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

High glucose inhibits neural differentiation by excessive autophagy <em>via</em> peroxisome proliferator-activated receptor gamma

The high prevalence of prediabetes and diabetes globally has led to the widespread occurrence of severe complications, such as diabetic neuropathy, which is a result of chronic hyperglycemia. Studies have demonstrated that maternal diabetes can lead to neural tube defects by suppressing neurogenesis during neuroepithelium development. While aberrant autophagy has been associated with abnormal neuronal differentiation, the mechanism by which high glucose suppresses neural differentiation in stem cells remains unclear. Therefore, we developed a neuronal cell differentiation model of retinoic acid induced P19 cells to investigate the impact of high glucose on neuronal differentiation in vitro. Our findings indicate that high glucose (HG) hinders neuronal differentiation and triggers excessive. Furthermore, HG treatment significantly reduces the expression of markers for neurons (Tuj1) and glia (GFAP), while enhancing autophagic activity mediated by peroxisome proliferator-activated receptor gamma (PPARγ). By manipulating PPARγ activity through pharmacological approaches and genetically knocking it down using shRNA, we discovered that altering PPARγ activity affects the differentiation of neural stem cells exposed to HG. Our study reveals that PPARγ acts as a downstream mediator in high glucose-suppressed neural stem cell differentiation and that refining autophagic activity via PPARγ at an appropriate level could improve neuronal differentiation efficiency. Our data provide novel insights and potential therapeutic targets for the clinical management of gestational diabetes mellitus.

Expression analysis of SOX14 during retinoic acid induced neural differentiation of embryonal carcinoma cells and assessment of the effect of its ectopic expression on SOXB members in HeLa cells

SOX14 is a member of the SOXB2 subgroup of transcription factors implicated in neural development. Although the first SOX14 gene in vertebrates was cloned and characterized more than a decade ago and its expression profile during development was revealed in various animal model systems, the role of this gene during neural development is largely unknown. In the present study we analyzed the expression of SOX14 in human NT2/D1 and mouse P19 pluripotent embryonal carcinoma cells. We demonstrated that it is expressed in both cell lines and upregulated during retinoic acid induced neural differentiation. We showed that SOX14 was expressed in both neuronal and non-neuronal differentiated derivatives, as revealed by immunocytochemistry. Since it was previously proposed that increased SOXB2 proteins level interfere with the activity of SOXB1 counteracting partners, we compared expression patterns of SOXB members during retinoic acid induction of embryonal carcinoma cells. We revealed that upregulation of SOX14 expression is accompanied by alterations in the expression patterns of SOXB1 members. In order to analyze the potential cross-talk between them, we generated SOX14 expression construct. The ectopic expression of SOX14 was demonstrated at the mRNA level in NT2/D1, P19 and HeLa cells, while an increased level of SOX14 protein was detected in HeLa cells only. By transient transfection experiments in HeLa cells we showed for the first time that ectopic expression of SOX14 repressed SOX1 expression, whereas no significant effect on SOX2, SOX3 and SOX21 was observed. Data presented here provide an insight into SOX14 expression during in vitro neural differentiation of embryonal carcinoma cells and demonstrate the effect of its ectopic expression on protein levels of SOXB members in HeLa cells. Obtained results contribute to better understanding the role of one of the most conserved SOX proteins.

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.

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.

Gap junction blockage interferes with neuronal and astroglial differentiation of mouse P19 embryonal carcinoma cells

