Analysis of Coronary Vessels in Cleared Embryonic Hearts

Whole mount visualization of the embryonic coronary plexus from which the capillary and arterial networks will form is rendered problematic using standard ...

UCL Institute of Child Health

The PLZF gene is translocated in a subset of all-trans-retinoic acid resistant acute promyelocytic leukaemia (APL) cases, encodes a DNA binding transcription factor and is expressed highly in haematopoietic progenitor cells as well-developing central nervous system (CNS). The spatially restricted and temporally dynamic pattern of PLZF expression in the developing CNS suggested that it might play a role in the circuitry regulating hindbrain segmentation. We have now identified a PLZF binding site (PLZF-RE) in an enhancer region of Hoxb2 that itself is required for directing high-level expression in rhombomers 3 and 5 of the developing hindbrain. The wild-type r3/r5 enhancer linked to a heterologous promoter was responsive to regulation by PLZF, and this activity was lost in variants containing a mutated PLZF-RE. Compared with the wild-type protein, the binding of the APL-associated reciprocal RARalpha-PLZF fusion to PLZF-RE was much stronger, suggesting that the N-terminal PLZF sequences missing from the fusion may play a role in the regulation of DNA binding. Consistent with this, the N-terminal POZ domain was required for cooperative binding of PLZF to a multimerized PLZF-RE. In the context of the r3/r5 enhancer, the PLZF-RE cooperated for PLZF binding with an additional A/T-rich motif positioned downstream of the PLZF-RE. This A/T motif was previously shown to be essential for the regulation of Hoxb2 expression in r3 and r5 in cooperation with another Krüppel-like zinc finger protein Krox 20. The presence of both the PLZF-RE and the A/T-rich motif was required for a maximal effect of PLZF on a heterologous promoter and was essential in vivo to direct the expression of a lacZ reporter in the chick neural tube. Hence, both PLZF and Krox20 cooperate with a common A/T motif in mediating in vivo activity of the Hoxb2 enhancer. Our findings indicate that Hoxb2 is a direct target for regulation by PLZF in the developing CNS and suggest that deregulation of Hox gene expression may contribute to APL pathogenesis.

Regulation of Hoxb2 by APL-associated PLZF protein.

Both Tbx1 and retinoic acid (RA) are key players in embryonic pharyngeal development; loss of Tbx1 produces DiGeorge syndrome-like phenotypes in mouse models as does disruption of retinoic acid homeostasis. We have demonstrated that perturbation of retinoic acid levels in the avian embryo produces altered Tbx1 expression. In vitamin A-deficient quails, which lack endogenous retinoic acid, Tbx1 expression patterns were disrupted early in development and expression was subsequently lost in all tissues. "Gain-of-function" experiments where RA-soaked beads were grafted into the pharyngeal region produced localized down-regulation of Tbx1 expression. In these embryos, analysis of Shh and Foxa2, upstream control factors for Tbx1, suggested that the effect of RA was independent of this regulatory pathway. Real-time polymerase chain reaction analysis of retinoic acid-treated P19 cells showed a dose-dependent repression of Tbx1 by retinoic acid. Repression of Tbx1 transcript levels was first evident after 8-12 hr in culture in the presence of retinoic acid, and to achieve the highest levels of repression, de novo protein synthesis was required.

Retinoic acid down-regulates Tbx1 expression in vivo and in vitro.

The development of pharyngeal arch derivatives in mouse and zebrafish embryos depends on the activity of the transcription factor Tbx1. We cloned the Xenopus laevis orthologue of Tbx1 (XTbx1) and show that the pattern of expression is similar to that in other vertebrate species. Zygotic transcripts are first detected shortly after the mid-blastula transition and are localized to the presumptive mesoderm at mid-gastrula stages. XTbx1 expression persists in the lateral plate mesoderm at neurula stages and is found in the pharyngeal arches and otic vesicles from early tail bud stages onward. We demonstrate that XTbx1 is a transcriptional activator and that this trans-activation requires the C-terminal region of the protein. A dominant interfering mutant of XTbx1 disrupts the development of Xenopus head structures and pharyngeal arch derivatives. Lineage labeling reveals a requirement for XTbx1 function in cells that contribute to the pharyngeal mesoderm and for fgf8 expression.

XTbx1 is a transcriptional activator involved in head and pharyngeal arch development in Xenopus laevis.

