Pharyngeal arch artery defects and lethal malformations of the aortic arch and its branches in mice deficient for the Hrt1/Hey1 transcription factor

The aortic arch and major branch arteries are formed from the three pairs of pharyngeal arch arteries (PAAs) during embryonic development. Their morphological defects are clinically observed as isolated diseases, as a part of complicated cardiovascular anomalies or as a manifestation of multi-organ syndromes such as 22q11.2 deletion syndrome. Although numerous genes have been implicated in PAA formation and remodeling, detailed mechanisms remain poorly understood. Here we report that the mice null for Hrt1/Hey1, a gene encoding a downstream transcription factor of Notch and ALK1 signaling pathways, show perinatal lethality on the C57BL/6N, C57BL/6N × C57BL/6J or C57BL/6N × 129X1/SvJ background. Hrt1/Hey1 null embryos display abnormal development of the fourth PAA (PAA4), which results in congenital vascular defects including right-sided aortic arch, interruption of the aortic arch and aberrant origin of the right subclavian artery. Impaired vessel formation occurs randomly in PAA4 of Hrt1/Hey1 null embryos, which likely causes the variability of congenital malformations. Endothelial cells in PAA4 of null embryos differentiate normally but are structurally disorganized at embryonic day 10.5 and 11.5. Vascular smooth muscle cells are nearly absent in the structurally-defective PAA4, despite the appropriate migration of cardiac neural crest cells into the fourth pharyngeal arches. Endothelial expression of Jag1 is down-regulated in the structurally-defective PAA4 of null embryos, which may be one of the mechanisms underlying the suppression of vascular smooth muscle cell differentiation. While the direct downstream phenomena of the Hrt1/Hey1 deficiency remain to be clarified, we suggest that Hrt1/Hey1-dependent transcriptional regulation has an important role in PAA formation during embryonic development.

An important role of endothelial hairy-related transcription factors in mouse vascular development

The Hairy-related transcription factor family of Notch- and ALK1-downstream transcriptional repressors, called Hrt/Hey/Hesr/Chf/Herp/Gridlock, has complementary and indispensable functions for vascular development. While mouse embryos null for either Hrt1/Hey1 or Hrt2/Hey2 did not show early vascular phenotypes, Hrt1/Hey1; Hrt2/Hey2 double null mice (H1(ko) /H2(ko) ) showed embryonic lethality with severe impairment of vascular morphogenesis. It remained unclear, however, whether Hrt/Hey functions are required in endothelial cells or vascular smooth muscle cells. In this study, we demonstrate that mice with endothelial-specific deletion of Hrt2/Hey2 combined with global Hrt1/Hey1 deletion (H1(ko) /H2(eko) ) show abnormal vascular morphogenesis and embryonic lethality. Their defects were characterized by the failure of vascular network formation in the yolk sac, abnormalities of embryonic vascular structures and impaired smooth muscle cell recruitment, and were virtually identical to the H1(ko) /H2(ko) phenotypes. Among signaling molecules implicated in vascular development, Robo4 expression was significantly increased and activation of Src family kinases was suppressed in endothelial cells of H1(ko) /H2(eko) embryos. The present study indicates an important role of Hrt1/Hey1 and Hrt2/Hey2 in endothelial cells during early vascular development, and further suggests involvement of Robo4 and Src family kinases in the mechanisms of embryonic vascular defects caused by the Hrt/Hey deficiency.

Interferon-gamma induced medulloblastoma in the developing cerebellum

We have generated a mouse model system with a high incidence of medulloblastoma, a malignant neoplasm believed to arise from immature precursors of cerebellar granule neurons. These animals ectopically express interferon-gamma (IFN-gamma) in astrocytes in the CNS in a controlled manner, exploiting the tetracycline-controllable system. More than 80% of these mice display severe ataxia and develop cerebellar tumors that express synaptophysin, the mouse atonal homolog MATH1, sonic hedgehog (SHH), and Gli1. IFN-gamma-induced tumorigenesis in these mice is associated with increased expression of SHH, and SHH induction and tumorigenesis are dependent on signal transducer and activator of transcription 1 (STAT1). When IFN-gamma expression is shut down with doxycycline at postnatal day 12 (P12), the clinical symptoms dissipate and the mice do not develop tumors, whereas if transgene expression is shut down at P16, the clinical symptoms and tumors progress to lethality, indicating that IFN-gamma is required for tumor induction but not progression. The tumors that occur in the continued presence of IFN-gamma display extensive necrosis and apoptosis as well as macrophage and lymphocytic infiltration, whereas the tumors that develop in mice in which IFN-gamma expression is shut down at P16 do not. Thus, IFN-gamma expression in the perinatal period can induce SHH expression and medulloblastoma in the cerebellum by a STAT1-dependent mechanism, and its continued presence appears to promote a host response to the tumor.