During embryonic development, cells not only increase in number, they also undergo specialization and differentiate into diverse cell types that are organized into different tissues and organs. Nervous system development, for example, involves a complex series of events such as neuronal and astroglial differentiation that are coordinated among adjacent cells. The organization of growth and differentiation may be mediated, at least partly, by exchange of small ions and molecules via intercellular gap junction channels. These structures are mode of connexons (hemichannels), which are hexameric assemblies of the gap junction proteins, connexins. We investigated the role of intercellular communication in neuronal and astroglial differentiation by using a gap junction blocking agent, carbenoxolone (CBX), in comparison to its inactive (control) analog, glycyrrhizic acid (GZA). We used the mouse P19 embryonal carcinoma cell line, which differentiates into neurons and astrocytes upon retinoic acid (RA) induction. Our results show that both GZA- and CBX-treated cells express alpha 1 connexin (connexin43). The level of alpha 1 connexin decreases upon RA induction. CBX treated cells show significant reduction in both neuronal (5-fold) and astrocytic (13-fold) differentiation compared with those of control. These results clearly indicate that the blockage of gap junction-mediated intercellular communication interferes with differentiation of P19 cells into neurons and astrocytes.

The gain-of-function Chinese hamster ovary mutant LEC11B expresses one of two Chinese hamster FUT6 genes due to the loss of a negative regulatory factor

The LEC11 Chinese hamster ovary (CHO) gain-of-function mutant expresses an alpha(1,3)fucosyltransferase (alpha(1,3)Fuc-T) activity that generates the LeX, sialyl-LeX, and VIM-2 glycan determinants and has been extensively used for studies of E-selectin ligand specificity. In order to identify regulatory mechanisms that control alpha(1,3)Fuc-T expression in mammals, mechanisms of FUT gene expression were investigated in LEC11 cells and two new, independent mutants, LEC11A and LEC11B. Northern and ribonuclease protection analyses, using probes that span the coding region of a cloned CHO FUT gene, detected transcripts in each LEC11 mutant but not in CHO cells or other gain-of-function CHO mutants that express a different alpha(1,3)Fuc-T activity. Coding region sequence analysis and alpha(1,3)Fuc-T acceptor specificity comparisons with recombinant human Fuc-TV and Fuc-TVI showed that the cloned FUT gene is orthologous to the human FUT6 gene. Southern analyses identified two closely related FUT6 genes in the Chinese hamster, whose evolutionary relationships are discussed. The blots showed that rearrangements had occurred in LEC11A and LEC11 genomic DNA, consistent with a cis mechanism of FUT6 gene activation in these mutants. By contrast, somatic cell hybrid analyses revealed that LEC11B cells express FUT6 gene transcripts due to the loss of a trans-acting, negative regulatory factor. Sequencing of reverse transcriptase-polymerase chain reaction products identified unique 5'- and 3'-untranslated region sequences in FUT6 gene transcripts from each LEC11 mutant. Northern and Southern analyses with gene-specific probes showed that LEC11A cells express only the cgFUT6A gene (where cg is Cricetulus griseus), whereas LEC11 and LEC11B cells express only the cgFUT6B gene. In LEC11A x LEC11B hybrid cells, the cgFUT6A gene was predominantly expressed, as predicted if a trans-acting negative regulatory factor functions to suppress cgFUT6B gene expression in CHO cells. This factor is predicted to be a cell type-specific regulator of FUT6 gene expression in mammals.

Meis2, a novel mouse Pbx-related homeobox gene induced by retinoic acid during differentiation of P19 embryonal carcinoma cells

We report the cDNA cloning, partial genomic organization, and expression pattern of Stra10, a novel retinoic acid-inducible gene in P19 embryonal carcinoma cells. Four murine cDNA isoforms have been isolated, which are likely to result from alternative splicing. The predicted protein sequences exhibit approximately 85% identity with the Pbx-related Meis1 homeobox gene products, which are involved in myeloid leukemia in BXH-2 mice, and one of the Stra10 isoforms corresponds to the recently published Meis2 sequence (Nakamura et al. [1996] Oncogene 13:2235-2242). The Meis2 homeodomain is identical to that of Meis1, and is most closely related to those of the Pbx/TGIF homeobox gene products. By in situ hybridization analysis, we show that the Meis2 gene displays spatially restricted expression patterns in the developing nervous system, limbs, face, and in various viscera. In adult mice, Meis2 is mainly expressed in the brain and female genital tract, with a different distribution of the alternative splice forms in these organs.

