Factors in serum regulate muscle development in P19 cells

Bmp Signaling Regulates Hand1 in a Dose-Dependent Manner during Heart Development

The bone morphogenetic protein (Bmp) signaling pathway and the basic helix–loop–helix (bHLH) transcription factor Hand1 are known key regulators of cardiac development. In this study, we investigated the Bmp signaling regulation of Hand1 during cardiac outflow tract (OFT) development. In Bmp2 and Bmp4loss-of-function embryos with varying levels of Bmp in the heart, Hand1 is sensitively decreased in response to the dose of Bmp expression. In contrast, Hand1 in the heart is dramatically increased in Bmp4 gain-of-function embryos. We further identified and characterized the Bmp/Smad regulatory elements in Hand1. Combined transfection assays and chromatin immunoprecipitation (ChIP) experiments indicated that Hand1 is directly activated and bound by Smads. In addition, we found that upon the treatment of Bmp2 and Bmp4, P19 cells induced Hand1 expression and favored cardiac differentiation. Together, our data indicated that the Bmp signaling pathway directly regulates Hand1 expression in a dose-dependent manner during heart development.

FoxO1: a novel insight into its molecular mechanisms in the regulation of skeletal muscle differentiation and fiber type specification

FoxO1, a member of the forkhead transcription factor forkhead box protein O (FoxO) family, is predominantly expressed in most muscle types. FoxO1 is a key regulator of muscle growth, metabolism, cell proliferation and differentiation. In the past two decades, many researches have indicated that FoxO1 is a negative regulator of skeletal muscle differentiation while contrasting opinions consider that FoxO1 is crucial for myoblast fusion. FoxO1 is expressed much higher in fast twitch fiber enriched muscles than in slow muscles and is also closely related to muscle fiber type specification. In this review, we summarize the molecular mechanisms of FoxO1 in the regulation of skeletal muscle differentiation and fiber type specification.

Histone deacetylase inhibitor valproic acid as a small molecule inducer to direct the differentiation of pluripotent stem cells

Pluripotent stem cells can be directed into myogenic differentiation by small molecular inducers, which preferentially activate muscle-specific transcription networks. Here we describe how to efficiently direct the differentiation of pluripotent P19 cells into skeletal muscle lineage by using histone deacetylase inhibitor, valproic acid and the ligand of retinoic acid receptor, all-trans retinoic acid.

Skeletal myosin light chain kinase regulates skeletal myogenesis by phosphorylation of MEF2C

The MEF2 factors regulate transcription during cardiac and skeletal myogenesis. MEF2 factors establish skeletal muscle commitment by amplifying and synergizing with MyoD. While phosphorylation is known to regulate MEF2 function, lineage-specific regulation is unknown. Here, we show that phosphorylation of MEF2C on T(80) by skeletal myosin light chain kinase (skMLCK) enhances skeletal and not cardiac myogenesis. A phosphorylation-deficient MEF2C mutant (MEFT80A) enhanced cardiac, but not skeletal myogenesis in P19 stem cells. Further, MEFT80A was deficient in recruitment of p300 to skeletal but not cardiac muscle promoters. In gain-of-function studies, skMLCK upregulated myogenic regulatory factor (MRF) expression, leading to enhanced skeletal myogenesis in P19 cells and more efficient myogenic conversion. In loss-of-function studies, MLCK was essential for efficient MRF expression and subsequent myogenesis in embryonic stem (ES) and P19 cells as well as for proper activation of quiescent satellite cells. Thus, skMLCK regulates MRF expression by controlling the MEF2C-dependent recruitment of histone acetyltransferases to skeletal muscle promoters. This work identifies the first kinase that regulates MyoD and Myf5 expression in ES or satellite cells.

MyoD directly up-regulates premyogenic mesoderm factors during induction of skeletal myogenesis in stem cells

