Cell-cell interaction modulates myoD-induced skeletal myogenesis of pluripotent P19 cells in vitro

Myogenic potential of mouse embryonic stem cells lacking functional Pax7 tested in vitro by 5-azacitidine treatment and in vivo in regenerating skeletal muscle

Regeneration of skeletal muscle relies on the presence of satellite cells. Satellite cells deficiency accompanying some degenerative diseases is the reason for the search for the "replacement cells" that can be used in the muscle therapies. Due to their unique properties embryonic stem cells (ESCs), as well as myogenic cells derived from them, are considered as a promising source of therapeutic cells. Among the factors crucial for the specification of myogenic precursor cells is Pax7 that sustains proper function of satellite cells. In our previous studies we showed that ESCs lacking functional Pax7 are able to form myoblasts in vitro when differentiated within embryoid bodies and their outgrowths. In the current study we showed that ESCs lacking functional Pax7, cultured in vitro in monolayer in the medium supplemented with horse serum and 5azaC, expressed higher levels of factors associated with myogenesis, such as Pdgfra, Pax3, Myf5, and MyoD. Importantly, skeletal myosin immunolocalization confirmed that myogenic differentiation of ESCs was more effective in case of cells lacking Pax7. Our in vivo studies showed that ESCs transplanted into regenerating skeletal muscles were detectable at day 7 of regeneration and the number of Pax7-/- ESCs detected was significantly higher than of control cells. Our results support the concept that lack of functional Pax7 promotes proliferation of differentiating ESCs and for this reason more of them can turn into myogenic lineage.

Myogenic Differentiation of Mouse Embryonic Stem Cells That Lack a Functional Pax7 Gene

The transcription factor Pax7 plays a key role during embryonic myogenesis and sustains the proper function of satellite cells, which serve as adult skeletal muscle stem cells. Overexpression of Pax7 has been shown to promote the myogenic differentiation of pluripotent stem cells. However, the effects of the absence of functional Pax7 in differentiating embryonic stem cells (ESCs) have not yet been directly tested. Herein, we studied mouse stem cells that lacked a functional Pax7 gene and characterized the differentiation of these stem cells under conditions that promoted the derivation of myoblasts in vitro. We analyzed the expression of myogenic factors, such as myogenic regulatory factors and muscle-specific microRNAs, in wild-type and mutant cells. Finally, we compared the transcriptome of both types of cells and did not find substantial differences in the expression of genes related to the regulation of myogenesis. As a result, we showed that the absence of functional Pax7 does not prevent the in vitro myogenic differentiation of ESCs.

Molecular and cellular regulation of skeletal myogenesis

Since the seminal discovery of the cell-fate regulator Myod, studies in skeletal myogenesis have inspired the search for cell-fate regulators of similar potential in other tissues and organs. It was perplexing that a similar transcription factor for other tissues was not found; however, it was later discovered that combinations of molecular regulators can divert somatic cell fates to other cell types. With the new era of reprogramming to induce pluripotent cells, the myogenesis paradigm can now be viewed under a different light. Here, we provide a short historical perspective and focus on how the regulation of skeletal myogenesis occurs distinctly in different scenarios and anatomical locations. In addition, some interesting features of this tissue underscore the importance of reconsidering the simple-minded view that a single stem cell population emerges after gastrulation to assure tissuegenesis. Notably, a self-renewing long-term Pax7+ myogenic stem cell population emerges during development only after a first wave of terminal differentiation occurs to establish a tissue anlagen in the mouse. How the future stem cell population is selected in this unusual scenario will be discussed. Recently, a wealth of information has emerged from epigenetic and genome-wide studies in myogenic cells. Although key transcription factors such as Pax3, Pax7, and Myod regulate only a small subset of genes, in some cases their genomic distribution and binding are considerably more promiscuous. This apparent nonspecificity can be reconciled in part by the permissivity of the cell for myogenic commitment, and also by new roles for some of these regulators as pioneer transcription factors acting on chromatin state.

Enhancing myogenic differentiation of pluripotent stem cells with small molecule inducers

Pluripotent stem cells are able to differentiate into many types of cell lineages in response to differentiation cues. However, a pure population of lineage-specific cells is desirable for any potential clinical application. Therefore, induction of the pluripotent stem cells with lineage-specific regulatory signals, or small molecule inducers, is a prerequisite for effectively directing lineage specification for cell-based therapeutics. In this article, we provide in-depth analysis of recent research findings on small molecule inducers of the skeletal muscle lineage. We also provide perspectives on how different signaling pathways and chromatin dynamics converge to direct the differentiation of skeletal myocytes.