Developmental expression pattern of Stra6, a retinoic acid-responsive gene encoding a new type of membrane protein

Retinoic acid plays important roles in development, growth and differentiation by regulating the expression of target genes. A new retinoic acid-inducible gene, Stra6, has been identified in P19 embryonal carcinoma cells using a subtractive hybridization cDNA cloning technique. Stra6 codes for a very hydrophobic membrane protein of a new type, which does not display similarities with previously characterized integral membrane proteins. Stra6, which exhibits a specific pattern of expression during development and in the adult, is strongly expressed at the level of blood-organ barriers. Interestingly, in testis Sertoli cells, Stra6 has a spermatogenic cycle-dependent expression which is lost in testes of RAR alpha null mutants where Stra6 is expressed in all tubules. We suggest that the Stra6 protein may be a component of an as yet unidentified transport machinery.

Muscle isoactin expression during in vitro differentiation of murine embryonic stem cells

Embryonic stem (ES) cells are pluripotent cells derived from mouse blastocysts. ES cells can differentiate into complex embryoid bodies (EBs) which exhibit many of the characteristics of 4-10-d embryos, including areas which rhythmically contract. The expression of the four muscle isoactins was examined in EBs by using transcript-specific probes for each of the muscle actin mRNAs and selectively reactive MAbs to muscle actins. Northern blot analyses from undifferentiated ES cells and EBs after 5, 10, 15, and 20 d in suspension culture demonstrated that no muscle actin transcripts could be detected in the undifferentiated cells, whereas during differentiation, the vascular and enteric smooth muscle isoactin mRNAs were easily detected. To further define the pattern of expression polymerase chain reaction analyses were carried out on RNA isolated from individual EBs. The data indicated that all four muscle-specific actin genes are transcribed. We also demonstrated the presence of muscle actins in at least two distinct cell populations within the EBs using selectively reactive MAbs. Fibroblast-like cells exhibit significant levels of the two smooth muscle actins (vascular and enteric) localized to stress fibers. In addition, one or both of the striated muscle actins (cardiac and skeletal) are expressed in cardiomyocyte-like cells. As is the case in embryonic heart, alpha-smooth muscle actin and the striated muscle actin(s) are incorporated into well organized sarcomeres in these cardiomyocyte-like cells. Thus, differentiating EBs provide an in vitro system to study both striated and smooth muscle cell gene expression.

Preferential differentiation of P19 mouse embryonal carcinoma cells into smooth muscle cells. Use of retinoic acid and antisense against the central nervous system-specific POU transcription factor Brn-2

Investigation of the molecular mechanisms that control smooth muscle cell (SMC) development and differentiation is a prerequisite in understanding the regulatory mechanisms of physiological and pathological SMC-associated vascular processes. The pluripotent murine embryonal carcinoma P19 cell, whose developmental potential resembles that of early embryonic cells, can develop into cell types derived from the neuroectoderm, mesoderm, and endoderm. In the present study, we have shown a unique strategy to enhance SMC differentiation in P19 cells. Under chemical induction of high concentrations of retinoic acid (1 micromol/L), P19 cells showed optimum differentiation into SMCs. Because the P19 cells thus induced also showed differentiation into neuronal cells, a strategy to block neuronal lineage differentiation was developed using a stable transformant antisense RNA construct against Brn-2, a neuronal lineage-specific POU-domain transcription factor; thus, by specifically inhibiting neuronal differentiation, enhanced SMC differentiation by P19 cells was attained. SMC expression was confirmed by immunohistochemical staining, RNA analysis (RNase protection assay), and protein analysis (Western blot) using SMC-specific markers (eg, SM1 and calponin) and alpha-smooth muscle actin. Our results show that the pathway of SMC differentiation may provide an in vitro system useful in the investigation of SMC regulatory mechanisms (eg, transcriptional regulation) and in the further understanding of SMC development and differentiation.

