The role of aggregation in embryonal carcinoma cell differentiation

Retinoids: Mechanisms of Action in Neuronal Cell Fate Acquisition

Neuronal differentiation has been shown to be directed by retinoid action during embryo development and has been exploited in various in vitro cell differentiation systems. In this review, we summarize the role of retinoids through the activation of their specific retinoic acid nuclear receptors during embryo development and also in a variety of in vitro strategies for neuronal differentiation, including recent efforts in driving cell specialization towards a range of neuronal subtypes and glial cells. Finally, we highlight the role of retinoic acid in recent protocols recapitulating nervous tissue complexity (cerebral organoids). Overall, we expect that this effort might pave the way for exploring the usage of specific synthetic retinoids for directing complex nervous tissue differentiation.

Melatonin promotes the acquisition of neural identity through extracellular-signal-regulated kinases 1/2 activation

Melatonin, a major pineal secretory product, exerts a range of physiological and neuroprotective effects. However, the functional significance of melatonin in determining neural identity, and the mechanisms by which this may occur, is unknown. In this study, P19 cells were used as a model system and cell behavior was monitored. Our data show that melatonin plays an important role in determining cell fate during neural commitment and promoting the differentiation of pluripotent P19 cells (Oct4(+) Sox2(+) ) into neural stem cells (Oct4(-) Sox2(+) ). This promotion appears to coincide with the activation of the MT1 receptor and phosphorylation of extracellular-signal-regulated kinases 1/2 (ERK1/2). Furthermore, our results show that melatonin regulates neural fate specification of P19 cells through two distinct mechanisms: the promotion of nuclear localization of ERK1/2 and upregulation of Sox2 transcription, and suppression of Smad1-induced expression of mesodermal-specific genes, such as Bra.

Neural epidermal growth factor-like like protein 2 (NELL2) promotes aggregation of embryonic carcinoma P19 cells by inducing N-cadherin expression

NELL2 was first identified as a mammalian homolog of chick NEL (Neural EGF-like) protein. It is almost exclusively expressed in neurons of the rat brain and has been suggested to play a role in neural differentiation. However, there is still no clear evidence for the detailed function of NELL2 in the differentiation of neurons. In this study, we identified NELL2 function during neural differentiation of mouse embryonic carcinoma P19 cells. Endogenous expression of NELL2 in the P19 cells increased in parallel with the neuronal differentiation induced by retinoic acid (RA). We found that the mouse NELL2 promoter contains RA response elements (RAREs) and that treatment with RA increased NELL2 promoter activity. Transfection of P19 cells with NELL2 expression vectors induced a dramatic increase in cell aggregation, resulting in the facilitation of neural differentiation. Moreover, NELL2 significantly increased N-cadherin expression in the P19 cell. These data suggest that NELL2 plays an important role in the regulation of neuronal differentiation via control of N-cadherin expression and cell aggregation.

Directed differentiation of size-controlled embryoid bodies towards endothelial and cardiac lineages in RGD-modified poly(ethylene glycol) hydrogels

Recent advances in stem cell research have demonstrated the importance of microenvironmental cues in directing stem cell fate towards specific cell lineages. For instance, the size of the embryoid body (EB) was shown to play a role in stem cell differentiation. Other studies have used cell adhesive RGD peptides to direct stem cell fate towards endothelial cells. In this study, materials and cell-based approaches are combined by using microwell arrays to produce size-controlled EBs and encapsulating the resulting aggregates in high molecular weight PEG-4 arm acrylate with and without conjugated RGD to study their effect on stem cell differentiation in a 3D microenvironment. Increasing EB size is observed along with a decrease in the total number of EBs in pristine PEG hydrogel, regardless of the initial EB size. In correlation with this aggregation, EBs in PEG show enhanced cardiogenic differentiation compared to RGD-PEG hydrogel. Both aggregation and cardiogenic differentiation are significantly reduced when RGD peptides are introduced to the microenvironment, while endothelial cell differentiation is accelerated by 3 to 5 days, depending on the EB size, and doubled over the course of cell culture for both EB sizes. Presented results indicate that RGD sequence has a dominant effect in driving endothelial cell differentiation in size-controlled EBs, while pristine multi-arm, high molecular weight PEG can induce cardiogenic differentiation, possibly through EB aggregation. The photopatternable nature of the hydrogel used in this study enabled patterning of such domains devoid or abundant of cell attachment sequences. Therefore, these hydrogels can potentially be used for spatially patterned embryonic stem cell differentiation, which may be beneficial for tissue engineering and regenerative medicine applications.

In vitro cellular models for cardiac development and pharmacotoxicology

Permanent cultures of cardiac cells described so far have limited value for studying cell biology and pharmacology of the developing heart because of the loss of proliferative capacity and cardiac-specific properties of cardiomyocytes during long-term cultivation. Pluripotent embryonic carcinoma (EC) and embryonic stem (ES) cells cultivated as permanent lines offer a new approach for studying cardiogenic differentiation in vitro. We describe cardiogenesis in vitro by differentiating EC and ES cells by way of embryo-like aggregates (embryoid bodies) into spontaneously beating cardiomyocytes. During cardiomyocyte differentiation three distinct developmental stages were defined by expression of specific action potentials and ionic currents measured by the whole-cell patch-clamp technique. Whereas early differentiated cardiomyocytes are characterized by action potentials and ionic currents typical for early pacemaker cells, terminally differentiated cardiomyocytes show action potentials and ionic currents inherent to ventricular-, atrial- or sinus nodal-like cells. These functional characteristics are in accordance with the expression of alpha- and beta-cardiac myosin heavy chain at early differentiation stages and the additional expression of ventricular-specific MLC-2V and atrial-specific ANF genes at terminal stages demonstrated by reverse transcription polymerase chain reaction (RT-PCR) analysis. Pharmacological studies performed by measuring chronotropic responses and by analysing the Ca(2+) channel activity correspond to data obtained with cardiac cells from living organisms. For testing the influence of exogenous compounds on cardiac differentiation the teratogenic compound retinoic acid (RA) was applied during distinct stages of embryoid body development. A temporally controlled influence of RA on cardiac differentiation and expression of cardiac-specific genes was found. We conclude that ES cell-derived cardiomyocytes provide an excellent cellular model to study early cardiac development and to perform pharmacological and embryotoxicological investigations.

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.

Inhibition of Rho kinase regulates specification of early differentiation events in P19 embryonal carcinoma stem cells

BACKGROUND

The Rho kinase pathway plays a key role in many early cell/tissue determination events that take place in embryogenesis. Rho and its downstream effector Rho kinase (ROCK) play pivotal roles in cell migration, apoptosis (membrane blebbing), cell proliferation/cell cycle, cell-cell adhesion and gene regulation. We and others have previously demonstrated that inhibition of ROCK blocks endoderm differentiation in embryonal carcinoma stem cells, however, the effect of ROCK inhibition on mesoderm and ectoderm specification has not been fully examined. In this study, the role of ROCK within the specification and differentiation of all three germ layers was examined.

