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

Pax3, neural crest and cardiovascular development

The understanding of cardiovascular development has begun a transformation from the descriptive science of anatomy and embryology to a molecular understanding of the cellular and subcellular events leading to proper cardiac morphogenesis. Powerful tools available to molecular geneticists have identified numerous examples of specific gene defects that result in predictable cardiovascular abnormalities. Not only have certain genes been "knocked out" (mutated by homologous recombination in embryonic stem cells), but also single gene defects have been found to underlie the cardiovascular derangements observed in certain inbred mouse lines. Such is the case for the mouse mutant Splotch, which was first described in 1954 as a spontaneously occurring mutation resulting in a white belly spot. More recently, the genetic defect of all of the various Splotch alleles has been found to be due to mutations or deletions of a gene called Pax-3. In the homozygous state, these mutations result in embryonic lethality at about day 13.5 of mouse embryogenesis (E13.5). These embryos display abnormalities strikingly reminiscent of human DiGeorge syndrome. These include outflow tract abnormalities of the heart, such as double-outlet right ventricle (DORV) and persistent truncus arteriosus (PTA), as well as abnormalities of the great vessels and the thyroid and parathyroid glands. These defects suggest an underlying abnormality of neural crest, including its contribution to the cardiovascular system. © 1996, Elsevier Science Inc. (Trends Cardiovasc Med 1996;6:255-261).

Induction of a high population of neural stem cells with anterior neuroectoderm characters from epiblast-like P19 embryonic carcinoma cells

The epiblast, derived from the inner cell mass (ICM), represents the final embryonic founder cell population of mouse embryo and can give rise to all germ layer lineages including the neuroectoderm. The generation of neural stem cells from epiblast-like cells is of great value for studying the mechanism of neural determination during gastrulation stages of embryonic development. Mouse embryonic carcinoma (EC) P19 cells are equivalent to the epiblast of early post-implantation blastocysts. In this study, we establish a feasible induction system that allows rapid and efficient derivation of a high percentage ( approximately 95%) of neural stem cells from P19 EC cell in N2B27 serum-free medium. The induced neural stem cells bear anterior neuroectoderm characters, and can be efficiently caudalized by retinoic acid (RA). These neural stem cells have multilineage potential to differentiate into neurons, astrocytes, and oligodendrocytes. Mechanistic analysis indicates that inhibition of the bone morphogenetic protein (BMP) pathway may be the main reason for N2B27-neural induction, and that fibroblast growth factor (FGF) signaling is also involved in this process. This method will provide an in vitro system to dissect the molecular mechanisms involved in neural induction of early mouse embryos.

Retinoic acid regulation of eye and testis-specific transcripts within a complex locus

We previously used a yeast-based enhancer trap to identify a strong, retinoic acid response element (RARE). We have now characterized testis and eye transcripts that are adjacent to this regulatory element. Bioinformatics analysis of expressed sequence tag (EST) clones and RNase protection, reverse transcription-PCR, and Northern blot assays indicate that these two RNAs are transcribed from the same locus on opposite template strands. This positions the RARE upstream of the testis transcript and downstream of the eye transcript. Additionally, these two RNAs are embedded within the third intron of the 329kbp gene that encodes the Zinc Finger and BTB domain containing 7C protein (Zbtb7C). We present evidence indicating that the testis transcript is expressed primarily in spermatocytes and/or early round spermatids. Furthermore, our analyses of transcript levels in eyes and testes isolated from vitamin A deficient mice or from mice with defects in retinoid storage or signaling indicate that retinoids are required for expression in vivo.

Molecular evidence from ascidians for the evolutionary origin of vertebrate cranial sensory placodes

Cranial sensory placodes are specialised areas of the head ectoderm of vertebrate embryos that contribute to the formation of the cranial sense organs and associated ganglia. Placodes are often considered a vertebrate innovation, and their evolution has been hypothesised as one key adaptation underlying the evolution of active predation by primitive vertebrates. Here, we review recent molecular evidence pertinent to understanding the evolutionary origin of placodes. The development of vertebrate placodes is regulated by numerous genes, including members of the Pax, Six, Eya, Fox, Phox, Neurogenin and Pou gene families. In the sea squirt Ciona intestinalis (a basal chordate and close relative of the vertebrates), orthologues of these genes are deployed in the development of the oral and atrial siphons, structures used for filter feeding by the sessile adult. Our interpretation of these findings is that vertebrate placodes and sea squirt siphon primordia have evolved from the same patches of specialised ectoderm present in the common ancestor of the chordates.