Ganglioneuromatous paraganglioma of the cauda equina—a pathological case study

This study presents a rare case of compound paraganglioma/ganglioneuroma with comprehensive immunohistochemical studies that reveal strong cytokeratin expression in all components. A 74-year-old woman presented with a mass lesion of the cauda equina. The 1.8-cm tumor showed 3 histomorphologically and immunohistochemically distinct components: typical paragangliomatous neuroendocrine areas, mature ganglion cell-like neuronal areas, and a "neuromatous" proliferation of Schwann cells with admixed axons. As often seen in cauda equina paragangliomas, the neuroendocrine cells were cytokeratin-positive. In addition, immunoreactivity for cytokeratins was also observed in the neurons and axons. This tumor illustrates the broad spectrum of divergent differentiation that can be seen in cells of sympathoadrenal lineage.

Tetralogy of fallot and other congenital heart defects in Hey2 mutant mice

Congenital malformations of the heart and circulatory system are the most common type of human birth defect. Recent studies have implicated the Notch signaling pathway in human cardiac development by demonstrating abnormalities of the JAG1 gene as the basis for Alagille syndrome and some cases of isolated tetralogy of Fallot or pulmonic stenosis. How the Notch pathway acts in cardiac development remains unknown, but the Hey family of basic helix-loop-helix (bHLH) transcription factors are candidates for mediating Notch signaling in the developing cardiovascular system. Here, we use gene targeting to determine the developmental functions of mouse Hey2, a Hey family member that is expressed during the embryonic development of the heart, arteries, and other organs. Homozygotes for the Hey2 mutant allele display a spectrum of cardiac malformations including ventricular septal defects, tetralogy of Fallot, and tricuspid atresia, defects that resemble those associated with mutations of human JAG1. These results establish Hey2 as an important regulator of cardiac morphogenesis and suggest a role for Hey2 in mediating or modulating Notch signaling in the developing heart.

A repertoire of differentially expressed transcription factors that offers insight into mechanisms of human cytotrophoblast differentiation

During human placental development, specialized cells allocated to the extraembryonic lineage (cytotrophoblasts) invade the uterus, anchoring the conceptus to the decidua and tapping a supply of maternal blood. This unusual behavior requires cytotrophoblasts to assume highly specialized characteristics; some are commonly associated with tumor cells, while others are typical of endothelia. Here we investigated the transcriptional mechanisms that control cytotrophoblast differentiation/invasion. Specifically, we examined the cells' expression of a number of transcription factors, at the RNA level, as they differentiated along the invasive pathway in vitro. Since basic helix-loop-helix (bHLH) proteins play important roles in murine trophoblast differentiation, we first examined their expression by cytotrophoblasts. As in murine placental development, expression of the human homologue of Mash-2 was confined to progenitor cells. But expression of Hand-1, which promotes differentiation of murine trophoblast giant cells, was not detected. We also found that cytotrophoblasts upregulated the expression of bHLH/PAS factors that function in adaptive responses to hypoxia, including hEPAS-1, which is expressed primarily in endothelial cells. Quite unexpectedly, we discovered that cytotrophoblasts express high levels of mRNA encoding the human homologue of the Drosophila neuronal fate gene, glial cells missing-1 (gcm-1). We also found evidence of crosstalk between the bHLH and GCM-1 regulatory networks. Together, these results offer insights into the transcriptional mechanisms that govern cytotrophoblast differentiation/invasion. Interestingly, these mechanisms suggest analogies with those that govern differentiation of murine stem cells allocated to both the intra- and extraembryonic lineages.

The developmental expression pattern of a new murine homeo box gene: Hox-2.5

To examine the possible role of homeo box genes in murine development we have studied the structure and expression pattern of Hox-2.5, a newly isolated homeo box gene that maps to the left end of the mouse Hox-2 locus on chromosome 11. The sequence of the Hox-2.5 homeo box has been determined. It is highly homologous to Hox-1.7 and Hox-3.2, demonstrating extended conservation among three homeo box complexes in the mouse. Northern and in situ hybridization analyses of Hox-2.5 demonstrate a novel, regionally restricted pattern of expression in developing mesoderm and neurectoderm. We detect localized Hox-2.5 transcripts as early as 8.5 days postcoitum. The expression pattern of Hox-2.5 was analyzed over the next 3 days of ontogeny, as well as in later embryonic, newborn, and adult stages. Three-dimensional reconstruction of Hox-2.5 transcript localization within the central nervous system of early embryos clearly illustrates the neural expression domain. Although the Hox-2.5 expression pattern is regionally restricted during all of these stages of development, the pattern changes along the anteroposterior and dorsoventral axes of the CNS as the embryo undergoes complex morphogenetic movements and cytodifferentiation.