METHODOLOGY/PRINCIPAL FINDINGS

P19 cells were treated with the specific ROCK inhibitor Y-27623, and increase in differentiation efficiency into neuro-ectodermal and mesodermal lineages was observed. However, as expected a dramatic decrease in early endodermal markers was observed when ROCK was inhibited. Interestingly, within these ROCK-inhibited RA treated cultures, increased levels of mesodermal or ectodermal markers were not observed, instead it was found that the pluripotent markers SSEA-1 and Oct-4 remained up-regulated similar to that seen in undifferentiated cultures. Using standard and widely accepted methods for reproducible P19 differentiation into all three germ layers, an enhancement of mesoderm and ectoderm differentiation with a concurrent loss of endoderm lineage specification was observed with Y-27632 treatment. Evidence would suggest that this effect is in part mediated through TGF-β and SMAD signaling as ROCK-inhibited cells displayed aberrant SMAD activation and did not return to a 'ground' state after the inhibition had been removed.

CONCLUSIONS/SIGNIFICANCE

Given this data and the fact that only a partial rescue of normal differentiation capacity occurred when ROCK inhibition was alleviated, the effect of ROCK inhibition on the differentiation capacity of pluripotent cell populations should be further examined to elucidate the role of the Rho-ROCK pathway in early cellular 'fate' decision making processes.

TGF-β isoforms inhibit IGF-1-induced migration and regulate terminal differentiation in a cell-specific manner

Following muscle injury, the damaged tissue and influx of inflammatory cells stimulate the secretion of growth factors and cytokines to initiate repair processes. This release of chemotactic signaling factors activates resident precursor cells and stimulates their mobilization and migration to the site of injury where terminal differentiation can occur. The three transforming growth factor-β (TGF-β) isoforms, and insulin-like growth factor-1 (IGF-1) are among the known regulatory factors released following muscle damage. We investigated the effect of recombinant active TGF-β1, -β2, -β3 and IGF-1 on C2C12 skeletal muscle satellite cell and P19 embryonal carcinoma cell terminal differentiation and migration. C2C12 myoblast fusion as well as P19 embryoid body formation and myogenic differentiation was assessed following 72 h TGF-β treatment (5 ng/ml), whereas the effect of the TGF-β isoforms on migration was determined following 7 h incubation. Our results showed that TGF-β decreases C2C12 myoblast fusion in an isoform-independent manner, whereas in the P19 cell lineage, results demonstrate that TGF-β1 specifically and significantly increased P19 embryoid body formation, but not expression of Connexin-43 or Myosin Heavy Chain. IGF-1 significantly increased migration compared to TGF-β isoforms, which, on their own, had no significant effect on the mobilization of either C2C12 or P19 cells. TGF-β isoforms decreased IGF-1-induced migration of both cell lineages. By distinguishing the factors involved in, and the molecular signals required for, myoblast recruitment during repair processes, strategies can be developed towards improved cell-mediated therapies for muscle injury.

Loss of a cohesin-linked suppressor APRIN (Pds5b) disrupts stem cell programs in embryonal carcinoma: an emerging cohesin role in tumor suppression

Cohesins appear to have critical functions beyond mitotic cohesion. Our data on a cohesin-associated Pds5-paralog, APRIN, indicate a novel cohesin role in stem cell differentiation. APRIN/Pds5B is lost in many cancers and it is a putative tumor suppressor. Its mutations in the germ line, however, generate birth defects. We reasoned that as both cancer and birth defects share disrupted stem cell differentiation, the data suggest an APRIN/Pds5B cohesin function in stem cells. We used an embryonal carcinoma stem cell model and show here that (i) APRIN expression is precisely coordinated with stem cell differentiation; (ii) this coordination involves surface-contact and endocrine pathways; and (iii) APRIN/Pds5b coordination is critical in stem/progenitor exit decisions. APRIN knockdown disrupted Oct4, Nanog and SOX2 patterns, differentiation failed and the resulting immature proliferative cells did not progress beyond proneural progenitor phase. Furthermore, the phenotype-blocked progenitor exit (Mash-1(+)); failed E-cadherin exit (E-Cadh(low+)); incomplete N-cadherin transition (N-Cadh(low+)); retained proliferative capacity (c-myc(+)); irregular stemness (SOX2(late++)) and lost response to contact and hormonal cues-shares similarities with cancer-initiating cells. The data suggest novel APRIN/Pds5B-linked cohesin roles in stem/progenitor programs and a new mechanism in tumor suppression.

P19 neuronal differentiation and retinoic acid metabolism as criteria to investigate atrazine, nitrite, and nitrate developmental toxicity

Atrazine and nitrogenous fertilizers are agrochemical contaminants frequently detected in water systems in North America. Several studies reported their ability to affect amphibian and mammalian development. Retinoids, supplied in the diet or synthesized by cells, are essential to embryogenesis. Disturbance of their homeostasis may lead to teratogenic effects. Retinoic acid (RA) is a major retinoid regulator of cell proliferation and differentiation. Previous studies reported alterations of retinoid stores in bullfrogs of Yamaska River subwatersheds (Québec, Canada), a region of intensive agricultural activities associated with atrazine, nitrate, and nitrite contaminants. These contaminants could affect RA metabolism and RA-mediated processes. Mouse P19 embryonic stem cells, which can differentiate to neurons in response to RA, were used to test this hypothesis. Cells were cultured in the absence or presence of contaminants during neuroinduction with RA and assayed by flow cytometry for expression of stage-specific embryonic antigen-1 (SSEA1) (embryonic marker) and betaIII-tubulin (neuronal marker). Cell cultures were also analyzed for RA metabolism by high performance liquid chromotagraphy (HPLC). Downregulation of SSEA1 paralleled betaIII-tubulin upregulation in an RA concentration-dependent manner. Atrazine, nitrate, and nitrite did not affect differentiation at environmentally encountered micromolar concentrations. However, low molar nitrite prevented RA-induced SSEA1 downregulation and decreased betaIII-tubulin appearance. Decreased cell viability/proliferation accompanied these differentiation effects. P19 cells metabolized RA to polar retinoids. RA metabolism was not affected at any concentration of atrazine, nitrate, or nitrite. Environmentally relevant levels of these contaminants, thus, had no gross effect on neurodifferentiation and RA catabolism of embryonic stem cells. P19 cell-based bioassays may provide valuable tools in monitoring developmental toxicity.