Gain- and loss-of-function experiments have illustrated that the family of myogenic regulatory factors is necessary and sufficient for the formation of skeletal muscle. Furthermore, MyoD required cellular aggregation to induce myogenesis in P19 embryonal carcinoma stem cells. To determine the mechanism by which stem cells can be directed into skeletal muscle, a time course of P19 cell differentiation was examined in the presence and absence of exogenous MyoD. By quantitative PCR, the first MyoD up-regulated transcripts were the premyogenic mesoderm factors Meox1, Pax7, Six1, and Eya2 on day 4 of differentiation. Subsequently, the myoblast markers myogenin, MEF2C, and Myf5 were up-regulated, leading to skeletal myogenesis. These results were corroborated by Western blot analysis, showing up-regulation of Pax3, Six1, and MEF2C proteins, prior to myogenin protein expression. To determine at what stage a dominant-negative MyoD/EnR mutant could inhibit myogenesis, stable cell lines were created and examined. Interestingly, the premyogenic mesoderm factors, Meox1, Pax3/7, Six1, Eya2, and Foxc1, were down-regulated, and as expected, skeletal myogenesis was abolished. Finally, to identify direct targets of MyoD in this system, chromatin immunoprecipitation experiments were performed. MyoD was observed associated with regulatory regions of Meox1, Pax3/7, Six1, Eya2, and myogenin genes. Taken together, MyoD directs stem cells into the skeletal muscle lineage by binding and activating the expression of premyogenic mesoderm genes, prior to activating myoblast genes.

Effective and steady differentiation of a clonal derivative of P19CL6 embryonal carcinoma cell line into beating cardiomyocytes

The P19CL6 cell line is a useful model to study cardiac differentiation in vitro. However, large variations were noticed in the differentiation rates among previous reports as well as our individual experiments. To overcome the unstable differentiation, we established P19CL6-A1, a new clonal derivative of P19CL6 that could differentiate into cardiomyocytes more efficiently and stably than the parent using the double stimulation with 5-Aza and DMSO based on the previous report. We also introduced a new software, Visorhythm, that can analyze the temporal variations in the beating rhythms and can chart correlograms displaying the oscillated rhythms. Using P19CL6-A1-derived cardiomyocytes and the software, we demonstrated that the correlograms could clearly display the enhancement of beating rates by cardiotonic reagents. These indicate that a combination of P19CL6-A1 and Visorhythm is a useful tool that can provide invaluable assistance in inotropic drug discovery, drug screening, and toxicity testing.

Cross talk between hedgehog and bone morphogenetic proteins occurs during cardiomyogenesis in P19 cells

Hedgehog (Hh) signaling plays a role in heart morphogenesis and can initiate cardiomyogenesis in P19 cells. To determine if Hh signaling is essential for P19 cell cardiomyogenesis, we determined which Hh factors are expressed and the effect of Hh signal transduction inhibitors. Here, we find that the Hh gene family and their downstream mediators are expressed during cardiomyogenesis but an active Hh signaling pathway is not essential. However, loss of Hh signaling resulted in a delay of BMP-4, GATA-4, Gli2, and Meox1 expression during cardiomyogenesis. By using Noggin-overexpressing P19 cells, we determined that Hh signaling was not active during Noggin-mediated inhibition of cardiomyogenesis. Thus, there is cross talk between the Hh and BMP signaling pathways and the Hh pathway appears important for timely cardiomyogenesis.

Extrinsic regulation of cardiomyocyte differentiation of embryonic stem cells

Cardiovascular disease is one of leading causes of death throughout the U.S. and the world. The damage of cardiomyocytes resulting from ischemic injury is irreversible and leads to the development of progressive heart failure, which is characterized by the loss of functional cardiomyocytes. Because cardiomyocytes are unable to regenerate in the adult heart, cell-based therapy of transplantation provides a potential alternative approach to replace damaged myocardial tissue and restore cardiac function. A major roadblock toward this goal is the lack of donor cells; therefore, it is urgent to identify the cardiovascular cells that are necessary for achieving cardiac muscle regeneration. Pluripotent embryonic stem (ES) cells have enormous potential as a source of therapeutic tissues, including cardiovascular cells; however, the regulatory elements mediating ES cell differentiation to cardiomyocytes are largely unknown. In this review, we will focus on extrinsic factors that play a role in regulating different stages of cardiomyocyte differentiation of ES cells.

Proteomics of RNA polymerase II holoenzymes during P19 cardiomyogenesis

The embryonal carcinoma P19 model has allowed the elucidation of a role for several transcription factors in cell differentiation. Here, the regulation of the RNA polymerase II machinery has been explored through its association with multifunctional complexes involved in transcription. An interaction proteomics analysis of TFIIS-purified RNA polymerase II (RNAPII) holoenzymes during cardiomyogenesis is described. Modifications of protein complexes that may be associated with transcriptionally active and activator responsive RNAPII holoenzymes were detected in a serum and DMSO dependent manner. Subunits of the PAF1 and Mediator complexes were correlated with holoenzymes from non-differentiated and terminally differentiated P19 cultures respectively. Moreover, high levels of nucleolin were identified in all forms of holoenzymes by two-dimensional gel electrophoresis, and suggest that nucleolin could bind to RNAPII and TFIIS. Several proteins that were identified in the RNAPII holoenzymes are known to have functions in mRNA processing and may bind to nucleolin. A novel function for nucleolin is proposed as a possible pivotal platform between transcription, mRNA processing and export.