Hox/Pbx and Brn binding sites mediate Pax3 expression in vitro and in vivo

Pax3 is a paired-homeodomain class transcription factor that serves a role in dorsal-ventral and medial-lateral patterning during vertebrate embryogenesis. Its expression is localized to dorsal domains within the developing neural tube and lateral domains within the developing somite. Additionally, modulation of its expression occurs along the rostral-caudal axis. Previous studies [Development 124 (1997) 617] have localized sequence elements required for expression of Pax3 in the neural tube and neural crest to a 1.6 kbp promoter fragment. In the present study, four discrete DNA elements within the 1.6 kbp promoter fragment are shown by electrophoretic mobility shift assays (EMSA) to exhibit sequence specific interactions with proteins present in nuclear extracts from P19 EC cells induced to express Pax3 by treatment with retinoic acid (RA). Proteins interacting at each of these elements are identified based on biochemical purification using DNA affinity chromatography or a candidate approach. These identifications were confirmed by the ability of specific antibodies to super-shift DNA-protein complexes in EMSA. Two of the four DNA sequence elements are shown to interact with the neural specific Pou-domain class III transcription factors Brn1 and Brn2. The remaining sites contain either consensus binding elements for heterodimers of Pbx and an anterior set of Hox family members, from paralogous groups 1-5, or monomeric Meis and are shown to interact with members of the Pbx and Meis families. Ectopic expression of Brn2 plus HoxA1 but not either factor alone, is sufficient to induce efficient expression from the endogenous Pax3 promoter in P19 EC stem cells under conditions where they would not otherwise express Pax3. Finally, in transgenic mice, mutation of either of the Pou-domain protein binding sites results in reduced expression throughout the neural tube while mutation of the Pbx/Hox binding site results in loss of expression in the anterior domain in which Hox family members from paralogous groups 1-5 are expressed. These observations demonstrate that binding elements for both neural and anterior-posterior position specific transcription factors mediate domains of Pax3 expression.

Oxidant regulation of gene expression and neural tube development: Insights gained from diabetic pregnancy on molecular causes of neural tube defects

AIMS/HYPOTHESIS

Maternal diabetes increases oxidative stress in embryos. Maternal diabetes also inhibits expression of embryonic genes, most notably, Pax-3, which is required for neural tube closure. Here we tested the hypothesis that oxidative stress inhibits expression of Pax-3, thereby providing a molecular basis for neural tube defects induced by diabetic pregnancy.

METHODS

Maternal diabetes-induced oxidative stress was blocked with alpha-tocopherol (vitamin E), and oxidative stress was induced with the complex III electron transport inhibitor, antimycin A, using pregnant diabetic or non-diabetic mice, primary cultures of neurulating mouse embryo tissues, or differentiating P19 embryonal carcinoma cells. Pax-3 expression was assayed by quantitative RT-PCR, and neural tube defects were scored by visual inspection. Oxidation-induced DNA fragmentation in P19 cells was assayed by electrophoretic analysis.

RESULTS

Maternal diabetes inhibited Pax-3 expression and increased neural tube defects, and alpha-tocopherol blocked these effects. In addition, induction of oxidative stress with antimycin A inhibited Pax-3 expression and increased neural tube defects. In cultured embryo tissues, high glucose-inhibited Pax-3 expression, and this effect was blocked by alpha-tocopherol and GSH-ethyl ester, and Pax-3 expression was inhibited by culture with antimycin A. In differentiating P19 cells, antimycin A inhibited Pax-3 induction but did not induce DNA strand breaks.

CONCLUSION/INTERPRETATION

Oxidative stress inhibits expression of Pax-3, a gene that is essential for neural tube closure. Impaired expression of essential developmental control genes could be the central mechanism by which neural tube defects occur during diabetic pregnancy, as well as other sources of oxidative stress.

Role for Hes1-induced phosphorylation in Groucho-mediated transcriptional repression

Transcriptional corepressors of the Groucho/transducin-like Enhancer of split (Gro/TLE) family regulate a number of developmental pathways in both invertebrates and vertebrates. They form transcription repression complexes with members of several DNA-binding protein families and participate in the regulation of the expression of numerous genes. Despite their pleiotropic roles, little is known about the mechanisms that regulate the functions of Gro/TLE proteins. It is shown here that Gro/TLEs become hyperphosphorylated in response to neural cell differentiation and interaction with the DNA-binding cofactor Hairy/Enhancer of split 1 (Hes1). Hyperphosphorylation of Gro/TLEs is correlated with a tight association with the nuclear compartment through interaction with chromatin, suggesting that hyperphosphorylated Gro/TLEs may mediate transcriptional repression via chromatin remodeling mechanisms. Pharmacological inhibition of protein kinase CK2 reduces the Hes1-induced hyperphosphorylation of Gro/TLEs and causes a decrease in the chromatin association of the latter. Moreover, the transcription repression activity of Gro/TLEs is reduced by protein kinase CK2 inhibition. Consistent with these observations, Gro/TLEs are phosphorylated in vitro by purified protein kinase CK2. Taken together, these results implicate protein kinase CK2 in Gro/TLE functions. They suggest further that this kinase is involved in a hyperphosphorylation mechanism activated by Hes1 that promotes the transcription repression functions of Hes1-Gro/TLE protein complexes.