Aggregated P19 mouse embryonal carcinoma cells as a simple in vitro model to study the molecular regulations of mesoderm formation and axial elongation morphogenesis

Because of their capacity to give rise to various types of cells in vitro, embryonic stem and embryonal carcinoma (EC) cells have been used as convenient models to study the mechanisms of cell differentiation in mammalian embryos. In this study, we explored the mouse P19 EC cell line as an effective tool to investigate the factors that may play essential roles in mesoderm formation and axial elongation morphogenesis. We first demonstrated that aggregated P19 cells not only exhibited gene expression patterns characteristic of mesoderm formation but also displayed elongation morphogenesis with a distinct anterior-posterior body axis as in the embryo. We then showed by RNA interference that these processes were controlled by various regulators of Wnt signaling pathways, namely beta-catenin, Wnt3, Wnt3a, and Wnt5a, in a manner similar to normal embryo development. We further showed by inhibitor treatments that the axial elongation morphogenesis was dependent on the activity of Rho-associated kinase. Because of the convenience of these experimental manipulations, we propose that P19 cells can be used as a simple and efficient screening tool to assess the potential functions of specific molecules in mesoderm formation and axial elongation morphogenesis.

Three-dimensional extracellular matrix-directed cardioprogenitor differentiation: systematic modulation of a synthetic cell-responsive PEG-hydrogel

We show that synthetic three-dimensional (3D) matrix metalloproteinase (MMP)-sensitive poly(ethylene glycol) (PEG)-based hydrogels can direct differentiation of pluripotent cardioprogenitors, using P19 embryonal carcinoma (EC) cells as a model, along a cardiac lineage in vitro. In order to systematically probe 3D matrix effects on P19 EC differentiation, matrix elasticity, MMP-sensitivity and the concentration of a matrix-bound RGDSP peptide were modulated. Soft matrices (E=322+/-64.2 Pa, stoichiometric ratio: 0.8), mimicking the elasticity of embryonic cardiac tissue, increased the fraction of cells expressing the early cardiac transcription factor Nkx2.5 around 2-fold compared to embryoid bodies (EB) in suspension. In contrast, stiffer matrices (E=4,036+/-419.6 Pa, stoichiometric ratio: 1.2) decreased the number of Nkx2.5-positive cells significantly. Further indicators of cardiac maturation were promoted by ligation of integrins relevant in early cardiac development (alpha(5)beta(1,) alpha(v)beta(3)) by the RGDSP ligand in combination with the MMP-sensitivity of the matrix, with a 6-fold increased amount of myosin heavy chain (MHC)-positive cells as compared to EB in suspension. This precisely controlled 3D culture system thus may serve as a potential alternative to natural matrices for engineering cardiac tissue structures for cell culture and potentially therapeutic applications.

The effect of Cyclosporine A on cardiomyocytes differentiation

Cyclosporine A (CsA) is a powerful immunosuppressive drug which significantly improved the success of organ transplantation; however, the major limiting factors for the drug's clinical use are its long and short term adverse effects. The present study was conducted to examine, in a dose-dependent manner, in a model of cardiogenesis, the effect of CsA on cardiomyocytes differentiation.

A chip-based platform for the in vitro generation of tissues in three-dimensional organization

We describe a multi-purpose platform for the three-dimensional cultivation of tissues. The device is composed of polymer chips featuring a microstructured area of 1-2 cm(2). The chip is constructed either as a grid of micro-containers measuring 120-300 x 300 x 300 microm (h x l x w), or as an array of round recesses (300 microm diameter, 300 microm deep). The micro-containers may be separately equipped with addressable 3D-micro-electrodes, which allow for electrical stimulation of excitable cells and on-site measurements of electrochemically accessible parameters. The system is applicable for the cultivation of high cell densities of up to 8 x 10(6) cells and, because of the rectangular grid layout, allows the automated microscopical analysis of cultivated cells. More than 1000 micro-containers enable the parallel analysis of different parameters under superfusion/perfusion conditions. Using different polymer chips in combination with various types of bioreactors we demonstrated the principal suitability of the chip-based bioreactor for tissue culture applications. Primary and established cell lines have been successfully cultivated and analysed for functional properties. When cells were cultured in non-perfused chips, over time a considerable degree of apoptosis could be observed indicating the need for an active perfusion. The system presented here has also been applied for the differentiation analysis of pluripotent embryonic stem cells and may be suitable for the analysis of the stem cell niche.

BMP induction of cardiogenesis in P19 cells requires prior cell-cell interaction(s)

Mouse P19 embryonal carcinoma cells undergo cardiogenesis in response to high density and DMSO. We have derived a clonal subline that undergoes cardiogenesis in response to high density, but without requiring exposure to DMSO. The new subline retains the capacity to differentiate into skeletal muscle and neuronal cells in response to DMSO and retinoic acid. However, upon aggregation, these Oct 4-positive cells, termed P19-SI because they "self-induce" cardiac muscle, exhibit increased mRNAs encoding the mesodermal factor Brachyury, cardiac transcription factors Nkx 2.5 and GATA 4, the transcriptional repressor Msx-1, and cytokines Wnt 3a, Noggin, and BMP 4. Exposure of aggregated P19-SI cells to BMP 4, a known inducer of cardiogenesis, accelerates cardiogenesis, as determined by rhythmic beating and myosin staining. However, cardiogenesis is severely inhibited when P19-SI cells are aggregated in the presence of BMP 4. These results demonstrate that cell-cell interaction is required before P19-SI cells can undergo a cardiogenic response to BMP 4. A concurrent increase in the expression of Msx-1 suggests one possible process underlying the inhibition of cardiogenesis. The phenotype of P19-SI cells offers an opportunity to explore new aspects of cardiac induction.

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.

Embryonic and adult stem cell-derived cardiomyocytes: lessons from in vitro models

For years, research has focused on how to treat heart failure by sustaining the overloaded remaining cardiomyocytes. Recently, the concept of cell replacement therapy as a treatment of heart diseases has opened a new area of investigation. In vitro-generated cardiomyocytes could be injected into the heart to rescue the function of a damaged myocardium. Embryonic and/or adult stem cells could provide cardiac cells for this purpose. Knowledge of fundamental cardiac differentiation mechanisms unraveled by studies on animal models has been improved using in vitro models of cardiogenesis such as mouse embryonal carcinoma cells, mouse embryonic stem cells and, recently, human embryonic stem cells. On the other hand, studies suggesting the existence of cardiac stem cells and the potential of adult stem cells from bone marrow or skeletal muscle to differentiate toward unexpected phenotypes raise hope and questions about their potential use for cardiac cell therapy. In this review, we compare the specificities of embryonic vs adult stem cell populations regarding their cardiac differentiation potential, and we give an overview of what in vitro models have taught us about cardiogenesis.