Nitric oxide signaling in oxytocin-mediated cardiomyogenesis

Oxytocin (OT), a hormone recently identified in the heart, induces embryonic and cardiac somatic stem cells to differentiate into cardiomyocytes (CM), possibly through nitric oxide (NO). We verified this hypothesis using P19 cells and P19 Clone 6 derivatives expressing a green fluorescent protein (GFP) reporter linked to cardiac myosin light chain-2v promoter. OT treatment of these cells induced beating cell colonies that were fully inhibited by N,G-nitro-L-arginine-methyl-ester (L-NAME), an inhibitor of NO synthases (NOS), partially reduced by 1400W, an inhibitor of inducible NOS, and ODQ, an inhibitor of NO-sensitive guanylyl cyclases. The NO generator S-nitroso-N-acetylpenicillamine (SNAP) reversed the L-NAME inhibition of cell beating and GFP expression. In OT-induced cells, L-NAME significantly decreased transcripts of the cardiac markers Nkx2.5, MEF2c, alpha-myosin heavy chain, and less, GATA4, endothelial NOS, and atrial natriuretic peptide, as well as the skeletal myocyte (SM) marker myogenin. Image analysis of OT-induced P19Cl6-GFP cells revealed ventricular CM coexpressing sarcomeric alpha-actinin and GFP, with some cells exclusively expressing alpha-actinin, most likely of the SM phenotype. The OT-mediated production of CM, but not SM, was diminished by L-NAME. In P19 cells, exogenously added OT stimulated the expression of its own transcript, which was reduced in the presence of L-NAME. Surprisingly, L-NAME alone decreased the expression of anti-stage specific embryonic antigen-1 marker of the undifferentiated state and induced some beating colonies as well as GFP in P19Cl6-GFP cells. Collectively, our data suggest that the pleiotropic action of NO is involved in the initiation of CM differentiation of P19 cells and maintenance of their undifferentiated state.

HDAC activity regulates entry of mesoderm cells into the cardiac muscle lineage

Class II histone deacetylases (HDAC4, HDAC5, HDAC7 and HDAC9) have been shown to interact with myocyte enhancer factors 2 (MEF2s) and play an important role in the repression of cardiac hypertrophy. We examined the role of HDACs during the differentiation of P19 embryonic carcinoma stem cells into cardiomyoctyes. Treatment of aggregated P19 cells with the HDAC inhibitor trichostatin A induced the entry of mesodermal cells into the cardiac muscle lineage, shown by the upregulation of transcripts Nkx2-5, MEF2C, GATA4 and cardiac α-actin. Furthermore, the overexpression of HDAC4 inhibited cardiomyogenesis, shown by the downregulation of cardiac muscle gene expression. Class II HDAC activity is inhibited through phosphorylation by Ca2+/calmodulin-dependent kinase (CaMK). Expression of an activated CaMKIV in P19 cells upregulated the expression of Nkx2-5, GATA4 and MEF2C, enhanced cardiac muscle development, and activated a MEF2-responsive promoter. Moreover, inhibition of CaMK signaling downregulated GATA4 expression. Finally, P19 cells constitutively expressing a dominant-negative form of MEF2C, capable of binding class II HDACs, underwent cardiomyogenesis more efficiently than control cells, implying the relief of an inhibitor. Our results suggest that HDAC activity regulates the specification of mesoderm cells into cardiomyoblasts by inhibiting the expression of GATA4 and Nkx2-5 in a stem cell model system.