Wnt signaling regulates the function of MyoD and myogenin

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

Pax3 is required for enteric ganglia formation and functions with Sox10 to modulate expression of c-ret

Hirschsprung disease and Waardenburg syndrome are human genetic diseases characterized by distinct neural crest defects. Patients with Hirschsprung disease suffer from gastrointestinal motility disorders, whereas Waardenburg syndrome consists of defective melanocyte function, deafness, and craniofacial abnormalities. Mutations responsible for Hirschsprung disease and Waardenburg syndrome have been identified, and some patients have been described with characteristics of both disorders. Here, we demonstrate that PAX3, which is often mutated in Waardenburg syndrome, is required for normal enteric ganglia formation. Pax3 can bind to and activate expression of the c-RET gene, which is often mutated in Hirschsprung disease. Pax3 functions with Sox10 to activate transcription of c-RET, and SOX10 mutations result in Waardenburg-Hirschsprung syndrome. Thus, Pax3, Sox10, and c-Ret are components of a neural crest development pathway, and interruption of this pathway at various stages results in neural crest-related human genetic syndromes.

Expression of PAX3 in Ewing's sarcoma family of tumors

The Ewing's sarcoma family of tumors (ESFT) is the second most common pediatric malignancy originating in the bone and is characterized by the t(11; 22) translocation. PAX3, a member of the paired box family of genes, is expressed during embryonal development of neural crest cells and is involved in the t(2; 13) translocation found in alveolar rhabdomyosarcoma. Since ESFTs are believed to be derived from neural crest tissue, we screened a series of Ewing's sarcoma and peripheral neuroectodermal tumor cell lines and tumor specimens for expression of PAX3. We found expression of PAX3 in most, but not all, of the specimens analyzed, including cell lines and patient material.

Positive and negative DNA sequence elements are required to establish the pattern of Pax3 expression

The transcription start site and DNA sequence elements required for the induction of Pax3 expression in differentiating P19 embryonal carcinoma cells have been localized. These elements consist of a promoter and additional elements located within 1.6 kbp 5′ to the transcription start site. Sequence elements within this 1.6 kbp region are also sufficient to mediate the induction and dorsal restriction of Pax3 in the neural tube and somites of transgenic mice throughout the hindbrain and trunk. Additional elements required for expression anterior to the hindbrain and in migrating myoblasts are located within 14 kbp 5′ to the transcription start site. This region also contains element(s) that repress Pax3 expression in the ventral body wall mesoderm of the tail bud.

Regulation of Oct-4 gene expression during differentiation of EC cells

The stem cell-specific factor Oct-4 is expressed in undifferentiated embryonal carcinoma and embryonic stem cells and is quickly down regulated upon RA-induced differentiation. Irrespective of the direction of differentiation, Oct-4 repression in P19 EC cells requires treatment with high doses of either all-trans or 9-cis RA. Unlike in P19 cells, no RA-induced down regulation of Oct-4 expression is observed in the P19-derived RA-resistant RAC65 cells. However, in these cells Oct-4 promoter repression can be rescued in a RA-dependent manner by cotransfection of RAR alpha 2 or RAR beta 2 but not RARr gamma 1, matching previously reported transactivation properties of these receptor types. In the vicinity of the transcription initiation site of the Oct-4 gene, three Hormone Response Element HRE half sites are present which are arranged as direct repeats with different spacing. In vitro translated RAR and RXR proteins bind to this HRE as heterodimers with low affinity, in such a way that all three HRE half sites contribute to complex formation. Although P19 EC cells contain weak binding activity interacting with the Oct-4 promoter HRE, strong binding activity is observed in nuclear extracts from RA-treated P19 cells. This binding activity was shown to correspond to COUP-TFs but not nuclear RA receptors. Moreover, the presence of these binding factors in nuclear extracts corresponds to silencing of Oct-4 expression. These results implicate RA and the action of its nuclear receptors in silencing Oct-4 expression upon differentiation of EC cells. The observed silencing is most likely not exerted by direct binding of RARs to the Oct-4 proximal promoter HRE. Our results support models in which different nuclear receptor complexes sequentially occupy different sites in the Oct-4 promoter HRE to silence Oct-4 expression during RA-induced differentiation.