Reprogramming in inter-species embryonal carcinoma-somatic cell hybrids induces expression of pluripotency and differentiation markers

Somatic cell reprogramming holds great promise for the development of novel cellular therapeutics. A number of sources of reprogramming potential have been identified, including oocytes, embryonic germ (EG) cells and embryonic stem (ES) cells. However, each of these sources of reprogramming factors is problematic, since they are either not freely available or have special growth requirements. Embryonal carcinoma (EC) cells are another source of pluripotent cells that, unlike ES and EG cells, do not usually require special growth conditions. Since they share many of the key characteristics of ES cells, such as pluripotency, EC cells may provide a readily amenable alternative source of reprogramming factors and could serve as a model for ES cells in this respect. Here we show that mouse EC cells can also function as donors of reprogramming factors. PEG-mediated fusion between murine EC cells (P19) and the cells of a human T-lymphoma cell line (CEM-GFP) resulted in inter-species hybrid colony formation. Colonies of hybrid cells displayed heterogeneity in cellular morphology as well as in their pattern of human gene expression. Expression of two human transcription factors characteristic of undifferentiated pluripotent stem cells, Oct-4 and Sox-2, was detected in the hybrid cells, demonstrating activation of endogenous human markers of pluripotency. Simultaneously, down-regulation of CD45, a marker present in lymphocytic cells, was observed in some hybrids. The detection of human specific markers of differentiation, such as nestin, lamininbeta1, and collagen IValpha1, indicates that fusion resulted in reprogramming of the human cells to reflect the differentiation potential of the murine EC partner.

A Wnt- and beta -catenin-dependent pathway for mammalian cardiac myogenesis

Acquisition of a cardiac fate by embryonic mesodermal cells is a fundamental step in heart formation. Heart development in frogs and avians requires positive signals from adjacent endoderm, including bone morphogenic proteins, and is antagonized by a second secreted signal, Wnt proteins, from neural tube. By contrast, mechanisms of mesodermal commitment to create heart muscle in mammals are largely unknown. In addition, Wnt-dependent signals can involve either a canonical beta-catenin pathway or other, alternative mediators. Here, we tested the involvement of Wnts and beta-catenin in mammalian cardiac myogenesis by using a pluripotent mouse cell line (P19CL6) that recapitulates early steps for cardiac specification. In this system, early and late cardiac genes are up-regulated by 1% DMSO, and spontaneous beating occurs. Notably, Wnt3A and Wnt8A were induced days before even the earliest cardiogenic transcription factors. DMSO induced biochemical mediators of Wnt signaling (decreased phosphorylation and increased levels of beta-catenin), which were suppressed by Frizzled-8Fc, a soluble Wnt antagonist. DMSO provoked T cell factor-dependent transcriptional activity; thus, induction of Wnt proteins by DMSO was functionally coupled. Frizzled-8Fc inhibited the induction of cardiogenic transcription factors, cardiogenic growth factors, and sarcomeric myosin heavy chains. Likewise, differentiation was blocked by constitutively active glycogen synthase kinase 3beta, an intracellular inhibitor of the Wntbeta-catenin pathway. Conversely, lithium chloride, which inhibits glycogen synthase kinase 3beta, and Wnt3A-conditioned medium up-regulated early cardiac markers and the proportion of differentiated cells. Thus, Wntbeta-catenin signaling is activated at the inception of mammalian cardiac myogenesis and is indispensable for cardiac differentiation, at least in this pluripotent model system.

Microarray analysis of global changes in gene expression during cardiac myocyte differentiation

Significant progress has been made in defining pathways that mediate the formation of the mammalian heart. Little is known, however, about the genetic program that directs the differentiation of cardiac myocytes from their precursor cells. A major hindrance to this kind of investigation has been the absence of an appropriate cell culture model of cardiac myocyte differentiation. Recently, a subline of P19 cells (P19CL6) was derived that, following dimethyl sulfoxide (DMSO) treatment, differentiate efficiently over 10 days into spontaneously beating cardiac myocytes. We demonstrate that these cells are indeed cardiac myocytes as they express cell type-specific markers and exhibit electrophysiological properties indicative of cardiac myocytes. The requirement for DMSO stimulation in this paradigm was shown to be limited to the first 4 days, suggesting that critical events in the differentiation process occur over this interval. To uncover relationships among known genes and identify novel genes that mediate cardiac myocyte differentiation, a detailed time course of changes in global gene expression was carried out using cDNA microarrays. In addition to the activation of genes encoding cardiac transcription factors and structural proteins, increases were noted in the expression of multiple known genes and expressed sequence tags (ESTs). Analysis of the former suggested the involvement of a variety of signaling pathways in cardiac myocyte differentiation. The 16 ESTs whose expression was increased during the early, stimulus-dependent phase of cardiac myocyte differentiation may be novel regulators of this process. Thus this first report of large-scale changes in gene expression during cardiac myocyte differentiation has delineated relationships among the expression patterns of known genes and identified a number of novel genes that merit further study.

Human septin 3 on chromosome 22q13.2 is upregulated by neuronal differentiation

An expression sequence tag identified in a screen for genes upregulated by retinoic acid induced neuronal differentiation of the human teratocarcinoma cell line Ntera2/D1 was found in close genomic proximity to a region of high sequence homology to the septin subfamily of GTPase genes. We could show that the tag corresponds to the 3' untranslated region of this novel gene named septin 3 and cloned three isoforms A (2191 bp), B (4378 bp), and C (1896 bp) from human Ntera2/D1 cDNA. We present the genomic localization and organization on chromosome 22q13.2, a chromosomal hot spot for translocations implicated in leukemia. Interestingly, MSF the closest paralog of septin 3 is a fusion partner in a therapy-related acute myeloid leukemia. Quantitative PCR confirmed the upregulation of the putative septin by neuronal differentiation and northern blotting showed only one band corresponding to sep3B with a neurospecific expression pattern in adult human tissues.

Identification of genes up-regulated by retinoic-acid-induced differentiation of the human neuronal precursor cell line NTERA-2 cl.D1

The human teratocarcinoma cell line NTERA-2 cl.D1 (NT2 cells) can be induced with retinoic acid and cell aggregation to yield postmitotic neurones. This seems to model the in vivo situation, as high concentrations of retinoic acid, retinoic acid binding proteins, and receptors have been detected in the embryonic CNS and the developing spinal cord suggesting a role for retinoic acid in neurogenesis. Suppression subtractive hybridization was used to detect genes up-regulated by this paradigm of neuronal differentiation. Microfibril-associated glycoprotein 2 was found to be drastically up-regulated and has not been implicated in neuronal differentiation before. Suppression subtractive hybridization also identified DYRK4, a homologue of the Drosophila gene minibrain. Minibrain mutations result in specific defects in the development of the fly central nervous system. In adult rats, DYRK4 is only expressed in testis, but our results suggest an additional role for DYRK4 in neuronal differentiation. We have shown that suppression subtractive hybridization in conjunction with an efficient screening procedure is a valuable tool to produce a repertoire of differentially expressed genes and propose a new physiological role for several identified genes and expressed sequence tags.