Disruption of MEF2 activity in cardiomyoblasts inhibits cardiomyogenesis

Myocyte enhancer factors (MEF2s) bind to muscle-specific promoters and activate transcription. Drosophila Mef2 is essential for Drosophila heart development, however, neither MEF2C nor MEF2B are essential for the early stages of murine cardiomyogenesis. Although Mef2c-null mice were defective in the later stages of heart morphogenesis, differentiation of cardiomyocytes still occurred. Since there are four isoforms of MEF2 factors (MEF2A, MEF2B, MEF2C and MEF2D), the ability of cells to differentiate may have been confounded by genetic redundancy. To eliminate this variable, the effect of a dominant-negative MEF2 mutant (MEF2C/EnR) during cardiomyogenesis was examined in transgenic mice and P19 cells. Targeting the expression of MEF2C/EnR to cardiomyoblasts using an Nkx2-5 enhancer in the P19 system resulted in the loss of both cardiomyocyte development and the expression of GATA4, BMP4, Nkx2-5 and MEF2C. In transiently transgenic mice, MEF2C/EnR expression resulted in embryos that lacked heart structures and exhibited defective differentiation. Our results show that MEF2C, or genes containing MEF2 DNA-binding sites, is required for the efficient differentiation of cardiomyoblasts into cardiomyocytes, suggesting conservation in the role of MEF2 from Drosophila to mammals.

Hedgehog signaling induces cardiomyogenesis in P19 cells

Sonic Hedgehog (Shh) is a critical signaling factor for a variety of developmental pathways during embryogenesis, including the specification of left-right asymmetry in the heart. Mice that lack Hedgehog signaling show a delay in the induction of cardiomyogenesis, as indicated by a delayed expression of Nkx2-5. To further examine a role for Shh in cardiomyogenesis, clonal populations of P19 cells that stably express Shh, termed P19(Shh) cells, were isolated. In monolayer P19(Shh) cultures the Shh pathway was functional as shown by the up-regulation of Ptc1 and Gli1 expression, but no cardiac muscle markers were activated. However, Shh expression induced cardiomyogenesis following cellular aggregation, resulting in the expression of factors expressed in cardiac muscle including GATA-4, MEF2C, and Nkx2-5. Furthermore, aggregated P19 cell lines expressing Gli2 or Meox1 also up-regulated the expression of cardiac muscle factors, leading to cardiomyogenesis. Meox1 up-regulated the expression of Gli1 and Gli2 and, thus, can modify the Shh signaling pathway. Finally, Shh, Gli2, and Meox1 all up-regulated BMP-4 expression, implying that activation of the Hedgehog pathway can regulate bone morphogenetic protein signals. Taken together, we propose a model in which Shh, functioning via Gli1/2, can specify mesodermal cells into the cardiac muscle lineage.

Disruption of Meox or Gli activity ablates skeletal myogenesis in P19 cells

Gli2 and Meox1 are transcription factors that are expressed in the developing somite and play roles in the commitment of cells to the skeletal muscle lineage. To further define their roles in regulating myogenesis, the function of wild type and dominant-negative forms of Gli2 and Meox1 were examined in the context of differentiating P19 stem cells. We found that Gli2 overexpression up-regulated transcript levels of Meox1 and, conversely, Meox1 overexpression resulted in the upregulation of Gli2 transcripts. Furthermore, dominant-negative forms of either Meox1 or Gli2 disrupted the ability of P19 cells to commit to the muscle lineage and to properly express either Gli2 or Meox1, respectively. Finally, Pax3 transcripts were induced by Gli2 overexpression and lost in the presence of either mutants Meox1 or Gli2. Taken together, these results support the existence of a regulatory loop between Gli2, Meox1, and Pax3 that is essential for specification of mesodermal cells into the muscle lineage.

Regulation of insulin-like growth factor-dependent myoblast differentiation by Foxo forkhead transcription factors

Insulin-like growth factors promote myoblast differentiation through phosphoinositol 3-kinase and Akt signaling. Akt substrates required for myogenic differentiation are unknown. Forkhead transcription factors of the forkhead box gene, group O (Foxo) subfamily are phosphorylated in an insulin-responsive manner by phosphatidylinositol 3-kinase-dependent kinases. Phosphorylation leads to nuclear exclusion and inactivation. We show that a constitutively active Foxo1 mutant inhibits differentiation of C2C12 cells and prevents myotube differentiation induced by constitutively active Akt. In contrast, a transcriptionally inactive mutant Foxo1 partially rescues inhibition of C2C12 differentiation mediated by wortmannin, but not by rapamycin, and is able to induce aggregation-independent myogenic conversion of teratocarcinoma cells. Inhibition of Foxo expression by siRNA resulted in more efficient differentiation, associated with increased myosin expression. These observations indicate that Foxo proteins are key effectors of Akt-dependent myogenesis.