The 22q11 deletion syndrome (22q11DS; DiGeorge/velo-cardio-facial syndrome) primarily affects the structures comprising the pharyngeal arches and pouches resulting in arch artery, cardiac, parathyroid, thymus, palatal and craniofacial defects. Tbx1 haploinsufficiency is thought to account for the main structural anomalies observed in the 22q11DS. The Df1 deleted mouse provides a model for 22q11DS, the deletion reflecting Tbx1 haploinsufficiency in the context of the deletion of 21 adjacent genes. We examined the expression of genes in Df1 embryos at embryonic day (E) 10.5, a stage when the arch-artery phenotype is fully penetrant. Our aims were threefold, with our primary aim to identify differentially regulated genes. Second, we asked whether any of the genes hemizygous in Df1 were dosage compensated to wild type levels, and third we investigated whether genes immediately adjacent to the deletion were dysregulated secondary to a position effect. Utilisation of oligonulceotide arrays allowed us to achieve our aims with 9 out of 12 Df1 deleted genes passing the stringent statistical filtering applied. Several genes involved in vasculogenesis and cardiogenesis were validated by real time quantitative PCR (RTQPCR), including Connexin 45, a gene required for normal vascular development, and Dnajb9 a gene implicated in microvascular differentiation. There was no evidence of any dosage compensation of deleted genes, suggesting this phenomenon is rare, and no dysregulation of genes mapping immediately adjacent to the deletion was detected. However Crkl, another gene implicated in the 22q11DS phenotype, was found to be downregulated by microarray and RTQPCR.

Microarray analysis of the Df1 mouse model of the 22q11 deletion syndrome.

Histone acetyltransferase (HAT) activities of proteins such as p300, CBP, and P/CAF play important roles in activation of gene expression. We now show that the HAT activity of p300 can also be required for down-regulation of transcription by a DNA binding repressor protein. Promyelocytic leukemia zinc finger (PLZF), originally identified as a fusion with retinoic acid receptor alpha in rare cases of all-trans-retinoic acid-resistant acute promyelocytic leukemia, is a transcriptional repressor that recruits histone deacetylase-containing corepressor complexes to specific DNA binding sites. PLZF associates with p300 in vivo, and its ability to repress transcription is specifically dependent on HAT activity of p300 and acetylation of lysines in its C-terminal C2-H2 zinc finger motif. An acetylation site mutant of PLZF does not repress transcription and is functionally deficient in a colony suppression assay despite retaining its abilities to interact with corepressor/histone deacetylase complexes. This is due to the fact that acetylation of PLZF activates its ability to bind specific DNA sequences both in vitro and in vivo. Taken together, our results indicate that a histone deacetylase-dependent transcriptional repressor can be positively regulated through acetylation and point to an unexpected role of a coactivator protein in transcriptional repression.

Histone acetyltransferase activity of p300 is required for transcriptional repression by the promyelocytic leukemia zinc finger protein.

22q11-deletion (DiGeorge/velocardiofacial) syndrome (22q11DS) is modeled by mutation of murine transcription factor Tbx1. As part of efforts to identify transcriptional targets of Tbx1, we analyzed the transcriptome of the pharyngeal region of Df1/+;Tbx1+/- embryos at 9.5 days of embryonic development using two independent microarray platforms. In this model, embryos are null for Tbx1, with hemizygosity of genes in cis with Tbx1 on one chromosome providing a positive control for array sensitivity. Reduced mRNA levels of genes deleted from Df1 were detected on both platforms. Expression level filtering and statistical analysis identified several genes that were consistently differentially expressed between mutant and wild type embryos. Real-time quantitative PCR and in situ hybridization validated diminished expression of Pax9 and Gcm2, genes known to be required for normal thymus and parathyroid gland morphogenesis, whereas Pax1, Hoxa3, Eya1, and Foxn1, which are similarly required, were not down-regulated. Gbx2, a gene required for normal arch artery development, was down-regulated specifically in the pharyngeal endoderm and the posterior part of pharyngeal arch 1, and is a potential point of cross talk between the Tbx1 and Fgf8 controlled pathways. These experiments highlight which genes and pathways potentially affected by lack of Tbx1, and whose role may be explored further by testing for epistasis using mouse mutants.

Microarray analysis detects differentially expressed genes in the pharyngeal region of mice lacking Tbx1.

Cyp26a1, a gene required for retinoic acid (RA) inactivation during embryogenesis, was previously identified as a potential Tbx1 target from a microarray screen comparing wild-type and null Tbx1 mouse embryo pharyngeal arches (pa) at E9.5. Using real-time PCR and in situ hybridization analysis of Cyp26a1 and its two functionally related family members Cyp26b1 and c1, we demonstrate reduced and/or altered expression for all three genes in pharyngeal tissues of Tbx1 null embryos. Blockade of Cyp26 function in the chick embryo using R115866, a specific inhibitor of Cyp26 enzyme function, resulted in a dose-dependent phenocopy of the Tbx1 null mouse including loss of caudal pa and pharyngeal arch arteries (paa), small otic vesicles, loss of head mesenchyme and, at later stages, DiGeorge Syndrome-like heart defects, including common arterial trunk and perimembranous ventricular septal defects. Molecular markers revealed a serious disruption of pharyngeal pouch endoderm (ppe) morphogenesis and reduced staining for smooth muscle cells in paa. Expression of the RA synthesizing enzyme Raldh2 was also up-regulated and altered Hoxb1 expression indicated that RA levels are raised in R115866-treated embryos as reported for Tbx1 null mice. Down-regulation of Tbx1 itself was observed, in accordance with previous observations that RA represses Tbx1 expression. Thus, by specifically blocking the action of the Cyp26 enzymes we can recapitulate many elements of the Tbx1 mutant mouse, supporting the hypothesis that the dysregulation of RA-controlled morphogenesis contributes to the Tbx1 loss of function phenotype.