Differential expression of gap junctions in neurons and astrocytes derived from P19 embryonal carcinoma cells

The P19 embryonal carcinoma cell line represents a pluripotential stem cell that can differentiate along the neural or muscle cell lineage when exposed to different environments. Exposure to retinoic acid induces P19 cells to differentiate into neurons and astrocytes that express similar developmental markers as their embryonic counterparts. We examined the expression of gap junction genes during differentiation of these stem cells into neurons and astrocytes. Untreated P19 cells express at least two gap junction proteins, connexins 26 and 43. Connexin32 could not be detected in these cells. Treatment for 96 hr with 0.3 mM retinoic acid induced the P19 cells to differentiate first into neurons followed by astrocytes. Retinoic acid produced a decrease in connexin43 mRNA, protein, and functional gap junctions. Connexin26 message was not affected by retinoic acid treatment. The neurons that developed consisted of small round cell bodies extending two to three neurites and expressed MAP2. Connexin26 was detected at sites of cell-cell and cell-neurite contact within 3 days following differentiation with retinoic acid. The astrocytes were examined for production of their intermediate filament marker, glial fibrillary acidic protein (GFAP). GFAP was first detected at 8 days by Western blotting. In culture, astrocytes co-expressed GFAP and connexin43 similar to primary cultures of mouse brain astrocytes. These results suggest that differentiation of neurons and glial cells involves specific connexin expression in each cell type. The P19 cell line will provide a valuable model with which to examine the role gap junctions play during differentiation events of developing neurons and astrocytes.

Lysophosphatidylethanolamine acyltransferase activity is elevated during cardiac cell differentiation

We examined if elevation in lysophosphatidylethanolamine acyltransferase activity was associated with elevation in phosphatidylethanolamine content during differentiation of P19 teratocarcinoma cells into cardiac myocytes. P19 cells were induced to undergo differentiation into cardiac myocytes by the addition of 1% dimethylsulfoxide to the medium. Immunofluorescence microscopy revealed the presence of striated myosin at 8 days post-dimethylsulfoxide addition confirming differentiation into cardiac cells. The content of phosphatidylethanolamine was increased 2.1-fold (P<0.05) in differentiated cells compared to undifferentiated cells, whereas the content of phosphatidylcholine was reduced 29% (P<0.05). There were no alterations in the pool sizes of other phospholipids, including cardiolipin. The relative abundance of fatty acids in phospholipids of P19 cells was 18:1 > 18:0 > 16:1 = 18:2 > 16:0 = 14:0 > 20:4 and differentiation did not affect the relative amounts of these fatty acids within individual phospholipids. When cells were incubated with [1,3-(3)H]glycerol, radioactivity incorporated into phosphatidylethanolamine was elevated 5.8-fold, whereas radioactivity incorporated into phosphatidylcholine was unaltered. Ethanolaminephosphotransferase, cholinephosphotransferase and membrane CTP:phosphocholine cytidylyltransferase activities were elevated in differentiated cells compared to undifferentiated cells, whereas membrane and cytosolic phospholipase A2 activities were unaltered. Lysophosphatidylethanolamine acyltransferase activities were elevated 2.4-fold (P<0.05). Lysophosphatidylcholine acyltransferase, monolysocardiolipin acyltransferase, acyl-Coenzyme A synthetase and acyl-Coenzyme A hydrolase activities were unaltered in differentiated cells compared to undifferentiated cells. We postulate that during cardiac cell differentiation, the observed elevation in lysophosphatidylethanolamine acyltransferase activity accompanies the elevation in phosphatidylethanolamine mass, possibly to maintain the fatty acyl composition of this phospholipid within the membrane.

Accumulation of 210 kDa microtubule-interacting protein in differentiating P19 embryonal carcinoma cells

The MA-01 antigen, a thermolabile 210 kDa microtubule-interacting protein, is present in P19 embryonal carcinoma cells on microtubular structures as well as in cytosol. After aggregation of the cells and subsequent incubation with all-trans-retinoic acid (RA), the level of MA-01 expression increased approximately 10 times during 15 days. The increase started after 2 days of incubation with RA and preceded the appearance of neuron-specific tubulin betaIII, MAP2C and neurofilaments. Such elevated expression of MA-01 antigen was not detected in P19 cells treated with dimethylsulfoxide. These data indicate that enhanced expression of MA-01 antigen is one of the earliest events occurring in P19 cells during neuronal differentiation.

Elevation in phosphatidylethanolamine is an early but not essential event for cardiac cell differentiation

The biosynthesis of phosphatidylethanolamine was examined during differentiation of P19 teratocarcinoma cells into cardiac myocytes. P19 cells were induced to undergo differentiation into cardiac myocytes by the addition of dimethyl sulfoxide to the medium. Immunofluorescence labeling confirmed the expression of striated myosin 10 days postinduction of differentiation. The content of phosphatidylethanolamine increased significantly within the first 2 days of differentiation. [1,3-(3)H]Glycerol incorporation into phosphatidylethanolamine was increased 7.2-fold during differentiation, indicating an elevation in de novo synthesis from 1, 2-diacyl-sn-glycerol. The mechanism for the increase in phosphatidylethanolamine levels during cardiac cell differentiation was a 2.8-fold increase in the activity of ethanolaminephosphotransferase, the 1,2-diacyl-sn-glycerol utilizing reaction of the cytidine 5'-diphosphate-ethanolamine pathway of phosphatidylethanolamine biosynthesis. Incubation of P19 cells with the phosphatidylethanolamine biosynthesis inhibitor 8-(4-chlorophenylthio)-cAMP inhibited the differentiation-induced elevation in phosphatidylethanolamine levels but did not affect the expression of striated myosin. The results suggest that elevation in phosphatidylethanolamine is an early event of P19 cell differentiation into cardiac myocytes, but is not essential for differentiation to proceed.

Hsp25 and the p38 MAPK pathway are involved in differentiation of cardiomyocytes

The small heat-shock protein HSP25 is expressed in the heart early during development, and although multiple roles for HSP25 have been proposed, its specific role during development and differentiation is not known. P19 is an embryonal carcinoma cell line which can be induced to differentiate in vitro into either cardiomyocytes or neurons. We have used P19 to examine the role of HSP25 in differentiation. We found that HSP25 expression is strongly increased in P19 cardiomyocytes. Antisense HSP25 expression reduced the extent of cardiomyocyte differentiation and resulted in reduced expression of cardiac actin and the intermediate filament desmin and reduced level of cardiac mRNAs. Thus, HSP25 is necessary for differentiation of P19 into cardiomyocytes. In contrast, P19 neurons did not express HSP25 and antisense HSP25 expression had no effect on neuronal differentiation. The phosphorylation of HSP25 by the p38/SAPK2 pathway is known to be important for certain of its functions. Inhibition of this pathway by the specific inhibitor SB203580 prevented cardiomyocyte differentiation of P19 cells. In contrast, PD90589, which inhibits the ERK1/2 pathway, had no effect. Surprisingly, cardiogenesis was only sensitive to SB203580 during the first 2 days of differentiation, before HSP25 expression increases. In contrast to the effect of antisense HSP25, SB203580 reduced the level of expression of the mesodermal marker Brachyury-T during differentiation. Therefore, we propose that the p38 pathway acts on an essential target during early cardiogenesis. Once this initial step is complete, HSP25 is necessary for the functional differentiation of P19 cardiomyocytes, but its phosphorylation by p38/SAPK2 is not required.