Neurons derived from P19 embryonal carcinoma cells develop responses to excitatory and inhibitory neurotransmitters

Cells of the P19 line of embryonal carcinoma cells differentiate into neurons, astrocytes and oligodendrocytes following treatment with retinoic acid. The neurons from these differentiating P19 cultures synthesize a pattern of neurotransmitters that resembles that of neurons of the forebrain. We treated P19 cells with retinoic acid and then implanted them into the striatum of adult rats. After times ranging from 1 to 15 weeks post-implantation, brain slices containing the implanted tissue were prepared and used for intracellular recording of electrical activity and responsiveness to application of neurotransmitters. Within 2 weeks of implantation, the P19-derived neurons had developed responsiveness to the excitatory neurotransmitter glutamate and the inhibitory transmitters gamma-aminobutyric acid and glycine. These neurons also exhibited spontaneous synaptic potentials. The responses to glutamate appear to be mediated by N-methyl-D-aspartic acid as well as non-N-methyl-D-aspartic acid receptor subtypes. Gamma-aminobutyric acid evoked bicuculline-sensitive depolarizing responses in the younger grafts and biphasic depolarizing/hyperpolarizing responses in older ones. Responses to glycine were strychnine sensitive and also showed age-related changes from depolarizing to biphasic character. Synaptic potentials in the younger grafts were exclusively depolarizing, but in older ones both depolarizing and hyperpolarizing events were observed. The synaptic potentials appear to arise from synaptic connections between P19-derived neurons within the grafts. Many of the features of P19-derived neurons are similar to those of neurons in the developing forebrain.

Sequence and expression pattern of the Stra7 (Gbx-2) homeobox-containing gene induced by retinoic acid in P19 embryonal carcinoma cells

The cDNA sequence of Stra7, a retinoic acid (RA)-inducible gene in P19 embryonal carcinoma (EC) cells, was determined. The deduced Stra7 protein contains a homeodomain highly similar to that of the previously described chicken CHox7 gene product, and is highly conserved during evolution, from hemichordates to vertebrates. The mouse Stra7 cDNA corresponds to the full-length form of the 77 bp homeodomain-encoding cDNA fragment which was previously cloned and termed MMoxA or Gbx-2. Reverse-transcriptase-PCR analysis revealed the presence of Stra7/Gbx-2 transcripts in the adult brain, spleen, and female genital tract, whereas no expression could be observed in heart, liver, lung, kidney, or testes. In situ hybridization analysis showed a restricted expression pattern of Stra7/Gbx-2 in the three primitive germ layers during gastrulation. Restricted expression was also detected in the pharyngeal arches. Subsequently, there were specific expression domains in the developing central nervous system, at the midbrain/hindbrain boundary and later in the cerebellum anlage, in certain rhombomeres, in dorsal regions of the spinal cord, and in the developing dorsal thalamus and corpus striatum.

On the ontogeny of cardiac gene transcripts

As a prerequisite to investigating the specification and differentiation of cardiac tissue in vitro, the ontogeny of a number of putative cardiac-specific, and striated muscle-specific gene transcripts has been studied. The probes used include cDNAs of alpha-actins, myosin heavy chains, myosin light chains, alpha-tropomyosin, troponin-T and atrial natriuretic factor. The expression of these genes was monitored by Northern analysis of heart and various other tissues at three developmental ages, viz, adult, neonatal and mid-foetal. The aim of this exercise was to confirm the efficacy of a number of markers to represent a cardiac-specific subset of gene expression in our mammalian model, the guinea pig. Our results indicate predominantly cardiac expression for the mRNA transcripts of cardiac alpha-actin (c alpha-actin), cardiac myosin heavy chain-alpha (MHC alpha), cardiac myosin heavy chain-beta (MHC beta), myosin light chain-1A (MLC1A), myosin light chain-1V (MLC1V), alpha-tropomyosin (alpha TM), cardiac troponin-T (cTnT) and atrial natriuretic factor (ANF). Furthermore, cardiac-specific expression at the midfoetal time point was observed for five gene transcripts, MLC1V, MHC alpha, MHC beta, striated alpha TM and ANF. No genes were expressed exclusively in cardiac tissue; for example, expression of the genes for c alpha-actin, both cardiac MHCs, both MLCs, alpha TM and cTnT was evident in skeletal and vascular smooth muscles at some stages of development. An interesting difference between this species and those of previous studies was the minor contribution of skeletal alpha-actin to cardiac phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Unstable integration of transfected DNAs into embryonal carcinoma cells