Oxytocin induces differentiation of P19 embryonic stem cells to cardiomyocytes

We recently discovered the existence of the oxytocin/oxytocin receptor (OT/OTR) system in the heart. Activation of cardiac OTR stimulates the release of atrial natriuretic peptide (ANP), which is involved in regulation of blood pressure and cell growth. Having observed elevated OT levels in the fetal and newborn heart at a stage of intense cardiomyocyte hyperplasia, we hypothesized a role for OT in cardiomyocyte differentiation. We used mouse P19 embryonic stem cells to substantiate this potential role. P19 cells give rise to the formation of cell derivatives of all germ layers. Treatment of P19 cell aggregates with dimethyl sulfoxide (DMSO) induces differentiation to cardiomyocytes. In this work, P19 cells were allowed to aggregate from day 0 to day 4 in the presence of 0.5% DMSO, 10(-7) M OT and/or 10(-7) M OT antagonist (OTA), and then cultured in the absence of these factors until day 14. OT alone stimulated the production of beating cell colonies in all 24 independently growing cultures by day 8 of the differentiation protocol, whereas the same result was obtained in cells induced by DMSO only after 12 days. Cells induced with OT exhibited increased ANP mRNA, had abundant mitochondria (i.e., they strongly absorbed rhodamine 123), and expressed sarcomeric myosin heavy chain and dihydropyridine receptor-alpha 1, confirming a cardiomyocyte phenotype. In addition, OT as well as DMSO increased OTR protein and OTR mRNA, and OTA completely inhibited the formation of cardiomyocytes in OT- and DMSO-supplemented cultures. These results suggest that the OT/OTR system plays an important role in cardiogenesis by promoting cardiomyocyte differentiation.

Beta-catenin is essential and sufficient for skeletal myogenesis in P19 cells

Wnt1 and Wnt3a are signaling factors known to play a role in the induction of myogenesis in the myotome of the differentiating somite. Both factors may transduce their signal by a conserved pathway that leads to transcriptional regulation by beta-catenin/Lef1. beta-Catenin and Lef1 are found in the myotome prior to MyoD expression. We have utilized the P19 cell system to study the mechanisms by which Wnt3a may activate MyoD expression and subsequent skeletal muscle development. We have isolated P19 cell lines that stably express either Wnt3a or activated beta-catenin and found that aggregation of these cells results in the induction of myogenesis compared with control cells. Pax3, Gli2, Mox1, and Six1 were expressed during Wnt3a and beta-catenin-induced differentiation prior to MyoD expression. Furthermore, we have shown that the nuclear function of beta-catenin was essential for skeletal myogenesis in P19 cells by overexpression of a dominant negative beta-catenin/engrailed chimera. Primitive streak factors were present, but expression of Pax3, Mox1, Gli2, and Six1 was lost in these cells, indicating that nuclear beta-catenin is essential for specification of mesodermal precursors to the myogenic lineage. Therefore, Wnt signaling, acting via beta-catenin, is necessary and sufficient for skeletal myogenesis in P19 cells.

BMP signaling regulates Nkx2-5 activity during cardiomyogenesis

Nkx2-5 regulates the transcription of muscle-specific genes during cardiomyogenesis. Nkx2-5 expression can induce cardiomyogenesis in aggregated P19 cells but not in monolayer cultures. In order to investigate the mechanism by which cellular aggregation regulates Nkx2-5 function, we examined the role of bone morphogenetic protein 4 (BMP4). We showed that the expression of the BMP inhibitor, noggin, was sufficient to inhibit the induction of cardiomyogenesis by Nkx2-5 during cellular aggregation. Furthermore, soluble BMP4 could activate Nkx2-5 function in monolayer cultures, resulting in the formation of cardiomyocytes. Therefore, BMP signaling is necessary and sufficient for the regulation of Nkx2-5 activity during cardiomyogenesis in P19 cells.