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

P19 embryonal carcinoma cells can be induced to differentiate in culture to develop into a wide variety of cell types that include skeletal muscle. Skeletal myogenesis is controlled by transcription factors of the bHLH class, such as myoD. Expression of myoD from transfected genes did not induce significant amounts of myogenesis in P19 cells and it was possible to establish lines of undifferentiated P19[myoD] cells that express high levels of myoD mRNA. These P19[myoD] cells remained undifferentiated when cultured on solid surfaces but when allowed to aggregate, P19[myoD] cells differentiated efficiently into skeletal muscle. Aggregation did not increase the amount of myoD mRNA or the amount of myoD protein in P19[myoD] cells. The myoD protein was present in the nucleus in cells grown as attached or aggregated cultures and, in both culture conditions, the myoD protein was associated with transcription factors of the E2A family and was able to bind DNA at E-box sequences. Thus, the aggregation-induced myogenesis of P19[myoD] cells occurs in the absence of change in the myoD protein, suggesting that the cell-cell contact achieved in aggregates may result in the induction of an activity that increases accessibility of the myoD transcription factor to muscle-specific genes in chromatin.

Cholinergic properties of neurons differentiated from an embryonal carcinoma cell-line (P19)

P19 is a mouse-derived embryonal carcinoma cell-line capable of differentiation toward ectodermal, mesodermal and endodermal lineages. Following treatment with retinoic acid these cells differentiate into neurons, astrocytes and fibroblast-like cells. We induced P19 differentiation under conditions which lead to a homogeneous neuronal culture (> 95% neurons). Under these conditions, most cells (approximately 85%) express high levels of the cholinergic markers acetyl cholinesterase and choline acetyltransferase while approximately 10% of cells express the GABAergic marker glutamic acid decarboxylase. While the proportion of the GABAergic neurons is constant at different culture conditions, the cholinergic phenotype is suppressed at high cell densities. The cholinergic nature of P19 neurons is also evident in their ability to form contacts with a muscle cell-line--C2. At day 10 of differentiation cells are capable of depolarization-dependent acetylcholine release. The release is Ca2+ dependent, and drops to baseline levels at 0.5 mM Ca2+. The cells also respond to sub-nM levels of alpha-latrotoxin by acetylcholine release. All major proteins implicated in synapse functionality are expressed prior to day 10 at both at RNA and protein levels. However, the expression pattern of each gene is unique. The genes include cytoskeletal proteins, synaptic vesicle proteins and terminal specific proteins. We suggest that this cell-line can serve as an in-vitro model system for the study of neuronal phenotype acquisition. Under our conditions, the P19 cells can also provide a system in which to study the differentiation of functional cholinergic neurons.

Cells differentiating into neuroectoderm undergo apoptosis in the absence of functional retinoblastoma family proteins

The retinoblastoma (RB) protein is present at low levels in early mouse embryos and in pluripotent P19 embryonal carcinoma cells; however, the levels of RB rise dramatically in neuroectoderm formed both in embryos and in differentiating cultures of P19 cells. To investigate the effect of inactivating RB and related proteins p107 and p130, we transfected P19 cells with genes encoding mutated versions of the adenovirus E1A protein that bind RB and related proteins. When these E1A-expressing P19 cells were induced to differentiate into neuroectoderm, there was a striking rise in the expression of c-fos and extensive cell death. The ultrastructural and biochemical characteristics of the dying cells were indicative of apoptosis. The dying cells were those committed to the neural lineages because neurons and astrocytes were lost from differentiating cultures. Cell death was dependent on the ability of the E1A protein to bind RB and related proteins. Our results suggest that proteins of the RB family are essential for the development of the neural lineages and that the absence of functional RB activity triggers apoptosis of differentiating neuroectodermal cells.

Cardiomyocyte-like cells differentiated in vitro from embryonic carcinoma cells P19 are characterized by functional expression of adrenoceptors and Ca2+ channels

P19 embryonal carcinoma cells were differentiated via embryolike aggregates (embryoid bodies) into spontaneously beating myocytes. During the whole process of differentiation the functional expression of cardiac-specific receptors and ionic channels was characterized by measuring the chronotropic reactivity, action potentials, and ionic currents in response to various cardioactive drugs. Positive chronotropic effects obtained at different maximal effective concentrations of adrenoceptor-mediated agonists indicated differential adrenoceptor expression during the in vitro development of cardiomyocyte-like cells. No cardiac-specific response was obtained with the muscarinic cholinoceptor agonist carbachol. Single beating cells were enzymatically isolated and investigated by the patch-clamp technique. Pacemaker action potentials similar to those of embryonal cardiomyocytes exhibited amplitudes ranging from 50 to 85 mV. The action potentials were synchronous to the mechanical contractions and, comparable to the chronotropic effects, were modulated by BayK 8644, isradipine, and adrenaline. The functional expression of L-type Ca2+ channels was demonstrated by the Ca2+ channel blockers isradipine, nisoldipine, gallopamil, and diltiazem causing negative chronotropic responses, as well as by the Ca2+ channel activator BayK 8644 causing positive chronotropic responses. These effects gradually increased with time of differentiation. The expression of L-type Ca2+ channels and of nicotinic acetylcholine receptors was confirmed in voltage-clamp experiments. The study demonstrates that P19 embryonal carcinoma cells can be induced to differentiate into cardiomyocyte-like cells comparable to embryonal and neonatal heart cells lacking the muscarinic cholinoceptor response only.

Expression of the Brachyury gene during mesoderm development in differentiating embryonal carcinoma cell cultures

When aggregated and treated with dimethyl sulfoxide (DMSO), P19 embryonal carcinoma cells differentiate into cell types normally derived from the mesoderm and endoderm including epithelium and cardiac and skeletal muscle. The Brachyury gene is expressed transiently in these differentiating cultures several days before the appearance of markers of the differentiated cell types. The expression of Brachyury is not affected by DMSO but is induced by cell aggregation, which requires extracellular calcium. Expression of Brachyury is also induced by various members of the TGF beta family such as activin and bone morphogenetic proteins. D3 is a mutant clone of P19 cells selected for its failure to differentiate when aggregated in DMSO. Aggregated D3 cells express Brachyury mRNA suggesting that the mutation(s) responsible for the phenotype of D3 cells is downstream of the chain of events initiated by Brachyury expression.