Mouse and human pluripotent stem cells and the means of their myogenic differentiation

Pluripotent stem cells, such as embryonic stem cells and induced pluripotent stem cells, are an important tool in the studies focusing at the differentiation of various cell types, including skeletal myoblasts. They are also considered as a source of the cells that due to their pluripotent character and availability could be turned into any required tissue and then used in future in regenerative medicine. However, the methods of the derivation of some of cell types from pluripotent cells still need to be perfected. This chapter summarizes the history and current advancements in the derivation and testing of pluripotent stem cells-derived skeletal myoblasts. It focuses at the in vitro methods allowing the differentiation of stem cells grown in monolayer or propagated as embryoid bodies, and also at in vivo tests allowing the verification of the functionality of obtained skeletal myoblasts.

Gli2 and MEF2C activate each other's expression and function synergistically during cardiomyogenesis in vitro

The transcription factors Gli2 (glioma-associated factor 2), which is a transactivator of Sonic Hedgehog (Shh) signalling, and myocyte enhancer factor 2C (MEF2C) play important roles in the development of embryonic heart muscle and enhance cardiomyogenesis in stem cells. Although the physiological importance of Shh signalling and MEF2 factors in heart development is well known, the mechanistic understanding of their roles is unclear. Here, we demonstrate that Gli2 and MEF2C activated each other's expression while enhancing cardiomyogenesis in differentiating P19 EC cells. Furthermore, dominant-negative mutant proteins of either Gli2 or MEF2C repressed each other's expression, while impairing cardiomyogenesis in P19 EC cells. In addition, chromatin immunoprecipitation (ChIP) revealed association of Gli2 to the Mef2c gene, and of MEF2C to the Gli2 gene in differentiating P19 cells. Finally, co-immunoprecipitation studies showed that Gli2 and MEF2C proteins formed a complex, capable of synergizing on cardiomyogenesis-related promoters containing both Gli- and MEF2-binding elements. We propose a model whereby Gli2 and MEF2C bind each other's regulatory elements, activate each other's expression and form a protein complex that synergistically activates transcription, enhancing cardiac muscle development. This model links Shh signalling to MEF2C function during cardiomyogenesis and offers mechanistic insight into their in vivo functions.

Cdc42-mTOR signaling pathway controls Hes5 and Pax6 expression in retinoic acid-dependent neural differentiation

The conditional knockout of the small GTPase Cdc42 from neuroepithelial (NE) and radial glial (RG) cells in the mouse telencephalon has been shown to have a significant impact on brain development by causing these neural progenitor cells to detach from the apical/ventricular surface and to lose their cell identity. This has been attributed to the requirement for Cdc42 in establishing proper apical/basal cell polarity and cell-cell adhesions. In the present study, we provide new insights into the role played by Cdc42 in the maintenance of neural progenitor cells, using the mouse embryonal carcinoma P19 cell line as a model system. We show that the ability of P19 cells to undergo the transition from an Oct3/4-positive, undifferentiated status to microtubule-associated protein 2-positive neurons and glial fibrillary acidic protein-positive astrocytes, upon treatment with retinoic acid (RA), requires RA-induced activation of Cdc42 during the neural cell lineage specification phase. Experiments using chemical inhibitors and RNA interference suggest that the actions of Cdc42 are mediated through signaling pathways that start with fibroblast growth factors and Delta/Notch proteins and lead to Cdc42-dependent mTOR activation, culminating in the up-regulation of Hes5 and Pax6, two transcription factors that are essential for the maintenance of NE and RG cells. The constitutively active Cdc42(F28L) mutant was sufficient to up-regulate Hes5 and Pax6 in P19 cells, even in the absence of RA treatment, ultimately promoting their transition to neural progenitor cells. The ectopic Cdc42 expression also significantly augmented the RA-dependent up-regulation of these transcription factors, resulting in P19 cells maintaining their neural progenitor status but being unable to undergo terminal differentiation. These findings shed new light on how Cdc42 influences neural progenitor cell fate by regulating gene expression.

Functional skeletal muscle regeneration from differentiating embryonic stem cells

Little progress has been made toward the use of embryonic stem (ES) cells to study and isolate skeletal muscle progenitors. This is due to the paucity of paraxial mesoderm formation during embryoid body (EB) in vitro differentiation and to the lack of reliable identification and isolation criteria for skeletal muscle precursors. Here we show that expression of the transcription factor Pax3 during embryoid body differentiation enhances both paraxial mesoderm formation and the myogenic potential of the cells within this population. Transplantation of Pax3-induced cells results in teratomas, however, indicating the presence of residual undifferentiated cells. By sorting for the PDGF-alpha receptor, a marker of paraxial mesoderm, and for the absence of Flk-1, a marker of lateral plate mesoderm, we derive a cell population from differentiating ES cell cultures that has substantial muscle regeneration potential. Intramuscular and systemic transplantation of these cells into dystrophic mice results in extensive engraftment of adult myofibers with enhanced contractile function without the formation of teratomas. These data demonstrate the therapeutic potential of ES cells in muscular dystrophy.

