinants for embryonic skeletal muscle development by initiating myogenic regulatory factor (MRF) gene expression. We show that Pax3 and 7 participate in adult skeletal muscle plasticity during the initial responses to chronic overload (< or =7 days) and appear to coordinate MyoD expression, a member of the MRF family of genes. Pax3 and 7 mRNA were higher than control within 12 h after initiation of overload, preceded the increase in MyoD mRNA on day 1, and peaked on day 2. On days 3 and 7, Pax7 mRNA remained higher than control, suggesting that satellite cell self-renewal was occurring. Pax3 and 7 and MyoD protein levels were higher than control on days 2 and 3. These data indicate that Pax3 and 7 coordinate the recapitulation of developmental-like regulatory mechanisms in response to growth-inducing stimuli in adult skeletal muscle, presumably through activation of satellite cells.

Regenerative pharmacology in the treatment of genetic diseases: the paradigm of muscular dystrophy

Current evidence supports the therapeutic potential of pharmacological interventions that counter the progression of genetic disorders by promoting regeneration of the affected organs or tissues. The rationale behind this concept lies on the evidence that targeting key events downstream of the genetic defect can compensate, at least partially, the pathological consequence of the related disease. In this regard, the beneficial effect exerted on animal models of muscular dystrophy by pharmacological strategies that enhance muscle regeneration provides an interesting paradigm. In this review, we describe and discuss the potential targets of pharmacological strategies that promote regeneration of dystrophic muscles and alleviate the consequence of the primary genetic defect. Regenerative pharmacology provides an immediate and suitable therapeutic opportunity to slow down the decline of muscles in the present generation of dystrophic patients, with the perspective to hold them in conditions such that they could benefit of future, more definitive, therapies.