Discrete endogenous signals mediate neural competence and induction in P19 embryonal carcinoma stem cells

Endogenous signals capable of inducing neuroectodermal differentiation are expressed by differentiating P19 EC cells in vitro. The present study demonstrates that at least two discrete signals are required. One is expressed by isolated primitive streak mesoderm-like cell lines and has the capacity to induce the expression of Pax-3 but, alone, induces neural differentiation inefficiently. The second signal is not expressed by the primitive streak mesoderm-like cell line but is present in conditioned media from differentiating P19 EC cells following DMSO treatment. This signal does not induce either Pax-3 expression or morphological differentiation and does not commit stem cells to a neuroectodermal fate. Rather, it acts synergistically with the signal derived from the primitive streak mesoderm-like cells to increase the efficiency with which stem cells respond initially by Pax-3 expression and subsequently by differentiation towards neural lineages. The activity of this second signal can be replaced by forskolin and 3-isobutyl-1-methyl-xanthine suggesting that its effects are transduced by a cyclic nucleotide-dependent pathway.

From embryonal carcinoma cells to neurons: the P19 pathway

The differentiation of mammalian neurons during development is a highly complex process involving regulation and coordination of gene expression at multiple steps. The P19 mouse embryonal carcinoma cell line is a suitable model system with which to analyze regulation of neuronal differentiation. These multipotential cells can be maintained and propagated in tissue culture in an undifferentiated state. Exposure of aggregated P19 cells to retinoic acid results in the differentiation of cells with many fundamental phenotypes of mammalian neurons. Undifferentiated P19 cells are amenable to genetic manipulations such as transfection and establishment of stable clonal cell lines expressing introduced genes. Proteins that play a key role in the neuronal differentiation of P19 cells are beginning to be identified. These include retinoic acid receptors, the epidermal growth factor receptor and the transcription factors Oct-3 and Brn-2. The biological and technical advantages of this system should facilitate deeper analysis of the activities of proteins that play a role in neuronal differentiation.

Primitive streak mesoderm-like cell lines expressing Pax-3 and Hox gene autoinducing activities

Differentiating P19 embryonal carcinoma (EC) cells transiently express an endogenous activity capable of inducing Pax-3 expression in adjacent P19 stem cells (Pruitt, Development 116, 573–583, 1992). In the present study, expression of this activity in mesodermal cell lineages is demonstrated. First, expression of the mesodermal marker Brachyury correlates with expression of Pax-3-inducing activity. Second, the ability of leukemia inhibitory factor (LIF) to block mesoderm differentiation at two different points is demonstrated and correlated with the inhibition of Pax-3-inducing activity. Finally, two mesodermal cell lines that express Pax-3-inducing activity were derived from P19 EC cells. Each of these lines expresses high levels of the mesodermal marker Brachyury and high levels of Oct-3/4 (which is down-regulated at early times during mesoderm differentiation) suggesting that these lines are early mesodermal derivatives. Unlike EC or embryonic stem cell lines, each of the two mesodermal derivatives autoinduces Hox gene expression on aggregation even in the presence of LIF. Following aggregation, anterior-specific genes are expressed more rapidly than more posterior genes. These observations directly demonstrate the ability of murine mesodermal derivatives to autoinduce Hox gene expression in the absence of signals from other cell lineages. Similar to the Pax-3-inducing activity, signals from mesodermal cell lines were sufficient to induce HOX expression in adjacent P19 stem cells in cell mixing assays. These observations are consistent with the previous suggestion (Blum, M., Gaunt, S. J., Cho, K. W. Y., Steinbeisser, H., Blumberg, B., Bittner, D. and De Robertis, E. M. (1992) Cell 69, 1097–1106) that signals responsible for anterior-posterior organizer activity are localized to the anterior primitive streak mesoderm of the mouse embryo.

Transcriptional regulation of the development of neurons and glia

The past year has seen significant progress in the analysis of transcriptional regulation as it relates to neural development. Highlights include the identification and analysis of new homeobox genes that delimit developmental boundaries in the vertebrate forebrain, the study of upstream regulators of homeobox genes, the analysis of Pax genes that may contribute to specification of the vertebrate dorso-ventral neuraxis, and the functional analysis of transcription factors that are likely to specify particular neural cell types in both vertebrate and invertebrate nervous systems.