Cyp26 genes a1, b1 and c1 are down-regulated in Tbx1 null mice and inhibition of Cyp26 enzyme function produces a phenocopy of DiGeorge Syndrome in the chick.

Elucidating the gene regulatory networks that govern pharyngeal arch artery (PAA) development is an important goal, as such knowledge can help to identify new genes involved in cardiovascular disease. The transcription factor Tbx1 plays a vital role in PAA development and is a major contributor to cardiovascular disease associated with DiGeorge syndrome. In this report, we used various genetic approaches to reveal part of a signalling network by which Tbx1 controls PAA development in mice. We investigated the crucial role played by the homeobox-containing transcription factor Gbx2 downstream of Tbx1. We found that PAA formation requires the pharyngeal surface ectoderm as a key signalling centre from which Gbx2, in response to Tbx1, triggers essential directional cues to the adjacent cardiac neural crest cells (cNCCs) en route to the caudal PAAs. Abrogation of this signal generates cNCC patterning defects leading to PAA abnormalities. Finally, we showed that the Slit/Robo signalling pathway is activated during cNCC migration and that components of this pathway are affected in Gbx2 and Tbx1 mutant embryos at the time of PAA development. We propose that the spatiotemporal control of this tightly orchestrated network of genes participates in crucial aspects of PAA development.

Tbx1 controls cardiac neural crest cell migration during arch artery development by regulating Gbx2 expression in the pharyngeal ectoderm.

22q11.2 Deletion Syndrome (22q11DS) arises from an interstitial chromosomal microdeletion encompassing at least 30 genes. This disorder is one of the most significant known cytogenetic risk factors for schizophrenia, and can also cause heart abnormalities, cognitive deficits, hearing difficulties, and a variety of other medical problems. The Df1/+ hemizygous knockout mouse, a model for human 22q11DS, recapitulates many of the deficits observed in the human syndrome including heart defects, impaired memory, and abnormal auditory sensorimotor gating. Here we show that Df1/+ mice, like human 22q11DS patients, have substantial rates of hearing loss arising from chronic middle ear infection. Auditory brainstem response (ABR) measurements revealed significant elevation of click-response thresholds in 48% of Df1/+ mice, often in only one ear. Anatomical and histological analysis of the middle ear demonstrated no gross structural abnormalities, but frequent signs of otitis media (OM, chronic inflammation of the middle ear), including excessive effusion and thickened mucosa. In mice for which both in vivo ABR thresholds and post mortem middle-ear histology were obtained, the severity of signs of OM correlated directly with the level of hearing impairment. These results suggest that abnormal auditory sensorimotor gating previously reported in mouse models of 22q11DS could arise from abnormalities in auditory processing. Furthermore, the findings indicate that Df1/+ mice are an excellent model for increased risk of OM in human 22q11DS patients. Given the frequently monaural nature of OM in Df1/+ mice, these animals could also be a powerful tool for investigating the interplay between genetic and environmental causes of OM.

Hearing loss in a mouse model of 22q11.2 Deletion Syndrome.

The chemokine CXCL12 and its receptor CXCR4 have many functions during embryonic and post-natal life. We used murine models to investigate the role of CXCL12/CXCR4 signaling in cardiac development and found that embryonic Cxcl12-null hearts lacked intra-ventricular coronary arteries (CAs) and exhibited absent or misplaced CA stems. We traced the origin of this phenotype to defects in the early stages of CA stem formation. CA stems derive from the peritruncal plexus, an encircling capillary network that invades the wall of the developing aorta. We showed that CXCL12 is present at high levels in the outflow tract, while peritruncal endothelial cells (ECs) express CXCR4. In the absence of CXCL12, ECs were abnormally localized and impaired in their ability to anastomose with the aortic lumen. We propose that CXCL12 is required for connection of peritruncal plexus ECs to the aortic endothelium and thus plays a vital role in CA formation.

Sarah Ivins

Developmental Biology of Birth Defects

UCL Institute of Child Health

Analysis of Coronary Vessels in Cleared Embryonic Hearts

Sarah Ivins

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UCL Institute of Child Health

Analysis of Coronary Vessels in Cleared Embryonic Hearts

Developmental Biology of Birth Defects