The differentiation inducer, dimethyl sulfoxide, transiently increases the intracellular calcium ion concentration in various cell types

Dimethyl sulfoxide (DMSO) initiates a coordinated differentiation program in various cell types but the mechanism(s) by which DMSO does this is not understood. In this study, the effect of DMSO on intracellular calcium ion concentration ([Ca2+]i) was determined in primary cultures of chicken ovarian granulosa cells from the two largest preovulatory follicles of laying hens, and in three cell lines: undifferentiated P19 embryonal carcinoma cells, 3T3-L1 fibroblasts, and Friend murine erythroleukemia (MEL) cells. [Ca2+]i was measured in cells loaded with the Ca(2+)-specific fluoroprobe Fura-2. There was an immediate (i.e., within 5 sec), transient, two to sixfold increase in [Ca2+]i after exposing all cell types to 1% DMSO. DMSO was effective between 0.2 and 1%. The prompt DMSO-induced [Ca2+]i spike in all of the cell types was not prevented by incubating the cells in Ca(2+)-free medium containing 2 mM EGTA or by pretreating them with the Ca(2+)-channel blockers methoxyverapamil (D600; 100 microM), nifedipine (20 microM), or cobalt (5 mM). However, when granulosa cells, 3T3-L1 cells, or MEL cells were pretreated with lanthanum (La3+; 1 mM), which blocks both Ca2+ channels and membrane Ca2+ pumps, there was a sustained increase in [Ca2+]i in response to 1% DMSO. By contrast, pretreating P19 cells with La3+ (1 mM) did not prolong the DMSO-triggered [Ca2+]i transient. In all cases, the DMSO-induced [Ca2+]i surge was unaffected by pretreating the cells with the inhibitors of inositol phospholipid hydrolysis, neomycin (1.5 mM) or U-73, 122 (2.5 microM). These results suggest that DMSO almost instantaneously triggers the release of Ca2+ from intracellular stores through a common mechanism in cells in primary cultures and in cells of a variety of established lines, but this release is not mediated through phosphoinositide breakdown. This large, DMSO-induced Ca2+ spike may play a role in the induction of cell differentiation by DMSO.

Expression of Pax-3- and neuroectoderm-inducing activities during differentiation of P19 embryonal carcinoma cells

A P19 embryonal carcinoma stem cell line carrying an insertion of the E. coli LacZ gene in an endogenous copy of the Pax-3 gene was identified. Expression of the Pax-3/LacZ fusion gene in neuroectodermal and mesodermal lineages following induction of differentiation by chemical treatments (retinoic acid and dimethylsulfoxide) was characterized using this line and is consistent with the previous localization of Pax-3 expression in the embryo to mitotically active cells of the dorsal neuroectoderm and the adjacent segmented dermomyotome. Pax-3/LacZ marked stem cells were also utilized as target cells in mixing experiments with unmarked P19 cells that had been differentiated by pretreatment with chemical inducers. Induction of beta-galactosidase and neuroectodermal markers in the target cells demonstrates that: (1) some differentiated P19 cell derivatives transiently express endogenous Pax-3- and neuroectoderm-inducing activities, (2) undifferentiated target stem cells respond to these activities even in the presence of leukemia inhibitory factor and (3) the endogenous activities can be distinguished from, and are more potent than, retinoic acid treatment in inducing neuroectoderm. These observations demonstrate that P19 embryonal carcinoma cells provide a useful in vitro system for analysis of the cellular interactions responsible for neuroectoderm induction in mammals.

Inhibition of differentiation by leukemia inhibitory factor distinguishes two induction pathways in P19 embryonal carcinoma cells

The ability of leukemia inhibitory factor (LIF) to block differentiation of P19 embryonal carcinoma (EC) cells under a variety of induction conditions was determined. LIF inhibits differentiation under several conditions which lead to endodermal and mesodermal cell lineages including skeletal and cardiac muscle. In contrast, LIF does not block differentiation when cells are induced under conditions which lead to neuro-ectodermal cell types including neurons and astroglial cells. These studies demonstrate that P19 EC cell differentiation can be divided into LIF sensitive and insensitive pathways which correlate with differentiation of endodermal/mesodermal and neuro-ectodermal cell types, respectively. The effect of LIF on mRNA levels for several genes which have previously been implicated in mediating differentiation in P19 EC cells was determined. LIF has no effect on the mRNA levels for retinoic acid receptor (RAR) alpha, RAR beta, RAR gamma, jun A, jun D, c-fos, or fra-1. In contrast LIF stimulates jun B mRNA expression by a factor of four to six under all induction conditions.

pp60src tyrosine kinase modulates P19 embryonal carcinoma cell fate by inhibiting neuronal but not epithelial differentiation

P19 embryonal carcinoma cells provide an in vitro model system to analyze the events involved in neural differentiation. These multipotential stem cells can be induced by retinoic acid (RA) to differentiate into neural cells. We have investigated the ability of several variant forms of the protein-tyrosine kinase (PTK) pp60src to modulate cell fate determination in this system. Normally, P19 cells are induced to differentiate along a neural lineage when allowed to form extensive cell-cell contacts in large multicellular aggregates during exposure to RA. Through analysis of markers of epithelial (keratin and desmosomal proteins) and neuronal (neurofilament) cells we have found that RA-induced P19 cells transiently express epithelial markers before neuronal differentiation. Under these inductive conditions, expression of pp60v-src or expression of the neuronal variant pp60c-src+ inhibited neuronal differentiation, and resulted in maintained expression of an epithelial phenotype. Morphological analysis showed that expression of pp60src PTKs results in decreased cell-cell adhesion during the critical cell aggregation stage of the neural differentiation procedure. The effects of pp60v-src on cell fate and cell-cell adhesion could be mimicked by direct modulation of Ca+(+)- dependent cell-cell contact during RA induction of normal P19 cells. We conclude that the neural lineage of P19 cells includes an early epithelial intermediate and suggest that tyrosine phosphorylation can modulate cell fate determination during an early cell-cell adhesion- dependent event in neurogenesis.

Pluripotent mouse embryonic stem cells are able to differentiate into cardiomyocytes expressing chronotropic responses to adrenergic and cholinergic agents and Ca2+ channel blockers

A defined cultivation system was developed for the differentiation of pluripotent embryonic stem cells of the mouse into spontaneously beating cardiomyocytes, allowing investigations of chronotropic responses, as well as electrophysiological studies of different cardioactive drugs in vitro. The beta-adrenoceptor agonists (-)isoprenaline and clenbuterol, the mediators of cAMP metabolism, forskolin and isobutylmethylxanthine (IBMX), the alpha 1-adrenoceptor agonist (-)phenylephrine, and the heart glycoside digitoxin induced a positive, the muscarinic cholinoceptor agonist carbachol and L-type Ca2+ channel blockers nisoldipine, gallopamil and diltiazem induced a negative chronotropic response. In early differentiated cardiomyocytes beta 1-, alpha 1-, but not beta 2-adrenoceptors, cholinoceptors, as well as L-type Ca2+ channels participated in the chronotropic response. In terminally differentiated cardiomyocytes beta 2-adrenoceptors and digitoxin responses were also functionally expressed. The contractions of spontaneously beating cardiomyocytes were concomitant with rhythmic action potentials very similar to those described for embryonic cardiomyocytes and sinus-node cells. We conclude that cardiomyocytes differentiating from pluripotent embryonic stem cells are able to develop adrenoceptors and cholinoceptors and signal transduction pathways as well as L-type Ca2+ channels as a consequence of cell-cell interactions during embryoid body formation in vitro, independent of the development in living organisms. The cellular system described may be useful as in vitro assay for toxicological investigations of chronotropic drugs and a model system for studying commitment and cellular differentiation in vitro.