Molecular cloning and characterization of the Myf5 gene in sea perch (Lateolabrax japonicus)

The cDNA of myogenic factor (Myf5) was isolated from sea perch (Lateolabrax japonicus) using Reverse-transcription Polymerase Chain Reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). The 5' flanking sequence of the cDNA contains a TATA box, GC box, CAAT box, several E box sites and muscle-specific regulatory elements determined by genome walking. The Myf5 gene consists of 3 exons and 2 introns. The open reading frame was found to code a protein with 238 amino-acid residues, containing the conserved basic helix-loop-helix domain (bHLH). RT-PCR indicated the Myf5 was highly expressed in muscle, and weakly expressed in brain, eyes, spleen, gill, liver, kidney, intestine and heart. In early embryonic stages, Myf5 mRNA transcripts are highly detectable in the early gastrula stage while decreasing up to a low level at the late gastrula stage, subsequently greatly increased up to the highest level in the somites stage, then gradually decreases from the tail-bud stage to 15 d larvae after hatching, but they are still detectable. Further, Myf5 mRNA was expressed in several sea perch cell lines such as LJES1, LJHK, LJH-1, LJH-2, LJS, LJL, although its expression level varied greatly among different cell lines.

Cell aggregation-induced FGF8 elevation is essential for P19 cell neural differentiation

FGF8, a member of the fibroblast growth factor (FGF) family, has been shown to play important roles in different developing systems. Mouse embryonic carcinoma P19 cells could be induced by retinoic acid (RA) to differentiate into neuroectodermal cell lineages, and this process is cell aggregation dependent. In this report, we show that FGF8 expression is transiently up-regulated upon P19 cell aggregation, and the aggregation-dependent FGF8 elevation is pluripotent stem cell related. Overexpressing FGF8 promotes RA-induced monolayer P19 cell neural differentiation. Inhibition of FGF8 expression by RNA interference or blocking FGF signaling by the FGF receptor inhibitor, SU5402, attenuates neural differentiation of the P19 cell. Blocking the bone morphogenetic protein (BMP) pathway by overexpressing Smad6 in P19 cells, we also show that FGF signaling plays a BMP inhibition-independent role in P19 cell neural differentiation.

Overexpression of MyoD-inducible lysosomal sialidase (neu1) inhibits myogenesis in C2C12 cells

Lysosomal sialidase, encoded by neu1, is required for the removal of terminal sialic acid residues from a variety of sialoglycoconjugates. In humans, deficiency of this enzyme results in the inborn error of metabolism sialidosis, characterized by the accumulation of sialoglycoconjugates within the nervous system and in peripheral organs. A subset of sialidosis patients present with symptoms of profound muscle dysfunction, including progressive muscular atrophy. We have previously shown that the 5' regulatory region of murine neu1 is typical of skeletal muscle-specific genes due to the presence of several E-boxes and its responsiveness to stimulation by muscle regulatory factors (MRFs) such as MyoD. Here, we report that sialidase activity is increased 6-fold during the first 24 h of differentiation of C2C12 myoblasts followed by an attenuation to pre-differentiation levels by 48 h. We demonstrate that the lysosomal sialidase promoter is highly upregulated by MyoD through a mechanism that is dependent on the MyoD chromatin remodeling domain. We also show that the sialidase promoter is repressed by activated MEK. Inappropriate overexpression of sialidase 48 h after the onset of differentiation results in downregulation of myogenin as well as myosin heavy chain expression and in a halt of the differentiation cascade. This study indicates that lysosomal sialidase is a potent regulator of the early stages of myogenesis.