Plasmid DNA can be efficiently transfected into embryonal carcinoma cells but it is difficult to isolate clones of cells stably expressing genes present on the transfected plasmids. Even in clonal populations derived from transfected cells, the introduced genes are expressed in some but not all cells. Cotransfection with a region of the Pgk-1 gene results in more efficient, stable cotransformation due to increased numbers of copies of the transfected plasmids integrated into the genomic DNA. The PgK-1 genomic sequences did not allow the plasmid DNA to replicate autonomously but seemed to enhance the ligation of transfected plasmids before their integration into the host genome. Our results suggest a model in which the plasmid DNAs are able to integrate and subsequently excise from the host genome by recombination events enhanced by transcription through the tandemly repeated sequences of the transfected plasmids.

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.

Inactivation of MyoD in mice leads to up-regulation of the myogenic HLH gene Myf-5 and results in apparently normal muscle development

The myogenic basic HLH transcription factor family of genes, composed of MyoD, myogenin, Myf-5, and Myf-6, are thought to regulate skeletal muscle differentiation. To understand the role of MyoD in myogenesis, we have introduced a null mutation of MyoD into the germline of mice. Surprisingly, mice lacking MyoD are viable and fertile. Histological examination of skeletal muscle failed to reveal any morphological abnormalities in these mice. Furthermore, Northern analysis revealed normal levels of skeletal muscle-specific mRNAs. Significantly, Myf-5 mRNA levels are elevated in postnatal mutant mice. Normally, Myf-5 expression becomes markedly reduced at day 12 of gestation when MyoD mRNA first appears. This suggests that Myf-5 expression is repressed by MyoD. Our results indicate that MyoD is dispensable for skeletal muscle development in mice, revealing some degree of functional redundancy in the control of the skeletal myogenic developmental program.

Targeted inactivation of the muscle regulatory gene Myf-5 results in abnormal rib development and perinatal death

The Myf-5 gene, a member of the myogenic basic HLH factor family, has been inactivated in mice after homologous recombination in ES cells. Mice lacking Myf-5 were unable to breathe and died immediately after birth, owing to the absence of the major distal part of the ribs. Other skeletal abnormalities, except for complete ossification of the sternum, were not apparent. Histological examination of skeletal muscle from newborn mice revealed no morphological abnormalities. Northern blot analysis demonstrated normal levels of muscle-specific mRNAs including MyoD, myogenin, and Myf-6. However, the appearance of myotomal cells in early somites was delayed by several days. These results suggest that while Myf-5 plays a crucial role in the formation of lateral sclerotome derivatives, Myf-5 is dispensable for the development of skeletal muscle, perhaps because other members of the myogenic HLH family substitute for Myf-5 activity.

Inhibition of MAP2 expression affects both morphological and cell division phenotypes of neuronal differentiation

Expression of the differentiated neuronal phenotype is typically manifest in several properties: distinct morphologies and organizations of the underlying cytoskeleton; appearance of specific macromolecules; and cessation of cell division. All of these properties are induced in undifferentiated embryonal carcinoma cells exposed to retinoic acid. We show here that the mRNA and protein for the microtubule component MAP2 is also induced by retinoic acid. Stable transfectants of undifferentiated cells, constitutively expressing MAP2 antisense RNA, show significantly reduced levels of MAP2 antisense RNA, show significantly reduced levels of MAP2 protein upon induction compared with controls. These cells do express other neuronal markers, but they do not undergo normal morphological differentiation nor do they withdraw from the cell cycle. The results suggest that MAP2 expression may be necessary for both neurite extension and cessation of cell division.