Nkx2-5 activity is essential for cardiomyogenesis

The homeobox transcription factor tinman is essential for heart vessel formation in Drosophila. In contrast, mice lacking the murine homologue Nkx2-5 are defective in cardiac looping but not in cardiac myocyte development. The lack of an essential role for Nkx2-5 in cardiomyogenesis in mammalian systems is most likely the result of genetic redundancy with family members. In this study, we used a dominant negative mutant of Nkx2-5, created by fusing the repressor domain of engrailed 2 to the Nkx2-5 homeodomain, termed Nkx/EnR. Expression of Nkx/EnR inhibited Me(2)SO-induced cardiomyogenesis in P19 cells but not skeletal myogenesis. Nkx/EnR inhibited expression of cardiomyoblast markers, such as GATA-4 and MEF2C, but not of mesoderm markers, such as Brachyury T and Wnt5b, or of skeletal lineage markers, such as MyoD and Mox1. To identify the minimal region of Nkx2-5 that can trigger cardiomyogenesis, we analyzed the activity of various Nkx2-5 deletion mutants. The C-terminal domain was not necessary for the ability of Nkx2-5 to induce cardiomyogenesis and loss of this domain did not enhance myogenesis. Therefore, Nkx2-5 function is essential for commitment of mesoderm into the cardiac muscle lineage, and the N-terminal region, together with the homeodomain, is sufficient for cardiomyogenesis in P19 cells.

Pax3 is essential for skeletal myogenesis and the expression of Six1 and Eya2

Pax3 is a paired box transcription factor expressed during somitogenesis that has been implicated in initiating the expression of the myogenic regulatory factors during myogenesis. We find that Pax3 is necessary and sufficient to induce myogenesis in pluripotent stem cells. Pax3 induced the expression of the transcription factor Six1, its cofactor Eya2, and the transcription factor Mox1 prior to inducing the expression of MyoD and myogenin. Overexpression of a dominant negative Pax3, engineered by fusing the active transcriptional repression domain of mouse EN-2 in place of the Pax3 transcriptional activation domain, completely abolished skeletal myogenesis without inhibiting cardiogenesis. Expression of the dominant negative Pax3 resulted in a loss of expression of Six1, Eya2, and endogenous Pax3 as well as a down-regulation in the expression of Mox1. No effect was found on the expression of Gli2. These results indicate that Pax3 activity is essential for skeletal muscle development, the expression of Six1 and Eya2, and is involved in regulating its own expression. In summary, the combined approach of expressing both a wild type and dominant negative transcription factor in stem cells has identified a cascade of transcriptional events controlled by Pax3 that are necessary and sufficient for skeletal myogenesis.

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.

Myocyte enhancer factor 2C and myogenin up-regulate each other's expression and induce the development of skeletal muscle in P19 cells

Two families of transcription factors, myogenic regulatory factors (MRFs) and myocyte enhancer factor 2 (MEF2), function synergistically to regulate myogenesis. In addition to activating structural muscle-specific genes, MRFs and MEF2 activate each other's expression. The MRF, myogenin, can activate MEF2 DNA binding activity when transfected into fibroblasts and, in turn, the myogenin promoter contains essential MEF2 DNA binding elements. To determine which MEF2 is involved in this regulation, P19 cells stably expressing MyoD and myogenin were compared for their ability to activate the expression of MEF2 family members. There was very little cross-activation of MyoD expression by myogenin and vice versa. Myogenin expression, and not MyoD, was found to up-regulate MEF2C expression. MEF2A, -B, and -D expression levels were not up-regulated by overexpression of either MyoD or myogenin. To examine whether MEF2C can differentially regulate MyoD or myogenin expression, P19 cell lines overexpressing MEF2C were analyzed. MEF2C induced myogenesis in P19 cells and up-regulated the expression of myogenin with 25-fold greater efficiency than that of MyoD. Therefore, myogenin and MEF2C participate in a regulatory loop in differentiating stem cells. This positive regulation does not extend to MyoD or the other MEF2 family members. Consequently, MEF2C appears to play a specific role in early events of myogenesis.

Cloning, tissue distribution, subcellular localization and overexpression of murine histidine-rich Ca2+ binding protein

The histidine-rich Ca2+ binding protein (HRC) resides in the sarcoplasmic reticulum of muscle and binds Ca2+. Since Ca2+ concentrations can regulate gene expression via calcineurin, the mouse homologue of HRC (mHRC) was isolated and characterized. mHRC was detected in muscle progenitor cells, in primary clonal thymic tumors and a tumor cell line, suggesting a broader role for mHRC than in Ca2+ storage during muscle contraction. mHRC was present in the perinuclear region of myoblasts. To examine if it can regulate gene expression, mHRC was overexpressed in cells differentiating into cardiac and skeletal muscle. mHRC had no effect on cardiogenesis or myogenesis. Therefore, if mHRC plays a role in the regulation of gene expression during cellular differentiation, it does not appear to be either rate-limiting or inhibitory.