Structure and expression of Hox-2.2, a murine homeobox-containing gene

The Hox-2.2 gene is one of a cluster of homeobox-containing genes on mouse chromosome 11. A cDNA clone containing the Hox-2.2 homeobox has been isolated from an adult spinal cord library. Our analysis of the Hox-2.2 cDNA and genomic clones indicates that there are at least two oxons and one intron. The largest open reading frame includes the homeobox and codes for a 224 amino acid protein of molecular weight 25,312. Comparisons of the predicted Hox-2.2 protein with other homeodomain-containing proteins revealed four regions of sequence similiarity: an N-terminal octapeptide, a hexapeptide upstream of the homeodomain, the homeodomain, and a glutamic acid-rich region at the C terminus. Possible functions of these regions are discussed. The Hox-2.2 gene is expressed in 13.5-day embryos in the developing hindbrain and spinal cord. The expression patterns of Hox-2.2 and Hox-2.1 in 13.5-day embryos are compared.

Tissue specificity, localization in brain, and cell-free translation of mRNA encoding the A3 isoform of Na+,K+-ATPase

The isolation of multiple Na+,K+-ATPase cDNAs from rat brain has led to the discovery of a family of alpha-isoform genes. Using A1 (alpha), A2 (alpha+), and A3 (alpha III) Na+,K+-ATPase gene probes, we have analyzed the distribution of Na+,K+-ATPase mRNAs in adult and fetal rat tissues by RNA blot and hybridization histochemistry. A1 Na+,K+-ATPase mRNA was found ubiquitously among various tissues, with highest levels in transport epithelial and neural tissues. A2 mRNA was found in adult neural and muscle tissues, and A3 mRNA was found only in neural tissues and fetal heart muscle. Both A1 and A2 mRNAs were less abundant in fetal brain than in adult brain; in contrast, A3 mRNA was abundant at both stages. In situ mapping of brain areas that contain A3 mRNA suggests that this Na+,K+-ATPase isoenzyme is expressed predominantly by neural cells. Analysis of Na+,K+-ATPase proteins generated by cell-free translation of synthetic mRNAs suggests that the A3 protein has properties similar to A2 (alpha+).

Region-specific expression of two mouse homeo box genes

Mammalian homeo box genes have been identified on the basis of sequence homology to Drosophila homeotic and segmentation genes. These studies examine the distribution of transcripts from two mouse homeo box genes, Hox-2.1 and Hox-3.1, throughout the latter third of prenatal development. Transcripts from these genes are regionally localized along the rostro-caudal axis of the developing central nervous system, yielding expression patterns very similar to patterns of Drosophila homeotic gene expression.

Homeo box genes in murine development

Considerable information has accumulated on mouse homeo box gene organization and expression. Homeo box genes are expressed in a wide variety of tissues, developmental stages, and cell lines. How can this be interpreted in view of the relationship of these genes to Drosophila morphogenetic loci? One view is that homeo box genes control determinative decisions by modulating transcription of as yet unidentified target genes. Proponents of this view are faced with two tasks: to identify developmental processes that are controlled by homeo box genes, and to identify the target genes that mediate this control. Such target genes might be identified on the basis of in vitro homeo domain-DNA interactions. Candidate morphogenetic processes might be identified on the basis of the observed patterns of homeo box gene expression. It must be stressed that finding expression in a given tissue in no way demonstrates that the expression is necessary for the determination of that tissue. The role of Drosophila homeo box genes in determinative decisions is based upon analysis of mutants to demonstrate that the pattern of homeo box gene expression determines the morphogenetic outcome. To test whether the expression of a mouse homeo box gene is involved in a determinative decision, one must disrupt the normal pattern of expression of that gene and observe the resulting morphogenetic effect. In mouse this can be approached by looking for allelism with known morphogenetic loci, by isolating mutants in homeo box genes through large-scale mutagenesis screens, or by introducing altered homeo box genes into transgenic mice. One of the most intriguing possibilities is that homeo box genes are involved in regional specification along the anteroposterior axis. In situ hybridization and Northern blot analysis have demonstrated that at least four different homeo box genes display distinct regional patterns of expression along the anteroposterior axis of the developing CNS. The expression of each of these genes has a unique anterior boundary from which expression extends posteriorly within the CNS. Hox 1.5 expression has an anterior boundary within the hindbrain just posterior to the pontine flexure. The anterior boundary of Hox 2.1 expression lies more posteriorly within the medulla of the hindbrain. Weak expression of Hox 2.5 is detected in the spinal cord just posterior to the first cervical vertebra, and maximal expression is found posterior to the second cervical vertebra.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial restriction in expression of a mouse homoeo box locus within the central nervous system

A common feature of Drosophila homoeo box genes appears to be their spatially restricted expression patterns during morphogenesis. Using Northern blot analysis and in situ hybridization to mouse tissue sections, the spatially restricted expression of a newly identified mouse homoeo box locus, Hox-3, within the central nervous system of newborn and adult mice has been demonstrated.