Characterization of 5'-regulatory region of human myostatin gene: regulation by dexamethasone in vitro

We cloned and characterized a 3.3-kb fragment containing the 5'-regulatory region of the human myostatin gene. The promoter sequence contains putative muscle growth response elements for glucocorticoid, androgen, thyroid hormone, myogenic differentiation factor 1, myocyte enhancer factor 2, peroxisome proliferator-activated receptor, and nuclear factor-kappaB. To identify sites important for myostatin's gene transcription and regulation, eight deletion constructs were placed in C(2)C(12) and L6 skeletal muscle cells. Transcriptional activity of the constructs was found to be significantly higher in myotubes compared with that of myoblasts. To investigate whether glucocorticoids regulate myostatin gene expression, we incubated both cell lines with dexamethasone. On both occasions, dexamethasone dose dependently increased both the promoter's transcriptional activity and the endogenous myostatin expression. The effects of dexamethasone were blocked when the cells were coincubated with the glucocorticoid receptor antagonist RU-486. These findings suggest that glucocorticoids upregulate myostatin expression by inducing gene transcription, possibly through a glucocorticoid receptor-mediated pathway. We speculate that glucocorticoid-associated muscle atrophy might be due in part to the upregulation of myostatin expression.

Identification of a role for the sialomucin CD164 in myogenic differentiation by signal sequence trapping in yeast

Determination and differentiation of skeletal muscle precursors requires cell-cell contact, but the full range of cell surface proteins that mediate this requirement and the mechanisms by which they work are not known. To identify participants in cell contact-mediated regulation of myogenesis, genes that encode secreted proteins specifically upregulated during differentiation of C2C12 myoblasts were identified by the yeast signal sequence trap method (K. A. Jacobs, L. A. Collins-Racie, M. Colbert, M. Duckett, M. Golden-Fleet, K. Kelleher, R. Kriz, E. R. La Vallie, D. Merberg, V. Spaulding, J. Stover, M. J. Williamson, and J. M. McCoy, Gene 198:289-296, 1997), followed by RNA expression analysis. We report here the identification of CD164 as a gene expressed in proliferating C2C12 cells that is upregulated during differentiation. CD164 encodes a widely expressed cell surface sialomucin that has been implicated in regulation of cell proliferation and adhesion during hematopoiesis. Stable overexpression of CD164 in C2C12 and F3 myoblasts enhanced their differentiation, as assessed by both morphological and biochemical criteria. Furthermore, expression of antisense CD164 or soluble extracellular regions of CD164 inhibited myogenic differentiation. Treatment of C2C12 cells with sialidase or O-sialoglycoprotease, two enzymes previously reported to destroy functional epitopes on CD164, also inhibited differentiation. These data indicate that (i) CD164 may play a rate-limiting role in differentiation of cultured myoblasts, (ii) sialomucins represent a class of potential effectors of cell contact-mediated regulation of myogenesis, and (iii) carbohydrate-based cell recognition may play a role in mediating this phenomenon.

A key role for Pax7 transcripts in determination of muscle and nerve cells

Embryonic teratocarcinoma mouse cells (P19) and embryonic NIH3T3 fibroblasts were induced chemically to differentiate along neurogenic or myogenic lineages. The expression profiles of Pax7 alternate transcripts were then assessed by RNA isolation and RT-PCR. Only two transcripts, Pax7b and Pax7d, were expressed in the neurogenic lineage. By contrast, in adult skeletal muscle, four transcripts, Pax7a-d, were expressed in the myogenic lineage. Moreover, P19 cells were shown to undergo neural cell differentiation when stably transfected with a single Pax7 transcript, PAX7b, generated from human skeletal muscle. Our results suggest a key role for Pax7 transcripts in lineage determination.

Recruitment of (β)-catenin to cadherin-mediated intercellular adhesions is involved in myogenic induction

Cadherin-mediated cell adhesion is involved in muscle differentiation from early stages of myogenic induction to late stages of myoblast interaction and fusion. (β)-Catenin is a major constituent of cadherin-based adherens junctions and also serves as a signal transduction molecule that regulates gene expression during development. In this study, we explored the involvement of (β)-catenin in myogenic differentiation. We show here that shortly after a switch from growth to differentiation medium, (β)-catenin translocates to cell-cell junctions and its levels increase. We further show that elevation of (β)-catenin levels, induced either by inhibition of its breakdown, using LiCl, or by its overexpression, suppresses the formation of adherens junctions, resulting in a sharp decline in myogenin expression and an arrest of myogenic progression. Recruitment of (β)-catenin to adherens junctions after transfection with N-cadherin restores myogenin expression in the transfected cells. These results suggest that increased cadherin-mediated adhesion and translocation of (β)-catenin to adherens junctions are involved in activating the early steps of myogenic differentiation.