Search of the 5' untranslated region of the human cardiac actin gene for segments controlling translation

We have examined the possibility that the 5'-UT region of the human cardiac actin (CH-actin) mRNA is responsible for regulating translation of this transcript during skeletal myogenesis. Genes were constructed which consisted of the murine leukaemia virus promoter driving the Escherichia coli LacZ coding region with and without the CH-actin 5'-UT region. These constructs were transfected into L6 myoblasts that were subsequently differentiated into myotubes. The presence of the CH-actin 5'-UT region appeared to have no effect on expression of the LacZ reporter gene. EGTA blocks myogenesis and inhibits translation of muscle-specific transcripts (Endo, T. and Nadal-Ginard, B. (1987) Cell 49, 515-526) but EGTA had no effect on expression of the chimaeric LacZ transcripts. Thus, if the CH-actin transcript is subject to translational regulation, it must be mediated by sequences other than those of the 5'-UT region.

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.

Regulated expression of muscle-specific genes introduced into mouse embryonal stem cells: inverse correlation with DNA methylation

Pluripotent embryonal stem cell lines (ES) were isolated from cultured normal mouse blastocysts. These cells retained their capacity to differentiate into a great variety of cell types in cell cultures or in tumors formed after subcutaneous injection of the cells into nude mice. A chimeric actin/globin gene containing about two-thirds of the rat skeletal muscle actin gene and 730 bp of its 5' flanking region fused to the 3' end of the human embryonic epsilon-globin gene, was inserted into a plasmid containing a neomycin resistance gene (neor) whose transcription is regulated by the SV40 early control elements. The prokaryotic vector DNA sequences of this plasmid (pAG-Neo) were deleted and the two linked genes were introduced into the ES cells by electroporation. G418-resistant clones were isolated, amplified and injected subcutaneously into nude mice. From the teratocarcinoma-like tumors which developed we isolated myogenic as well as nonmyogenic cell lines. In cell lines derived from three independent transfected ES clones, expression of the actin/globin gene was developmentally regulated in myogenic cells. In contrast, in a number of experiments in which the actin/globin gene or other muscle-specific genes were introduced into the ES cells without the removal of the pBR sequences, no expression could be detected at any stage. Moreover, in the differentiated lines derived from these clones, G418 resistance was lost, and no neor transcripts could be detected. Southern-blot analysis of MSPI- or HpaII-digested DNA revealed extensive methylation in the clones that did not express the foreign DNA, whereas no significant methylation of the inserted DNA was observed in clones which expressed the transfected genes. Examination of the DNA extracted from transgenic mice carrying the same actin/globin gene revealed an inverse correlation between methylation of the exogenous gene and its potential to be expressed in the transgenic strain. However, no tissue-specific differences in methylation, related to the tissue specificity of expression of the exogenous gene, could be detected in these experiments. These results suggest that the process of methylation reported here is causally related to constitutive inactivation of the exogenous genes.

Expression of the human cardiac actin gene in differentiating rat skeletal myoblasts

The human cardiac-actin (CH-actin) gene was transfected into rat L6 skeletal myoblasts and stable transformants were isolated. The level of the CH-actin transcript varied between clones but changed little during the differentiation of myoblasts into multinucleate myotubes. Chimeric genes were constructed in which the CH-actin promoter, first non-coding exon (44 bp), and first intron (about 700 bp) were linked to the Herpes simplex virus thymidine kinase (tk) coding region. Clones of L6 cells transformed with these chimeric genes contained variable levels of actin-tk mRNA which changed little during differentiation. Thus, the activity of the CH-actin promoter appeared not to be up-regulated upon differentiation of myoblasts into myotubes. In clones of cells expressing the actin-tk mRNA, the TK protein was not detected in myoblasts but appeared in differentiating multinucleate myotubes. We interpret these results as suggesting developmentally regulated translation of the actin-tk mRNA. Since the first 44 nucleotides of the actin-tk mRNA were derived from the 5'-untranslated region of the CH-actin mRNA. These experiments suggest that translation of the actin-tk mRNA may be controlled by this region.