Differentiation of aggregated murine P19 embryonal carcinoma cells is induced by a novel visceral endoderm-specific FGF-like factor and inhibited by activin A

Aggregation of P19 embryonal carcinoma cells in the presence of a factor, secreted by the visceral endoderm-like cell line END-2, induces differentiation to cell types including visceral endoderm, mesoderm-derived muscle tissue and neurons. This factor is different from activin A, type beta transforming growth factors (TGF beta) and fibroblast growth factors (FGF) although its acid- and heat-lability and its stability in the presence of reducing agents resemble the properties of the FGFs. The END-2 factor is completely inhibited in its action by activin A. This inhibitory effect of activin A is not specific for the END-2 factor as retinoic acid (RA)-induced differentiation of aggregated P19 EC cells into neurons (10(-8) M RA) or mesoderm-derived muscle tissue (10(-9) M RA) is also completely inhibited by activin A. The results of this study suggest that the END-2 activity and activin A are intimately involved in the induction and regulation, respectively, of early differentiation processes in vertebrate embryogenesis.

Actin and myosin expression during development of cardiac muscle from cultured embryonal carcinoma cells

P19 embryonal carcinoma cells are multipotential stem cells that differentiate into striated muscle as well as some other cell types when aggregated and exposed to dimethyl sulfoxide (DMSO). Immunofluorescence experiments using monospecific antibodies indicated that the majority of muscle cells were mononucleate and contained four myosin isoforms normally found in cardiac muscle; atrial and ventricular myosin heavy chains, ventricular myosin light chain 1, and atrial myosin light chain 2. Northern blot analysis of RNA isolated from differentiating cultures indicated that cardiac actin and skeletal actin mRNAs were expressed at similar levels and with identical kinetics during the differentiation of P19-derived myocytes. These results demonstrate that most of the P19-derived myocytes are of the cardiac type and suggest that they closely resemble the cells of the early embryonic myocardium.

Smooth muscle actin expression during P19 embryonal carcinoma differentiation in cell culture

P19 embryonal carcinoma (EC) cells can be induced in vitro to differentiate into cells resembling those normally formed in the embryo. Among these cell types is one whose morphology is fibroblast-like. Using indirect immunofluorescence and Western blot analysis with antibodies directed against various isoforms of actin, many of these fibroblast-like cells were found to express smooth muscle actin isoforms. Northern blot analysis of RNA indicated the presence of a smooth muscle-specific isoform of myosin heavy-chain mRNA in immortal lines of these fibroblast-like cells. These results suggest that these fibroblast-like cells resemble fetal myofibroblastic or myoepithelial cells, which have a wide distribution during embryonic development.

States of developmental commitment of a mouse embryonal carcinoma cell line differentiating along a neural pathway

The embryonal carcinoma cell line PCC7-S-AzaR1 (clone 1009) has been shown to differentiate in the presence of all-trans retinoic acid and dibutyryl cAMP into cells of predominantly neural properties (Paulin, D., H. Jakob, F. Jacob, K. Weber, and M. Osborn. 1982. Differentiation. 22:90-99). By analyzing the marker expression of derivatives in further detail, we characterized the two major cell phenotypes as neuron- and fibroblast-like and the two minor ones as astroglia- and endothelial-like. The stability of developmental commitment of clone 1009 was tested by recloning. The isolated subclones exhibited different patterns of chemically induced derivatives, with some of them (denoted N-clones) producing only a single (neuronal) cell type. As shown by long-term cultures in the absence of retinoic acid, the properties of isolated subclones remained essentially stable. In contrast to the clones producing neuron-like and other derivatives upon induced differentiation, the (exclusively neuronal) derivatives of N-clones detached and died within a few days in culture. If maintained in the presence of other neural cell types, however, their survival was dramatically extended indicating a requirement for specific interactions with other cells of the same tissue. The patterns of derivatives obtained from N-clones depended on the chemical nature of the substrate on which they were grown. Thus, when seeded on laminin-coated surfaces before induced differentiation, N-clones developed not only to neuron-like derivatives but rather to the same four derivatives observed with the original cell pool. These and further results suggest a common cell lineage of the identified phenotypes. The isolated subclones of uninduced cells probably represent different states of commitment within the same developmental pathway. Their stability offers the opportunity to analyze the nature of cellular commitment on the cellular, molecular, and genetic levels. This makes the family of clones derived from PCC7-S-AzaR1 (clone 1009) cells an advantageous in vitro model of mammalian brain early ontogenesis.

Regulated expression of a transfected human cardiac actin gene during differentiation of multipotential murine embryonal carcinoma cells

P19 embryonal carcinoma (EC) cells are multipotential stem cells which can be induced to differentiate in vitro into a variety of cell types, including cardiac muscle cells. A cloned human cardiac actin (CH-actin) gene was transfected into P19 cells, and stable transformants were isolated. Low levels of CH-actin mRNA were present in transformed EC cells, but a marked increase in the level of CH-actin mRNA was found as these cells differentiated into cardiac muscle. The accumulation of CH-actin mRNA paralleled that of the endogenous mouse cardiac actin mRNA. A chimeric gene, which consisted of the CH-actin promoter linked to the herpes simplex virus thymidine kinase coding region, was constructed and transfected into P19 cells. In these transformants, the thymidine kinase protein was located almost exclusively in cardiac muscle cells and was generally not detectable in EC or other nonmuscle cells. These results suggest that the transfected CH-actin promoter functions in the appropriate developmental and tissue-specific manner during the differentiation of multipotential EC cells in culture.

Regulated expression of a transfected human cardiac actin gene during differentiation of multipotential murine embryonal carcinoma cells

P19 embryonal carcinoma (EC) cells are multipotential stem cells which can be induced to differentiate in vitro into a variety of cell types, including cardiac muscle cells. A cloned human cardiac actin (CH-actin) gene was transfected into P19 cells, and stable transformants were isolated. Low levels of CH-actin mRNA were present in transformed EC cells, but a marked increase in the level of CH-actin mRNA was found as these cells differentiated into cardiac muscle. The accumulation of CH-actin mRNA paralleled that of the endogenous mouse cardiac actin mRNA. A chimeric gene, which consisted of the CH-actin promoter linked to the herpes simplex virus thymidine kinase coding region, was constructed and transfected into P19 cells. In these transformants, the thymidine kinase protein was located almost exclusively in cardiac muscle cells and was generally not detectable in EC or other nonmuscle cells. These results suggest that the transfected CH-actin promoter functions in the appropriate developmental and tissue-specific manner during the differentiation of multipotential EC cells in culture.