Cell signaling switches HOX-PBX complexes from repressors to activators of transcription mediated by histone deacetylases and histone acetyltransferases

The Hoxb1 autoregulatory element comprises three HOX-PBX binding sites. Despite the presence of HOXB1 and PBX1, this enhancer fails to activate reporter gene expression in retinoic acid-treated P19 cell monolayers. Activation requires cell aggregation in addition to RA. This suggests that HOX-PBX complexes may repress transcription under some conditions. Consistent with this, multimerized HOX-PBX binding sites repress reporter gene expression in HEK293 cells. We provide a mechanistic basis for repressor function by demonstrating that a corepressor complex, including histone deacetylases (HDACs) 1 and 3, mSIN3B, and N-CoR/SMRT, interacts with PBX1A. We map a site of interaction with HDAC1 to the PBX1 N terminus and show that the PBX partner is required for repression by the HOX-PBX complex. Treatment with the deacetylase inhibitor trichostatin A not only relieves repression but also converts the HOX-PBX complex to a net activator of transcription. We show that this activation function is mediated by the recruitment of the coactivator CREB-binding protein by the HOX partner. Interestingly, HOX-PBX complexes are switched from transcriptional repressors to activators in response to protein kinase A signaling or cell aggregation. Together, our results suggest a model whereby the HOX-PBX complex can act as a repressor or activator of transcription via association with corepressors and coactivators. The model implies that cell signaling is a direct determinant of HOX-PBX function in the patterning of the animal embryo.

Wnt signaling regulates the function of MyoD and myogenin

The myogenic regulatory factors (MRFs), MyoD and myogenin, can induce myogenesis in a variety of cell lines but not efficiently in monolayer cultures of P19 embryonal carcinoma stem cells. Aggregation of cells expressing MRFs, termed P19[MRF] cells, results in an approximately 30-fold enhancement of myogenesis. Here we examine molecular events occurring during P19 cell aggregation to identify potential mechanisms regulating MRF activity. Although myogenin protein was continually present in the nuclei of >90% of P19[myogenin] cells, only a fraction of these cells differentiated. Consequently, it appears that post-translational regulation controls myogenin activity in a cell lineage-specific manner. A correlation was obtained between the expression of factors involved in somite patterning, including Wnt3a, Wnt5b, BMP-2/4, and Pax3, and the induction of myogenesis. Co-culturing P19[Wnt3a] cells with P19[MRF] cells in monolayer resulted in a 5- to 8-fold increase in myogenesis. Neither BMP-4 nor Pax3 was efficient in enhancing MRF activity in unaggregated P19 cultures. Furthermore, BMP-4 abrogated the enhanced myogenesis induced by Wnt signaling. Consequently, signaling events resulting from Wnt3a expression but not BMP-4 signaling or Pax3 expression, regulate MRF function. Therefore, the P19 cell culture system can be used to study the link between somite patterning events and myogenesis.

Vascular smooth muscle cells spontaneously adopt a skeletal muscle phenotype: a unique Myf5(-)/MyoD(+) myogenic program

Smooth and skeletal muscle tissues are composed of distinct cell types that express related but distinct isoforms of the structural genes used for contraction. These two muscle cell types are also believed to have distinct embryological origins. Nevertheless, the phenomenon of a phenotypic switch from smooth to skeletal muscle has been demonstrated in several in vivo studies. This switch has been minimally analyzed at the cellular level, and the mechanism driving it is unknown. We used immunofluorescence and RT-PCR to demonstrate the expression of the skeletal muscle-specific regulatory genes MyoD and myogenin, and of several skeletal muscle-specific structural genes in cultures of the established rat smooth muscle cell lines PAC1, A10, and A7r5. The skeletal muscle regulatory gene Myf5 was not detected in these three cell lines. We further isolated clonal sublines from PAC1 cultures that homogeneously express smooth muscle characteristics at low density and undergo a coordinated increase in skeletal muscle-specific gene expression at high density. In some of these PAC1 sublines, this process culminates in the high-frequency formation of myotubes. As in the PAC1 parental line, Myf5 was not expressed in the PAC1 sublines. We show that the PAC1 sublines that undergo a more robust transition into the skeletal muscle phenotype also express significantly higher levels of the insulin-like growth factor (IGF1 and IGF2) genes and of FGF receptor 4 (FGFR4) gene. Our results suggest that MyoD expression in itself is not a sufficient condition to promote a coordinated program of skeletal myogenesis in the smooth muscle cells. Insulin administered at a high concentration to PAC1 cell populations with a poor capacity to undergo skeletal muscle differentiation enhances the number of cells displaying the skeletal muscle differentiated phenotype. The findings raise the possibility that the IGF signaling system is involved in the phenotypic switch from smooth to skeletal muscle. The gene expression program described here can now be used to investigate the mechanisms that may underlie the propensity of certain smooth muscle cells to adopt a skeletal muscle identity.(J Histochem Cytochem 48:1173-1193, 2000)